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Power Theory for Petrol Heads!


They reckon that out of the audience for the gazillions of TV cookery shows bouncing around our atmosphere these days, not only will hardly any-one that watches them so avidly, ever, attempt one of the recipes demonstrated, MOST, and I mean over HALF wont even venture into the kitchen to make anything more adventurous than cornflakes!

When it comes to cars and motorbikes, by a similar set of statistics, MOST drivers will never even THINK about their engine, let alone get their hands dirty doing so much as check the oil and water! So, I doubt very many people who may read this will ever venture into their engine with ratchet and torque wrench and start pondering whether to use bigger valves, more valve lift or a longer duration to get more power..... of those few even remotely likely to start messing with their engine, I expect that 90% will go no further than considering a 'performance chip' for their engine management computer, and possibly a performance exhaust system and air filter! But then again, I do hope to inspire..... But still!

It is the perennial debate of bars, clubs, paddock and playground; POWER.

There are many more adages, nuggets of wisdom, debates, arguments, myths legends and bits of 'lore' fuelling the debates, and like a colon, EVERYONE has an opinion about 'power' and what aspect of it is important.

Now, the thing is, a lot of this 'lore' has some truth in it, but unfortunately so diluted and confused that often it gets completely lost and the 'logic' of the debates can often make a complete nonsense, because unfortunately, very few of people actually understand the concept of power....... and probably more significantly, few can translate that concept into the real world where the power an engine delivers, gets used to shove a vehicle along!

And, remember by the proportion of people that use their kitchen for no more than making a bowl of cornflakes, MOST of the rubbish spouted in debate about engines is provided very authoritatively by people who have NEVER built a high performance vehicle! MOST will base their experience on second hand reports, magazine articles, and something they might have done to a Mini, Escort or Yamaha twenty years ago, which of COURSE made it absolutely fantastic, and similar 'authoritative' opinion received from other similar 'experts'!

So, in this bit of diatribe I want to look at  a few things, and try and explode a few myths, looking at;  'Power', and what it really is, or more precisely, MIGHT be, and THRUST, which is the thing that is REALLY the important and controlling factor, I hardly EVER hear mentioned!

But more significantly, the way that engines 'behave' in real applications, and look at the matter of 'responsiveness' or 'tractability', which are things that are very subjective, and really influence the way a vehicle 'works', and what people often try and describe as 'useable-power'.... or 'power delivery' But lets start at the beginning....



This is the primary definition of power, and from it we get lots of derived formula and equations, that often do little to help explain or de-mystify the myths! But lets start here. First thing to acknowledge is that power is a RATE. That is to say its something to do with time, it is not a physical commodity, something you can see, touch, taste or smell! You cannot 'Feel the Power', in the same way you cant feel the interest rate on your credit card or mortgage..... well not directly, anyway!

Good anaolgy though. If I buy something on a credit card or finance deal, I never 'see' anything that is 24% APR. Its simply an imaginary commodity. What I 'see' is £'s coming out of my bank account.

Power is the same deal. If we have a 100BHP engine we don't FEEL the POWER what we feel is the FORCE that engine makes shoving us along, the engine is actually making FORCE, but what we experience is another 'rate' acceleration, in conjunction with that force, which is where much of the confusion lies.

Because, the 3.9l V8 in my Rangie is a 200 bhp engine, isn't it? Well, near enough. Thing IS, that is merely the quoted power rating; how much power the engine MIGHT make, at 'maximum', and then, when measured on a 'Brake Force Dynometer'... the 'Brake' in Brake Horse-Power.....




So, scrappy sketch time; we have a picture of a short wheel base Land-Rover, and a few arrows scrawled on it.

The first, just over the bonnet is marked 'Drag' and points at the windscreen.

Then there are a pair of red arrows, marked 'Thrust',  pointing in opposite directions down at the rear wheel (so this must be a Series Landy in Hi-Range then!)

And lastly, another marked 'Traction'.

Now, Thrust is the force that pushes a vehicle along, and is often called 'Motive Force' or 'Tractive Force' because it comes from the 'torque' acting on a wheel, and that 'Torque' actually gives us two forces, one pushing backwards against 'Traction', the other forwards against 'Drag'.

Which is why there are four arrows on the piccy. Newton gave us a law you are probably familiar with; 'Every Action has an Equal and Opposite Re-action', which basically means that every force has a partner working against it.

It's a bit like trying to run on a sheet of ice. Without any friction between your feet and the surface you are trying to run on, they just slip, and instead of moving forwards, your feet just flail about beneath you until you fall over!

So, dealing with 'Traction' first, the simplified explanation of Traction is that as long as you don't apply more thrust force than the road surface can react with traction, then all of your thrust can be used to push you along. But, as soon as your 'Thrust' exceeds the limit of traction, or your 'Grip', then the wheel will start to slip, and you cant react ANY thrust at all.

But as long as we have more grip than thrust, though, we have what is known as 'Ideal' conditions, and usually when looking at the science of stuff, we presume 'ideal conditions', to do the sums, then have to start doing a lot of fudging to make the theory fit the 'real world' because you rarely get 'ideal conditions' in it! But in this case we don't!

In the case of motor-vehicles, 99% of the time we actually DO get as close as makes no odds to ideal conditions! Of all the billions of vehicles on our roads, not MANY of them spend that much of their time, spinning wheels, skidding and sliding about, the tyres grip and everything works like the science says it should!

So for the most part we can pretty much ignore the balance between traction and thrust, presume 'ideal conditions' and simply worry about the tussle of forces between traction and drag.

Yes, I know that the 'interesting' bit is where we DON'T have ideal conditions, like trying to launch a motorbike rapidly off the line without the back wheel spinning, or driving a four-by-four over a muddy hill where there is bog all grip, but if you are interested in all that, what is known technically as 'boundary effects' or in English 'those places the science falls apart!' you may like to have a look at the article, Get A Grip. that deals with it in more detail!

So back to our forces, and another of Newton's laws of physics, forces are the things that make stuff move, more specifically accelerate.

Apply a force to a 'body' and the body will accelerate. That is the easy bit. More difficult bit is that if a body is NOT subjected to a force, it will move at constant speed.

Making a bit more sense of that, if we don't give the Landy in the piccy any thrust, it wont move. Apply some thrust, and it will accelerate. Take away the thrust, and it will 'Coast' until we apply some braking (reverse thrust!) then it will slow down.

So that's 'Thrust', as for 'Drag', strictly it is the 'TOTAL' resistance to motion.

As soon as our vehicle starts to move, we will get friction between the wheels and the road, and wind resistance from trying to shove air aside to push our vehicle through it, and put together, that all equals 'Drag'.

So, when we apply our 'Thrust', our Landy accelerates, but as soon as it has started moving, it becomes subject to drag which resists the thrust, and tries to slow it down!

Now comes the difficult bit, drag increases with speed, and exponentially, which is, if you go twice as fast you get more than twice as much drag, in fact a LOT more than twice as much drag.

Anyway, the way it works is this, you apply a thrust, and that makes you accelerate and gain speed. As you gain speed, you gain drag, and that opposes your thrust.

So at the start, ALL your thrust is used to accelerate, but once moving, you gain drag, so some of your thrust gets used to over come that drag and keep you moving at a constant speed, and only what is left over is available to make you accelerate.

So! IF you had a constant 'thrust' force, you would start by accelerating strongly, but as you gained speed, so you would gain drag, and your rate of acceleration would slowly bleed away until you reached a speed where the thrust force exactly matched the drag force and you had nothing left for acceleration.

Which is why I said that the really IMPORTANT thing about power is Thrust, because at the end of the day, whatever is going on under the bonnet or in the widgery bits in between, where it counts is where it is put to work, as 'Thrust' at the driven wheels. It is Thrust that makes you move, and I would hope that is what you are bothered about!

More thrust you have, the more acceleration you are going to get, the more drag you can over come, and ultimately the faster you will be able to go.

Getting Thrust

So how do you get 'Thrust'? If that is the 'stuff' that is important, where does it come from, and how do we get it to where it is useful? Well, it comes from the engine. Inside our engine we usually have some pistons and valves and stuff

The 'Useful' bit of what is going on though, is the 'Power Stroke', when the charge in the cylinder is set on fire and burned, heating the gasses in the chamber, making them expand and creating a pressure that pushes our piston down.

That ''pressure' is a force, and the piston directs that force into the con-rod, the con-rod directs it into the crank-shaft,. and that directs it into the gearbox and transmission, that eventually channel it (or most of it!) to our driven wheels.

Which introduces the 'Drive Train', or the route through all the widgety bits that our force takes from the combustion chamber to the driven wheel, which I don't want to get bogged down in, except to mention the 'see-saw' principle, or 'Leverage'

Forces & Torques

On their own, forces aren't all that hard to wrap your head round. Apply a force and it will try and move something, apply a resisting force and it will try and stop it. Whether the 'thing' moves depends on which is the bigger force.

Basically it's what's going on in a tug of war. Two teams pulling on opposite ends of a rope. The team that applies most force pulls the other team towards a line.

Taking forces and turning them into torques, is the 'see-saw' effect.

Two children on two ends of a see-saw; If they are both the same weight, then the beam 'balances' in the middle. If one is heavier than the other, then the heavier one goes down, the lighter one goes up. BUT, if you make the heavier one shuffle up the beam, so they are sitting closer to the pivot in the middle, you can get the beam to balance as though they were the same weight!

Because 'Torque', is a force multiplied by an amount of leverage

What's happening on the see saw is that the weight of each child is putting a force on the beam, which multiplied by their distance from the pivot, gives a 'torque' about the pivot. Heavy child with a little leverage gives the same torque as a lighter child with more leverage.

And basically, what that gives us is 'force multiplication', which applied to our engine and wheel, means that while we get our force from the pressure of the burning gasses in the cylinder, what we get at the driven wheels is that force multiplied or divided by whatever leverage the drive train applies to it.

Important fact for you; levers are force multipliers, gears are 'rotary levers'.

But there is a rule which says you don't get owt for nowt, and while you can use leverage to multiply forces, the trade off is, that to multiply a little force into a big one, you have to move the little force as many times further than the big one is bigger!


OK, well time to get busy. The important thing is force. What makes our vehicle move is the thrust force at the driven wheels, and we get that from the pressure force made by the gasses burning in our engine. Which, by using levers and gears and stuff, we can multiply to get pretty much as much force as we like at the driven wheels.

But, the thing is, while forces are the 'Important' bit, the things actually doing the shoving to make our vehicle move, once things are moving, we are looking at a much more complicated situation involving time, distance and speed, as well as forces, and this can be an awful lot more difficult to get to grips with.

In fact, Newton had to develop a whole new branch of mathematics, calculus, to deal with it all, which gave me a five year migraine to wrap my head around! But you can relax I'm not going to get into all that! What I AM going to get into is Newton's theory of the Conservation of Energy.

This is nice and simple, it says "Energy can neither be Created nor Destroyed, merely converted in form"

And this works brilliantly until you start doing nuclear physics and splitting the atom, when you have to wrap your head around the idea that all matter is energy..... but there aren't many nuclear cars around yet, so we don't have to worry about that!

OK, well there are many types of energy, but as far as we are concerned, the ones we are most worried about are:-

Calorific Energy - Which is the energy stored in a fuel. For our vehicle, this is the source of all the energy we have to mess around with, and we get it out of our fuel by burning the stuff, which leads me to the next kind of energy to bother us...

Heat - Pretty simple, burn a fuel and it gives off heat, which inside our engine makes the gasses in the engine expand and push a piston down a cylinder, giving us.........

Work - which is mechanical energy, things being made to move by forces, and I'll come back to this one in more detail in a second, because, having got our fuel to burn, release some heat, and do some work, if it cannot be destroyed, it HAS to go somewhere, and where it goes is important. That work, makes our vehicle move, and the energy associated with movement is......

Kinetic Energy - the energy a 'body' possesses by virtue of moving!

Kinetic Energy

Kinetic Energy = Mass x Velocity 2,

Now, I've put that equation in big bold words right away, but don't worry, I'm not going to start number crunching! We are going to 'Drive' that scrappy landy from before, and see what happens.

So, to begin with, our Landy is sat there, on the drive way, engine idling, body shaking, not going anywhere.

At this point, it has NO 'velocity', because it isn't moving, so it has NO Kinetic Energy, or kE, at least as a 'vehicle'.

INSIDE our Landy, the engine is running, so we know that there are pistons bobbing up and down, con-rods waggling, pushrods shuffling, rockers rocking, valves poppeting, and all manner of other widgety stuff doing whatever it does!

Individually, ALL those moving parts will have their own kE, and we know that the engine is burning fuel to make them move, but so far none of that energy is getting anywhere useful.

This is the start of the question of 'efficiency', which I don't REALLY want to get bogged down with in this article, but will deal with in brief later. For now, we are going to ignore all that, and JUST look at the vehicle as a 'body'. So, at rest engine running, all systems go, but no 'drive' the vehicle has NO kE.

OK, put the thing into first gear, open the throttle a bit, depress the clutch,  and get the thing rolling.

First of all, opening the throttle has bunged some more fuel into the engine. That fuel gets burned, releasing the calorific energy as heat, which our engine turns into work, the force in the cylinder being channelled through all the widgety stuff to the driven wheels to give us our thrust.

That thrust makes our vehicle accelerate, which means to acquire velocity, right? Our vehicle has mass, so that energy ends up getting turned into Kinetic Energy, as our vehicle goes faster.

OK, back in the beginning, I said, 'if you had a constant 'thrust' force, you would start by accelerating strongly, but as you gained speed, so you would gain drag, and your rate of acceleration would slowly bleed away until you reached a speed where the thrust force exactly matched the drag force and you had nothing left for acceleration.'

I also said, 'if we don't give the Landy in the piccy any thrust, it wont move. Apply some thrust, and it will accelerate. Take away the thrust, and it will 'Coast' until we apply some braking (reverse thrust!) then it will slow down.'

Alright, we have got our Landy up to some kind of 'top speed', where the thrust we have is exactly balanced by the drag, and we aren't going to go any faster.

At this point, the Kinetic Energy of the vehicle has maxed out. Remember, Kinetic Energy = Mass x Velocity 2,. The speed isn't changing, and the the vehicle's mass isn't going to change (or at least not by anything significant! It will get SLIGHTLY lighter as the fuel in the tank is burned and ejected through the exhaust pipe, but a few grams on a few tons is neither here nor there!) so the kE cannot change.

BUT, we have the throttle wide open and know we are burning a heck of a lot more fuel than when we were sat on the drive, and if we take our foot off the accelerator we slow down.... Energy cannot be created or destroyed, but we know we are bunging in a lot of fuel, hence energy, and that is NOT getting turned into Kinetic Energy.

AND, if we do take our foot off the accelerator, we don't 'coast' we slow down, which means that the Keneitc energy must be being lost, and it sure as heck isn't making it's way back through the engine and being put back in the fuel tank! What is happening?

The simple answer is 'work'


Work = Force x Distance Moved

Once moving at constant speed, our Landy is still subjected to forces; thrust, trying to push it forwards, and 'drag' trying to slow it down. Its no longer accelerating, because the two forces have balanced out, but they are still there, and the vehicle is moving.

Work, is Force times Distance Moved, so as soon as we have our Landy moving, and subject to drag, we are doing work.

When I described what was happening when we drove our scrappy landy, I said: "opening the throttle has bunged some more fuel into the engine. That fuel gets burned, releasing the calorific energy as heat, which our engine turns into work......" Pause for thought on that a second...then, "the force in the cylinder being channelled through all the widgety stuff to the driven wheels to give us our thrust."

So, the energy from our fuel gets turned into work, which arrives at the driven wheel as 'Thrust' to push our vehicle along, and there it does TWO things;

First it is used to overcome drag. So, from whatever thrust force we have, we have to take away whatever drag there is at that speed before we have any thrust left over to make our vehicle accelerate, which is the second thing that the thrust does.

Basically, by the Conservation of Energy; Calorific Energy IN = Work OUT - Kinetic Energy 'Stored'

Which is a little leap, and I say 'Kenetic Energy 'Stored' for good reason; back to driving our scrappy landy, get the thing up to top speed, thrust balances drag, and we have no acceleration. All the energy from the fuel is being used to overcome drag. Only while we were accelerating did any get transferred into Kinetic Energy.

Now, take your foot off the throttle, and you slow down, which means that the Kenetic Energy must be reducing, but it ALSO means that the drag force is now BIGGER than the thrust force......... Remember, forces make things accelerate, and slowing down is backwards acceleration.

BUT, our landy is still moving, AND it is still subject to drag, so the drag force times the distance moved is work, which if it ISN'T being taken from the thrust force, MUST be coming from the 'stored' Kinetic energy! Make sense?

Basically, the energy coming from the engine can do two things, do work over coming drag making the vehicle move, or additionally, make the vehicle accelerate, increasing it's kE.

When you decelerate, the energy keeping you moving comes from the loss in kE NOT from the energy coming from the engine, so the kE doesn't directly get turned back into Calorific Energy, but it does SAVE you having to take more calorific energy from the tank to keep the vehicle moving......

UNLESS you use your brakes to slow you down! Brakes are devices that apply an inordinate amount of artificial drag, and can deplete your stored Kinetic Energy in seconds during an emergency stop, usually converting all that energy into heat, via friction, which they usually simply 'dump' into the air around them!

Coffee Break Discussion!

Go put the kettle on, and have a think about what I've said so far. IF you have grasped most of it, you are about ten leaps ahead of most bar-room engine builders, and well on the way to an A-Level!

So far I have talked about forces, specifically Thrust, which I say is pretty important; Drag, which you have heard of, and are clamouring to ask all about aerodynamics and streamlining, and Traction, which you are probably also aware of and having had a quick peek at  Get A Grip. more than likely decided its a good idea to ignore for the moment!

What I HAVEN'T mentioned in any way shape or form, is what the article is supposed to all be about, which is POWER!

And for good reason. Power is an abstract commodity, and I'll explain what it is, why its a bit weird, why people talk about it the way they do, and how it REALLY works in a moment. The thing is, that the ideas about ENERGY are far, far more revealing, and explain an awful lot more than does power.

Basically, what makes things move is forces, and what is going on when things move is energy transfer.

We start with a force in our engine, through various widgety bits we get that force delivered to the driving wheels as Thrust; that gets things shifting, and how much shifts, and how fast all depends on how much energy is being moved around.

So; you know that you get thrust from your engine and that's what allows you to over come drag, doing work, and accelerate,  accumulating Kinetic energy. You also know, though I haven't mentioned it, the more power you have, the harder you can accelerate, and the faster you can go, but also the lighter your vehicle and or the more aerodynamic it is, so you can also accelerate harder or go faster. Then there is that nugget about power and economy..... This is the coffee break, so OK, lets have a LITTLE look at some of those ideas;

OK, well Thrust and Power ARE related, but not directly. I said earlier; "levers are force multipliers, gears are 'rotary levers'." so you can take any force, apply some leverage and get any other force.... in theory.

Archimedes put it as "Give me a lever long enough and I can move the world!" Which is true.... in theory, but in practice.. Not only would you need a lever long enough, but also one stiff enough.... other wise the force you apply to the lever aint going to move the world, just bend the lever! This is where Engineering departs from Science! Engineers like wot I iz, have to make science work in the real world, and what the lab-rats often come up with only works in their wonderfully idealised mathematical models, but anyway!

About the most modestly powered vehicle in existence is a moped. By European legislation, a moped is defined as a powered two wheel vehicle with an engine displacement not exceeding 50cc, a power rating not exceeding 3.5bhp, and restricted to a top speed of no more than 35mph or 55(ish)Km/h. And these specs were considerately framed around what engineers could reasonably achieve!

Now, applying a little bit of the science, you should be able to make a three and a half horse power moped do a hundred and fifty miles an hour IF you can set the gearing high enough.......... or so many teen-agers believe!

Only trouble is, at 150mph you'll have an awful lot of drag to contend with, remember, Drag increases exponentially with speed, so THAT is your first problem.

Gear the moped up so that it will turn a wheel fast enough to push 150miles of road beneath its tyre in one hour, and you have a VERY short lever, which means a pitiful amount of thrust.... so little that in reality you'd probably not be able to over come the drag at walking pace, let alone super-car velocities!

Ah! Well, IF we use a REALLY light rider, it'll go faster wont it? so all we need to do is make the thing as light as we possibly can, THEN it will be able to pull the really tall gearing, right?

Err.... sort of! Reason that lighter riders tend to go a bit faster on mopeds isn't REALLY anything to do with their weight. Weight comes into things ONLY where we are bothered about acceleration......

Yeah! well we have to 'Accelerate' to 150mph, don't we, and you said Kinetic Energy is Mass times Velocity Squared, SO if we have a lighter bike and rider, then the speed you can get HAS to be higher, dunnit!

Oh dear, a little knowledge is a DANGEROUS thing! That sort of logic is the cause of an AWFUL lot of errors in perceived 'wisdom' and lore!

This is an immutable FACT. Top speed is achieved when the maximum THRUST you can deliver to the driven wheels matches the DRAG at that speed!

Kinetic energy ONLY comes into it in as far as that is where the EXTRA energy NOT over coming drag, used to get the thing UP to that speed goes.... until you slow down again.....

AH! Yes, but what IF... we got the thing to top speed, then threw away some ballast, dumped some weight..... then we'd have the same Kinetic Energy, and less weight and we'd be able to accelerate to a higher top speed..........

NO! It doesn't work like that, or at least not for 'normal' vehicles. Take a moped, weighing something like 150lb, and a rider weighing another 150lb, and you have a combined weight of 300lb. Lets say that 75lb of the bike is 'ballast', and when you have reached your top speed (about 35mph!!!!) if you drop your ballast, it will KEEP its share of the Kinetic Energy that has been acquired, and it's momentum will keep it moving along side you until drag slows it down! In all probability, its actually likely to over-take you, because it'll be subject to less drag!

AH!, but what if we threw it backwards, or fired it off the pillion seat with a catapult? (You can tell I've had teen-age sons with mopeds, can't you!)

OK, well two things. First, it is STILL going to keep the kE it's acquired. Throwing it backwards is going to require MORE energy, either from your arm, or your catapult and it is THAT which MAY make you go a LITTLE bit faster!

And when you start chucking stuff out of the back of a moving vehicle, you are either an escaping Ram-Raider on an American 'Ride with the Cops' TV show, or getting into rocket science, and I don't want to go there right now!

Main reason that a smaller rider MIGHT go a bit faster, is the same reason that ducking down over the handlebars will; you have a smaller frontal area, so will have a bit less area for wind resistance to work on!

Yeah! Well THAT'S the other thing, IF we make a tear-drop streamline fairing, we can get the drag RIGHT down, and a tear-drop is a 'perfect' streamline, so we shouldn't get ANY drag, and should be able to go as fast as we want!

Now you are confusing Stream-lining with wind resistance and assuming that ALL of your drag comes from wind resistance!

First of all, 'stream-lining' is the idea that the 'slipperiness' of a shape effects how much resistance it will present to the passage of air around it. YES the more stream-lined a shape is, the less resistance it will offer....... note LESS not NONE!

The measure of how slippery a shape is is denoted by what is known as its cD or Co-efficient of Drag, and it is a 'fudge factor'. You take a perfectly un-streamlines shape, like a brick, and measure how much resistance that gives you, and assign a cD index of 100% to it. Any shape that betters the brick gets a minus value cD, anything worse, a plus value cD, which you can use in sums to account for any gain or loss in wind resistance that shape gives in the real world!

Streamlining works, but shapes ALWAYS have a frontal area, and that gives them a nominal wind resistance no matter how good their stream-lining. The frontal area of your moped, is about the same as that of any motorcycle, its pretty much fixed by the size of the person sitting astride it! Say 5'10" tall, and 22" wide.

So, if you put a stream line bubble around that rider, then the frontal area would at best, stay the same, and you could only hope to reduce the drag by whatever the improvement in your cD was. Lets say you get as good as any-one has, and get the wind resistance down by 30%. That is ONLY as much as if you got the rider to simply duck over the handlebars, reducing his frontal area from 5'10 x 22" to 4'1" x 22".

Fact; it takes about 50bhp to make a motorcycle sized shape go 100mph. On a 100mph motorcycle, lying over the tank, rather than sitting up 'prone', is worth ABOUT 4-5mph!

This moped is starting out with a top speed of 35mph, which is roughly 1/3 of 100mph. By that 'exponential' business to drag, though, the wind resistance at 100mph will be a LOT more than three times what you get at 35. So, reducing your wind resistance at 35mph will have a lot less significance to your speed, than the same reduction at 100.... but EVEN if we grant you a FULL 5mph increase in speed from a 30% reduction in wind resistance, the thrust your moped makes is STILL only going to get you to around 40mph!

OK, well lets take it a step further, and not JUST get your rider to lie over the tank, or add a stream-line bubble over him, lets go the whole hog and build a full on 'stream-liner' record breaker. Get your rider lying down, presenting a frontal area the width of their shoulders, about 22" and just tall enough for them to rest their head up enough to see where they are going, perhaps 22".

That would give you a frontal area of around 400" or about half that of your rider lying on the tank of a conventional machine, therefore subject to half the drag, but fully streamlined and as good as can be got, giving you a further 30% reduction in wind resistance.... or at least 30% less resistance than an un-streamlined shape of the same frontal area.

Now, this has been done many many times, and with the 50bhpish engine from a 100mph conventional motorcycle, such stream-liners have managed to get up to around, 180mph! Which is pretty incredible. But that is with a 50bhp engine, your moped has less than a tenth of that! You are NOT going to get it to better 150mph, and manage to get it to carry a pilot!

I think that the world Land Speed Record for a sub 50cc motorcycle, with full streamlining, has in recent years breached the 100mph threshold, just. Google it if you want to find out exactly what it stands at! But the thing is, that was NOT set with a MOPED engine, with no more than a 3.5bhp rating. It would have been a super-tuned 50cc engine, probably running on an exotic fuel, offering something closer to 30bhp, than to 3!

Yeah, well my mate took the head gasket out and fitted an extra base gasket and he reckoned his 50 would do 70!.......

Yes, and the fisherman I met while walking the dog reckoned he'd pulled Moby-Dick out of the cut! Geez! How many times have I heard those teenage fantasies!

When I was sixteen I did actually build a 'fast fifty', I did it just for fun, because I got a Yamaha DT50M for a fiver in a deal over an RD250, and thought it would be intriguing to see JUST how fast I could make it go, and my Granddad, Pops was amused by the notion too, and hand a big hand in it.

As you can imagine, a £5 moped, tuned and thrashed by a succession of probably eight or nine sixteen-year olds, is going to be a complete wreck, and that would have been being kind to mine! We started by stripping it down to it's component parts. It all needed a lot of attention, but the engine came in for rather a lot.

Ignoring Chip-Shop Racer wisdom, we started by building a 'tough bottom end', the first rule of competition engine building! You can find as much power as you like from valves and ports and stuff, but if the rest of the engine cant take it, it will break and you wont be going ANY speed!

The barrel and piston were completely shot though, so new ones were fitted, and initially left standard. With the fettled bottom end though, it was good for almost 40mph, so some 'tuning' was called for. I think I managed to just about find the power to reach a genuine 50mph, before I had knackered the 'new' barrel and piston! So a 'New' set was obtained, this time a 'big-bore' kit giving 65cc! Which was good enough to get me to around 55mph! A bit of messing with that though saw 60mph and the magic 70 the new target.

Didn't get there though! It would manage around 65mph, but by that point we'd tuned the nuts off the thing and were having a tough time holding it all together, one of the biggest problems being a propensity for it to blow its crank case seals! At which point the 'joke' was beginning to wear a bit thin.

So, err, yes! You CAN get a 'moped' to haul itself to 70ish mph, BUT, it's not as easy as knocking a washer out of the exhaust and messing with gaskets! And even when built to competition standards, a 70mph moped is a fragile beast that wont hang together for very long!

Which is all a long way advanced of where I was at, but does prove the point that the bottom line is if you want speed, you need power!


Power is Rate of Energy Transfer

OK, there it is. The answer! You now know what power is! Not very helpful though, is it! BUT, you can see why I spent so long waffling on about Kinetic energy and Work. You cant? OK, well lets explain, and do some messing around with that statement, and put it another way:-

Power = Force x Speed

If you want to know how I got to that, well, 'work' is energy, right? And Work is Force x Distance moved. So if we are transferring energy in the form of work, the 'rate' of energy transfer, which is what power is, is going to be the amount of force times the distance moved, divided by the time taken to move it... and if you divide time by distance, you have 'speed', so we can use that instead of time and distance and make the sums a whole lot simpler!..... yeah, I'll just get on with it shall I?

OK, well lets look at our scrappy Landy again for a second.

We have a few forces knocking around, and have mentioned that those forces will make it accelerate and acquire speed.

Leaping ahead to our 'top speed', we said that we reached top speed, when the thrust force balanced the drag force.

So, if we multiply our top speed by the drag at that speed, we get how much power is needed to achieve that speed. Simple enough?

So, if you have a vehicle, then it will go as fast as it has power to overcome drag.

If you want to make it go faster, then you are going to need more power, plain and simple...... Or you could try and reduce drag, which we looked at in the coffee break!

BUT, by the same token, if you DON'T want to go that fast, you DON'T need as much power. Pause for thought on that one a second, because it is such a simple bit of logic it is often over-looked!

You have an engine, and it has a rating of say 100bhp. That 'Rating' is the most power that engine can achieve, it is NOT how much power you get out of it ALL the time, in fact far from it.

Nearly all engines are described by their 'maximum power' rating, which nearly always only occurs only at a particular engine speed and under full throttle conditions. Over the rest of the engine's range of operating speeds, it wont be able to provide as much power, and of what it can offer at any engine speed, it will only offer that at full throttle.

And strictly you don't GET power out of an engine, you get 'WORK', Power is the SPEED you are getting that work out of the engine, NOT the work itself!

Which is why POWER is an abstract commodity. It's not a 'real' substance, its a rate, like the interest on your savings account or in my case, overdraft! The adverts say you 'GET' 3% or whatever, but have you ever SEEN a 3%? No. You cant have. Its a physical impossibility, its a rate, an abstract commodity, a mathematical convenience, a ratio.

As far as my overdraft is concerned, I'm charged 1% of my negative balance each month. When I come to pay them those charges, I don't hand over any tangible 'thing' called a 'percent', I hand over pound coins, MONEY!

With engine's it is the same. The engine is rated at so many horse-power, but what it chucks out is energy, work; higher the horse-power 'rating', the more work we can expect to get done in a set time......

BUT, work is force x distance, so an engine of a certain rating can deliver that work, either as a LOT of force, moved a short distance, or a little force moved a lot........

And starting at the back, you can see why this might be important. Going back to our scrappy Landy, We have a Drag force and A Thrust Force, and our Landy has a certain Speed.

Remember, its all about Energy Transfer, and right at the beginning we said that Energy cant be created or destroyed, so energy in, must equal energy out, Power is rate of energy transfer, so Power In must equal Power Out.

So if we know what the drag force is going to be at a certain speed, we can work out how much power we need to get the thrust to achieve that speed.

Shaft Power

Power = Torque x Revs

Well, this is a leap of the imagination, but basically if Power = Force x Speed is the 'Power 'Out', then we probably want to look at the Power 'In'. and it's coming from the engine, as 'Shaft Power', which is a little more complicated, because it's a revolving situation.

Again, Power = Torque x Revs is essentially the same sum as Power = Force x Speed, only torque is force twisted around a pivot, as I mentioned when I described the see-saw principle, a force times a length of leverage, and revs is the speed of that shaft.

Now, This equation, in the wrong hands is what gives rise to a LOT of lore being spouted on the topic of torque, and even more about engine architecture, so I'm going to detract for a second and apply the shaft Power equation DIRECTLY to a real engine!

Power = Cylinder Capacity x Cylinder Pressure x Crank Revs


And time for another Coffee Break!.... Go put the kettle on and come back with some brain lubrication!

Torque Talk!

Reason I've made a break here is that having introduced that equation, I can here the cogs of your mind grinding, and mulling over all the lore you have picked up about bore and stroke dimensions, and the perfectly sounding arguments that have been used to explain why some people suggest that torque is more important than power, and why long stroke engines are good for torque, short stroke engines good for revs.

Torque is a force times a length of leverage, right, and if you look at the graphic of our engine, we have a cylinder, full of pressure, acting on a piston, pushing on a crank-shaft, which is in effect a lever.

The 'Torque' on our crank-shaft, will then be the pressure in the cylinder, times the leverage of the crank-shaft, which is technically called it's 'throw' or 'offset'. Basically the distance between the centre of the big end bearing and the centre of the main bearing, and that is half of the quoted 'stroke' dimension.

OK, so for a given force, we can get a higher 'torque' if we have a longer crank shaft 'stroke' applying more leverage, right?

Now, if you take that nugget of an idea and run with it on its own, you can suggest that crank leverage, is what gives the 'force' you get at the driven wheel........ Hence long stroke engines give you more 'Thrust', or as that term isn't often recognised, simply 'better drive' or the 'sort' of 'power' you need to make acceleration and speed....

Which SORT of supports what I started talking about when I said that the important thing about power was thrust, except that it so HUGELY over-simplifies matters as to make a nonsense of them!

Remember all the widgety bits between the engine and the road, including the wheel itself, that can apply some leverage effect and completely alter the amount of force you get.... most notable amongst them being the gear-box!

Keeping it pretty simple for now, we have a charge in our cylinder and we are going to set fire to it to give us some pressure that is going to shove our piston down the bore and turn the crank shaft.

The bigger the cylinder, the more charge we are going to be able to suck in, so the more fuel that will be burned and the more heat generated. Turning the calorific energy of the fuel into heat is an energy transfer, so in a set time, the more calories we convert to heat, so the more power is available.

Next, cylinder pressure. The actual pressure generated when you set fire to the charge varies throughout the combustion stroke. When the piston is at the top of the cylinder, the volume of gasses is very small, so any expansion you get is going to be that much more magnified than when the piston is down the bottom, and the volume of the chamber that much bigger, however, for simplicities sake, we average out the pressure over the entire stroke, when we come to the power calculations.

But, you get a higher average combustion pressure from having more charge in the cylinder, from a higher initial compression ratio, and from a more complete burn, amongst other things. (like if you open the exhaust valve very early, and let some of the pressure out before its finished pushing the piston down!)

Lastly, the faster you turn the crank-shaft, the more engine cycles you are going to complete in a given time, so the more individual charges you are going to burn, and the faster you are going to convert the calorific energy in the fuel into work.

So you can see the sense in the sum, BUT, lets look at this bore/stroke ratio business.

The capacity of an engine,  it's 'swept volume' or 'displacement' depending on who you ask, is basically the volume of a cylinder; not the volume of the engine's cylinder, but that bit of it that the piston goes up and down.

The volume of a cylinder is the cross sectional area of the circle, (Pi times Diameter Squared, all divided by four), multiplied by the length of the cylinder.

Apply that to an engine, and the cross sectional area of the swept volume is the diameter of the cylinder, or the 'bore' dimension. The length of the swept volume is the distance the piston moves up and down the bore, the stroke. So

Cylinder Displacement = 1/4 x Pi x Bore2 x Stroke

So, if you mess about with that, you can see that you can get the same cylinder displacement either by having a big bore and short stroke, or by having a long stroke and small bore.

Just out of interest, an engine which as a bore that is the same dimension as it's stroke, is known as a 'square' engine. One with a stroke that is longer than the bore, is known as 'over-square', and one that has a stroke shorter than the bore, 'under-square'. Pretty simple, hugh?

Right, lets presume we have two engines, both the same cylinder capacity, but one over-square and one under-square. Lets also presume that they have the same compression ratio, and that they have combustion chambers, valves and ports that mean that they generate the same cylinder pressure during combustion, and for the purposes of this example, they are both turning the same crank speed. The only difference is the bore/stroke ratio.

Now pressure is force per unit area, and our cylinder pressure is acting on a piston of a certain area. In the case of the over-square engine, its a bigger piston, so we get a bigger force. In the under-square engine it's a smaller piston, so we get a smaller force.

But, when we come to turn that force into a 'torque', in our over-square engine, we have a shorter stroke, so our big force doesn't get multiplied by as much leverage. In our under-square engine though, our smaller force has the benefit of a longer stroke and more leverage, BUT, it doesn't matter, because if the cylinder pressure is the same in both cases, and the swept volume is the same in both cases, by a dint of maths, we find that we ALWAYS get the same torque!

So basically it SHOULD make naff all odds, whether an engine is over-square or under-square, as to how much torque you get at the crank-shaft, what counts is how much pressure you have in the cylinder! The simple fact is, the more pressure you get, the more torque you'll get, whatever the bore/stroke ratio may be!

Torquing Topography!

I don't really want to start delving much further into the topic here, because it's covered in more detail in the articles; Pots, Pans & Cams!, which gets into detail about how engines are arranged by way of the cylinders and valves and stuff, and what all the bits and pieces do, while The Chemistry of Combustion  gets into the business of turning the calorific energy in fuel into heat and thence pressure, while Tuning & Super Tuning  pulls it together again in looking at how that all effects the power and power delivery of an engine.

But there are a few points about the topography or architecture of an engine that are worth noting. If you have an under-square engine, for a given capacity, the piston has to travel further up and down the cylinder.

Now, the crank-shaft turns pretty much at constant speed, as far as we are concerned; but the piston goes up the bore, stops, then comes back down again, stops, and goes back up again. Two strokes every crank revolution.

With a longer stroke, this means two things, first for a given crank-speed, the piston has to have a higher average speed, and if it is going from a dead stop at the top and bottom of each stroke, that means it must be subject to a much higher acceleration between each stop. But not only the piston, but also the con-rod, which is accelerating not just up and down at the piston end, but up and down AND side to side at the crank-pin end.

Force = Mass x Acceleration, so the piston, and the con-rod are going to be subjected to much higher forces from having to do more harsh acceleration in a long-stroke engine, than in a short-stroke one of the same capacity.

Now, pistons tend to be made of aluminium, while con-rods, for the strength needed in them tend to be made of steel, which is a lot heavier. The smaller diameter piston of a long stroke engine tends not to be that much lighter than the larger aluminium piston of a short stroke engine, but the longer con-rod needed on a long stroke engine will tend to be a lot heavier, both because it is bigger, but also because it needs to be that much stronger to be able to withstand the added stresses placed on it.

So, short stroke engines have the advantages of seeing lower loadings on the critical bits of the engine; the piston, the con-rod and the crank-shaft. Which means that you can potentially get your engine to turn to higher rpm, and hence make more power from that higher engine speed, before bits start to break!

But there is another advantage; if you have a big-bore, short stroke engine, then the area you have over the piston is that much bigger, and that means that you have more room to fit more or bigger valves to let the charge in and the exhaust gas out more easily. That would promote better cylinder filling, which would aid power at any engine speeds, but particularly at higher ones.

Which suggests that short-stroke, big-bore engines have everything going for them; and certainly, that is the common perception, supported by trends in modern engine design that have seen strokes get shorter and cylinder heads gain more valves.......

But it isn't that clear cut, and the reasons for going to shorter stroke dimensions haven't always been to make more power from higher revs, a lot of more modern engines use shorter stroke engines but don't rev any higher than their longer stroke predecessors. In fact, in some cases, more modern, double overhead cam, 16 valve, fuel injected 'short stroke' engines, don't rev as highly, or make as much power than their longer stroke, 8 valve, push rod predecessors!

And, long stroke engines do have some advantages of their own. Provided you don't rev one to the point where bits start to snap, the loadings on the components of the engine can be quite acceptable.

Having a smaller bore, may mean having a smaller area in which you can fit valve area, but, because of the higher piston speeds, you tend to get more 'suck' through the valves, which goes some way to compensate, particularly at lower engine speeds, or at part throttle.

And, because piston gets out of the way quicker, you can to some degree, open the valves further or even a bit earlier, to compensate. David Vizzard comments in tuning BL's A-Series on how the 'long stroke' 1100 engine can respond well to using much wilder cam profiles than the shorter stroke engines, while Ford did likewise in the 'hot' versions of the 'long-stroke' 1600 cross-flow and CVH engines that often gave better power than the similar capacity, 'short stroke' Pinto or Zetec engines.

It is very much a case of swings and roundabouts, and basically when you start delving into it, there are so many variables in an engine design that you just cant make too many generalisations, and over-simplified statements like; 'short strokes and big valves make for high revs and no torque'; can be completely denied by far too many examples, as can such comment that; long-stroke engines make more 'useable' power.

Which is what I'm going to deal with now!

'Useable' Power





Right, to start off with, I'd better explain how you get a Dyno-Trace.

Basically you strap your engine to a machine called a 'Dynometer'; that machine applies a 'load' mimicking acceleration and or drag on a real vehicle, and measures the energy it has to dissipate, which gives you a rate of energy transfer, and hence the 'power' being offered.

The 'old fashioned' kind of Dynometer, which is still used most often in industry is the 'Engine Brake', from which we get the expression 'Brake Horse Power'; ie, Horse Power measured against a 'brake'! And put simply, that is exactly what one is, its a big brake, just like on your car's wheel, though usually a lot bigger, and these days usually a big electrical dynamo, rather than a friction brake!

Either way, you get your engine and attach the crank shaft to the brake, then 'run it up' and apply a braking force. React that braking force with a long lever and weights, and you have the 'torque' that the brake is applying, and if you have applied just enough braking force to hold the engine at a constant speed, you have a force balance, and the torque you are applying to the brake must be the same as that being delivered by the engine.

Power = Torque x Revs, remember, so record the engine revs (or brake revs, if it is easier!) record the brake-torque (or more usually the weights put on the beam, in which case you have to do a bit more maths to multiply that weight by the length of beam to get the torque) AND with some number crunching you get the power output!

Do that for a range of engine speeds, running the engine against the brake at full throttle to get as much power from it as you can at every speed, plot the results on a chart, join the dots and you have a power curve, like the one shown. Simple, innit!

Now, looking at this trace, you can see that the engine makes power from a tick-over engine speed of 750rpm, builds to a peak at 5000rpm, then trails off to a maximum engine speed of 6000rpm, where the thing either runs out of breath, or hits an imposed rev-limiter.

The MAXIMUM power it might provide is 100bhp, at it's peak of 5000rpm, and that is what would get quoted as it's power rating in the magazines or sales brochures, which on it's own is pretty meaningless, except that within reason, if we are comparing magazine articles or sales literature, we can PRESUME that the amount of 'USEABLE' power the engine can offer is pretty closely related to the MAX power Rating, and if we have an engine rated at 100bhp, an engine rated at 120bhp will PROBABLY offer more 'USEABLE' power as well as more 'MAX' power. Not necessarily, but it gives us a rough idea,

Now, this Dyno-Trace, is titled as being that of a "Screamer" type engine. That is one that makes its power by virtue of winding a lot of engine rpm's rather than necessarily making much pressure in its cylinders. Motorbike engines tend to have Dyno-Traces more akin to this type than car engines, and that is what this power chart was based on.

Take note, the 'Max' power rating is again, just about 100bhp (Well, it's a couple of ponies more if you look closely, but close enough!), this time offered at the heady engine speed of 12,250rpm!

Tick-over on this engine looks like about 1000rpm, but the power its offering down there is really scraping the barrel, it's hardly on the scale, is it? and it doesn't get much better going up the rev-range, until you get to nearly 7000rpm, when it suddenly starts to do something and JUST about gets over 20bhp.But then it kind of gets serious, and starts offering everything id didn't earlier on.

In comparison, the curve for the typical car engine, was offering 20bhp earlier than 1500rpm, and it ramped up nice and progressively to its peak.
















Another benefit of a small bore, short stroke engine, is on the Compression Ratio. The Compression Ratio is the ratio of combustion chamber volume compared to the swept volume of the cylinder. If you have a 500cc cylinder, and a 50cc combustion chamber, then you have a 10:1 compression ratio.

Your cylinder only sucks in a full charge, though under full throttle conditions, so you only compress 500cc of charge into your 50cc combustion chamber, at full throttle. The rest of the time, under 'part throttle conditions', which is during most driving situations, you are only sucking in fraction of the full 500cc of charge, so more often, you are compressing probably less than half the possible charge into a 50cc combustion chamber.

Now the compression ratio sets the 'base' pressure for combustion to increase on the power stroke. Higher the compression ratio, so the more pressure there is for combustion to multiply, so the more cylinder pressure, and hence power you should get, and generally, the higher the compression ratio, the better.

Only trouble is, the higher the compression ratio, the more likely you are to get a thing called 'pre-ignition', or 'knock'. The RON rating for fuel is actually an index of the fuel's resistance to pre-ignition, and generally, the higher the compression ratio so the higher RON fuel you need to use.

Now Diesel engines, actually rely on 'knock' as their ignition system! Basically, they have a very high Compression Ratio, and squashing the charge that much makes it get rather hot, and with a bit of help from a copper 'hot-spot' in the combustion chamber, the charge will ignite itself purely by reaching its 'flash-point'. And exactly the same thing can happen in a petrol engine, if you have the wrong fuel, the mixture is too weak, or a valve or the spark-plug gets too hot.

All right, well, the higher the Compression Ratio, the more likely you are to get 'Knock', but IF you don't expect to fill the cylinder completely all that often, what's the risk of getting knock when you do, IF you were to bump the compression ratio up a bit beyond what's 'safe' when your squashing a full charge?

I mean If 10:1 CR gives you the best pressure you can safely get without knock, then on a 500cc cylinder, you need a 50cc combustion chamber, but when you are using part throttle, squashing just 250cc or less of charge into the combustion chamber, you'll only be getting a 5:1 compression ratio, and that ISN'T going to give you as much cylinder pressure as you could have, especially as at part throttle, ytou are only burning halve the volume of charge....

So, IF you dropped the combustion chamber volume to 40cc, you'd have a 'theoretical' Compression Ratio of just over 12:1, and now, when you are working at part throttle, squashing just 250cc of charge into it, will get an 'effective' Compression Ratio of 6:1, which should give you a pretty healthy gain in combustion pressure, without very much risk of knock.

OK, at full throttle, you'll be a lot closer to encountering knock, but when will that happen? Mainly under acceleration, and for that short duration, the carburetion will enrich up the fuel/air mixture anyway, and that has a knock suppressing effect, and if you need a bit more, you COULD make the mixture a bit richer still......

Which is what engine makers actually do do, catching out an awful lot of amateur engine tuners who have presumed they can increase the compression ratio further still, and THEN run the engine more often at full throttle............. usually leading to melted pistons! (





Now before you come up with any more wonderfully imaginative theories, let me explain the principle of a rocket!

You start with a paper tube full of gun-powder. It weighs something like 100g (its only a little one, like you get in the selection box on bonfire night!) Light blue touch paper and stand well back. Gunpowder burns, ejecting  a stream of hot gasses at incredibly high speed.

Those gasses have mass and velocity so they have Kinetic Energy. They are travelling at INCREDIBLY high speed, so they actually have an awful LOT of kE. This is the complicated bit, like the Landy slowing down, that kE has to go somewhere, and it goes into doing 'work'; as the gas rushes out the exhaust of the rocket, it pushes against the air around it, which pushes back, shoving the rocket into the air...

 (Incredibly simplified, but you get the idea!) NOW! As the rocket burns gun-powder, it gets lighter, so the there is less mass to shift and for the same rate of 'ejection' or 'Thrust' so the rocket will accelerate faster.

It DOESN'T get faster because the Kinetic Energy it HAS is being shared by less mass, it accelerates faster because the FORCE is staying the same whilst the mass is reducing!

Its a common mistake, transposing two of Newton's Equations; the first, Kinetic Energy = Mass x Velocity2, and the other Force = Mass x Acceleration. And I'll look at that in more detail later, because 'our kid' WAS on the right lines!

Most people believe that lighter cars and motorbikes accelerate harder because Force = Mass x Acceleration, they don't; the reason they accelerate harder, is if you have a certain amount of 'THRUST', and use that for acceleration, then you have energy transfer going on, and for a given energy transfer, IF the vehicle is lighter, then it has to translate the energy into more speed to keep the sums straight.

I have a Land-Rover Series III, which weights just about two tons, and has an engine rated at around 75bhp. I also have a Honda 750 'Four' motorbike that also has about 75bhp, but weighs only 400lb, with me on it, lets call it about 600lb. Converting to metric and for easy reckoning, that's 2000Kg for the Landy and 250Kg for the bike. Or the Landy weighs 8 times what the bike does.

With roughly the same power, they can put roughly the same thrust to the driven wheels, and since the bike is eight times lighter than the Landy, IF the governing equation is F=m x a, then it should accelerate eight times faster, right?

Well, the traditional way of measuring acceleration is by the time taken to reach 60mph from a standing start. Now, the Land Rover wont like it, but I CAN make it do 60mph, JUST..... and I can tell you it takes a lot LESS than eight times what the bike does!

Bike will hit 60 from a standing start in something under five seconds, so the Land Rover should take around 8 times that, or about 40 seconds . I've not measured it, but in practice its more like 25-30 depending on how quickly I can waggle all the levers to go through all the gears.

So, the Landy's acceleration, is still much greater than predicted IF the law Force is Mass times Acceleration was the controlling rule.

It IS involved, but NOT the governing factor. Going back to driving our scrappy Landy, we had a thrust at the driven wheel, SOME of that thrust was used for acceleration, which increased the vehicle's kE, the rest was used to overcome increasing drag.....

Now, the 'governing' equation, during acceleration is; Kinetic Energy = Mass x Velocity2 . But, the faster you accelerate, so the quicker you acquire drag, and AS you acquire drag, so the more of your thrust is 'robbed' to do the work of overcoming that drag, and the less you have available to let you accelerate.







Hopping it Up!

Before getting 'into it', though, I'd like to mention a few things, first of which is, if you haven't already, you may want to have a look at You can do ANYTHING to a Landy! (or any other vehicle for that matter!) Because if your interest in power is to try and find some more of the stuff, well, it's a good place to start!

Basically, when it comes to 'messing' with your vehicle, almost anything is possible, but there are usually a lot of ways to achieve an end, but starting at the right place is usually far more important than the route! That Irishman asked for directions who said 'Well, I wouldn't wanna be a'startin here now!" wasn't as daft as he sounded!

In 'you can do anything', I make comment to the effect that before you start trying to modify your vehicle, have a good long think about why you want to modify it, and what you hope to achieve, then before actually doing ANYTHING take a good long look around and see if there is anything out there that will do what you want it to without any messing.

An example I gave was a mate, a lot of years ago who had a Vauxhall Chevette and the idea of tuning it up and fiddling with the suspension, because they wanted something more 'sporty', and drooled over an HS Chevette rally car, they believed they couldn't afford. But, after a lot of head scratching and research, and taking into account all the 'consequences', it actually proved a heck of a lot cheaper, and a lot less hassle to chop in the boggo Chevette for the drool worthy one!

Top-Gear have done a couple of features on the topic too, the first trying to get 'supercar' performance for under a grand, stripping out an old XJS Jag and fitting a Nitrous Oxide kit to it with spectacular results! And more recently, and a little more sensibly, trying to get Subaru Imprezza performance from a Renault people mover!

The Jag feature was heavily tong in cheek, but there was some useful lessons in it. If you want an awful lot of performance for not a lot of money, there are cars out there that offer it, and things like old XJS Jags, Nissan SX's or Toyota Supras, can give you a heck of a lot more bang for your buck than trying to hop up an old Ford Escort or Vauxhall Astra!

The Renault people mover feature was a bit more considered; they took a six year old 2.5l V6 thingies, and tried allsorts of stuff to make it go round their track as quick as an Imprezza. They had some good ideas, and threw a lot of money at the thing, but still failed, while in a couple of their tests, actually managed to make the thing SLOWER than it was to begin with..... something I actually DIDN'T find surprising!

Quickly, the first thing they did was throw some big brakes at the thing. Which didn't do much on their own, but did demand bigger wheels to clear them, and some sticky low profile tyres, which should have, but didn't, making the car considerable slower! Next they added some very expensive suspension, which did do a lot, and got them a few seconds a lap back, but still made the thing only as good as it was to begin with!

Finally, after they had over-ruled Jezza's demands for more power, they gave in and put the thing on the rolling road, only to discover that the six year old engine was down 60bhp on the 200 the makers specs said it should have delivered! Concluding that the biggest single improvement they could have made was simply to give the dang thing a proper service and get ALL the performance the thing should have had as standard. They did just that and were rewarded by a 60bhp or 45% power gain over what they started with!

Take note! It is a bit of old advice I was given MANY MANY years ago, and provided in that great tomb of automotive wisdom, David Visard's Tuning BL's A-Series.......

Before looking for BETTER than standard, make sure you have as GOOD as standard!

But moving on.... IF your starting point is a 'cooking model' saloon, like an Escort, Astra, there are even more possibilities. When I was a 'lad', old rear wheel drive Mk1 & Mk2 escorts were not the 'classics' they are now, they were just 'old bangers', but old bangers beloved of the 'lads' like me, because they were HUGELY messable with, and had a very impressive Rally pedigree.

What every 'lad' wanted, of course, was an RS2000 or 1600 Mexico, which were the Rally 'stars', or at least the clubman kings; the cars that Ford had actually achieved their greatest glory with were early 'Homologation Specials' with Cosworth BDA engines in them! But I digress!

Back in the mid Eighties, you could pick up an old MK1 or MK2 'Skort for a couple of hundred quid with tax and test on it. They offered three engine sizes of RWD 'Skort, 1100cc, 1300cc and 1600cc, all sharing the same 'X-Flo' engine block and cylinder head, the different capacities coming from using the same cylinder bore, but with different 'stroke' crank-shafts. Conveniently, nearly all the 'bits' in cross flow engines are interchangeable, but better still, there was a wealth of aftermarket 'performance' parts on offer to hop them up.

The 1.1 was a common starting point because it was the one most easily insured, at least in 'standard' trim, and in those days attitudes to letting insurance companies know about mods were quite a lot more relaxed on both sides!....

As a 'Class' contender in clubman competition, the 1100 Escort proved a very capable tool; basically, you fitted a 1600 engine's cylinder head, because it had bigger valves in it, then added a big, fee-flowing carburettor and exhaust system and a lumpy cam-shaft to make the thing turn some pretty crazy rpm, which the short stroke crank was happy to do.

For hot road use, though, more 'amateur' tuning usually wasn't so successful, mainly because the people that bought 1.1 Skorts as their starting point were on the most restrictive budget, and making a 1.1 screamer was an expensive business.

Most often they would leave the internals of the engine alone and fit a big carb and exhaust, that would make it very loud and very raucous, put a huge hole in mid range power, but NOT actually make it any faster, because it was still trying to 'breath' through tiny little valves! That was often tackled by those a little more clued up by fitting the cylinder head from a 1.3 or 1.6 model, that had bigger valves in them, but unfortunately were also chambered for a bigger combustion charge!

Each pot of a four cylinder 1600cc engine has a capacity of 400cc's, so to attain a 10:1 compression ratio it has a combustion chamber volume of 40cc, 1/10 the volume of the cylinder, right? OK, now the volume of an 1100cc engine's cylinder is just 275cc, so squeeze that into 40cc and you have a compression ratio of just under 7:1, which would negate more than any benefit from having bigger valves and almost any other 'tuning' goodies!

The 'Lads' that managed to make 1.1 'Skorts actually shift much better than standard, tended to do so by stuffing a 1.6l engine in them out of an old Cortina! If it came from a Mk4 Cortina though, it was an OHC 'Pinto' block, that looked nothing like the push-rod X-Flow, and was a bit hard for an insurance inspector to ignore after an accident..... so with that 'risk' accepted, many went the whole hog and stuffed in the 2.0l Pinto engine, if possible, the more highly tuned version from a Capri!

Of the hopped up saloons of my youth, the 'Hot-rods' with big engines from other models were probably some of the more successful, and usually the cheapest to build, for the simple reason that the 'standard' engine, although not standard to the car it was fitted in, delivered the goods.... until some-one came along and tried fitting big crabs and lumpy cam-shafts to it!

Anyway, as expounded upon in 'anything', when modding vehicles, you HAVE to consider the vehicle as a 'whole' not just the entity like engine or suspension you are interested in, and work towards making the complete package do what you want, so after messing with the motor, making the resultant mutant handle was often the next problem, which leads me back to the point about considering the alternatives.

Back to the example of the 'Skorts, the principle behind building a 'Skort RS 'Replica',  is pretty simple; you start where Dagenham did, with a standard Escort 1.1 body-shell; you want to make it fast, so you fit either a 1600cc cross-flow or 2.0l pinto engine, both easily purloined in a scrap yard from another Ford model. Next, you beef up the brakes and suspension, some bits for which, could again be taken from other stock Ford cars, or from the aftermarket goodie market.

But, at the end of the day, as many found, taking a scrapped (and presumably knackered) MK4 Cortina, and using it as the donor to turn a (probably little less knackered!) 1.1 Skort into an RS2000 Replica, takes a lot of hard work, and to make it work, quite a few quids worth of performance parts, and what you end up with at the end of the day USUALLY is 'not quite' as good as a real RS, not worth as much as a real RS, and probably cost MORE to build out of junk, than buying a real genuine RS which would work as it should, and probably had more life in it!

And these days, second hand cars are a heck of a lot cheaper, there is almost always at least one or two 'Hot' versions of every 'cooking model' saloon, and they nearly always don't attract premium prices. Added to that, tuning modern cars, with complex electronic engine management systems, double overhead cam shafts and multi-valve heads, and catalytic converters, is a lot harder, more expensive and more risky, than it was twenty tears ago, so even LESS viable.... UNLESS of course, you really know what you are doing!

Back to Top-Gear's 'Imprezza' performance challenge. Their goal was to make something go around their test track as fast as an Imprezza, but for half the cost, and well, it shouldn't have taken a rocket scientist to point out to that they could have bought a second hand Imprezza for half the cost of a new Imprezza, and with a good service, whatever time it achieved would have been 'as fast' as an Imprezza'!

Getting to the Point!






What is Power

Power has a strict scientific definition, "Power is the Ability to do Work", which is unfortunately not very enlightening, and probably demands the strict scientific definition of work, which is "Work is the energy needed to move a load" But, before getting 'practical' about the stuff, these definitions do give us a couple of useful facts we need to be aware of.

First, 'Work' is 'Energy', which can take an awful lot of different forms, depending on what it is doing. Second, 'Power' is not the energy itself, but the 'ability' of a device or system to deliver energy (or consume it!)

Note I say 'deliver' energy, not 'make'.... getting a bit 'deep' and heading off towards the realms of philosophical physics, but a bit of Newtonian Physics that even Einstein and Steven Hawkins agree on is that 'Energy cannot be created nor Destroyed, merely converted in form'.

In science, 'power' is simple; from the definitions that it is the ability to do work; that work is a load moved through a distance; and that in turn is 'energy'; it is simply and strictly; 'Power is Rate of Energy Transfer'

So, scientifically speaking, power is power; how fast you shove energy into something, or how fast you get energy out of something, and as such, there is only ONE kind of power, defined by the strict mathematical equation:- Power = Energy Transfer / time , and the only distinction that might be made is by a '+' or '-' symbol to denote whether it's going into something, or coming out, and even then, that depends really on which side of something you are looking at it from!

In / Out, Shake it All About!

All right, trying to keep things skipping along here and not get too heavy, I've said that power is energy transfer; so to get a better grasp on the stuff, we need to look at energy and how its shoved about!

Time for one of my scrappy sketches! Here we have a schematic of a motor-vehicle showing the main 'widgets' of it's workings; and in terms of 'energy' it goes something like this:-

ALL the energy we have to mess with comes from the fuel. In the fuel, it is stored as chemical 'potential' energy, or 'calorific value' if you prefer, and we don't get to see any of that energy until it gets burned.

So, the fuel is piped to where it is going to get burned in the engine, being mixed with air to make a 'charge' by the carburetion system as it goes.

Once in the engine, its set on fire, and burned, turning the 'charge' into 'exhaust gasses' and releasing an enormous amount of heat. That heat makes the exhaust gasses expand enormously, raising the pressure in the combustion chamber hugely, and that 'pressure' is used to push a piston down a cylinder and make 'stuff' move...... (the article "Suck; Squash; Bang; Blow! " explains the process in more detail, while, "The 4-Stroke Engine " and "The 2-Stroke Engine " explain the mechanics of the two more usual types of internal combustion engine), but basically, the "Engine" is a device to 'catch' the thermal energy released by 'combustion' and turn it into kinetic energy, which is the energy of 'motion'.

And having got stuff moving, the transmission, by way of the clutch, gearbox, prop-shaft, axle and driven wheels, direct that motion to the road, in order to get the vehicle itself moving, which is the 'Useful' bit of the process!

In / Out, Shake it all about? Well, you put "In' fuel, and you get "Out" motion of the vehicle, but that's not all. You also get out quite a bit of noise, which is also 'energy' and you get rather a lot of heat that doesn't manage to get 'caught' and turned into 'useful' motion. And of the 'motion' that is generated, not all of it is 'useful' and even of what is 'useful', not all of it is used to do the job intended, shoving the vehicle along the road!


This, raises the question of the "Efficiency" of the 'system', and scientifically, efficiency is a measure of how much you get out, compared to what you put in to get it. And Internal combustion engines are notoriously 'inefficient' devices. At least depending on how you measure it! Here lies the cause of many an argument.

The pie chart shows how it typically pans out from the potential energy in the fuel. through to the useful motive 'work' that shoves the vehicle along.

In between, you loose about 20% of the possible energy in the fuel, because it doesn't get completely burned.

Two thirds of the energy you do get from burning the fuel, though is lost as waste heat, either spent down the exhaust, or lost into the air through the cooling system or the walls of the engine and stuff.

Only a fifth of the possible energy, or a quarter of the actual energy from the fuel, makes its way to the wheels to push the vehicle along.

While the rest, about 8% of the 'potential' energy in the fuel, or about 10% of the heat actually released, is used to drive the support mechanisms of the vehicle, or lost through things wagging about or rubbing together.

There are a lot of arguments made on the back of these kind of 'statistics'; just remember, some-one famously described statistics as the art of lying with numbers! 20% efficient, doesn't sound all that great, but, it's actually NOT all that bad, as overall efficiency goes. I mean, a common a garden light bulb is only about 20% efficient at turning electricity into useful light, work out the losses from the power station and suddenly a car looks VERY efficient at what it does!

Anyway, there are a couple of things in there I want to elaborate on, and one of them is what is marked on the chart as 'Unburned Fuel'. This is a subject that gets a lot of attention, and it's commonly suggested that most vehicles are 'throwing away' a fifth of the fuel they use, without using it. This loss is the 'Combustion Efficiency', and the theory is, that because as much energy is wasted in combustion efficiency as you actually get out in useful work, then you could double your power or your economy....

Right, think on that a moment, and let me point you at "Wonder Fuel & Widgets!", and the sort of snake-oil marketing merchants that use that kind of argument to flog gizmo's that will give you hundreds of miles per gallon, or thousands of horse-power, or eliminate all wear and tear on you engine just by putting a magic pebble in your petrol tank or whatever!

A good one is the 100mpg carburettor, that some how manages to claw back this 'wasted' fuel. Well, even if it managed to completely eliminate the combustion inefficiency, and turn ALL of that 'wasted' fuel into useful effort, it would only be able to better 100mpg on a vehicle that bettered 50mpg to begin with........ And, the suggestion fails to point out that the 'combustion efficiency' is only the first 'cut' as it were.

Yes, about 20% of the potential energy in the fuel, never gets released as heat, but only 25% of the heat that IS released gets converted into useful work. So, if you DID eliminate the 'loss', and put 20% more heat into the engine, then you would only actually get 5% more useful work from it, because two thirds of that extra heat would still be lost in the 'Thermal Efficiency'!

But that is presuming that the 'Unburned fuel' really is, unburned fuel. It isn't. what it ACTUALLY is is the difference between the calculated, 'Calorific Value' of the fuel, and the amount of actual heat you got from burning it in an engine. See "The Chemistry of Combustion " for the gory details, but potting a lot of science into a few sentences;

Basically, the 'Calorific' value of the fuel is what you might get IF that fuel was burned under what is known as 'Ideal' conditions, following a nice neat chemical equation, making combustion 'products' (smoke or ash!) that release the most energy possible from burning.

At the 'nub' of the matter, the 'ideal' conditions presume the fuel is burned in pure oxygen, but in the real world, we tend to burn stuff in air, a mixture of gasses, of which only 15% is actually oxygen. Throwing a lot of nitrogen in to dilute the mix, and a few other compounds besides, putting the whole lot under pressure to begin with, and keeping it all rather a lot hotter than would be ideal, we get a combustion that doesn't follow the chemical equation, and we don't get ALL the possible 'potential' energy out of the burn.

So, you aren't ACTUALLY getting 'Unburned' fuel coming out of the exhaust pipe, what you are getting is the products of less than ideal combustion, and yes, IF you could get closer to ideal combustion, then you WOULD get more energy out of the fuel as heat.... but you still have the problem that your engine only manages to catch about a third of that heat! and THAT really is the BIG one.

Two thirds of the heat you get is just 'lost', dumped into the atmosphere, spent through the cooling system, or disappears down the exhaust pipe. If you could capture some of that 'wasted' energy, you might really be on to something.

The trouble is, though, its fundamental to the working of an engine that you have a temperature difference to get the charge in the cylinder to expand and give you some mechanical effort. Also fundamental is that thermal energy moves from hot to cold at a rate proportional to the difference in temperature.

In an engine, Petrol burns at something between 1,200 & 1,800 Deg C. The 'Ambient' temperature an engine sits in is normally around 15-20 Deg C. In really cold countries, it may be as low as -30DegC, and in really hot ones, as high as +60DegC, but, in the combustion chamber of an engine, you have temperatures hundreds of times that of ambient, and that heat is going to be pretty eager to escape!

Yes, IF you could keep more of that heat in the engine for longer, sure you should get more expansion out of it. But, there is actually an exponential at work in there, and the more heat you put in, proportionally the less expansion you get out, and practically, the burn temperatures in a combustion chamber, are often actually higher than the melting point of the metal the chamber is made of, and It's only the brief exposure to that heat that stops the engine melting from inside!





Is your head Hurting Yet?

OK, pulling the salient bits from what we've got so far; we have a vehicle, and its got a lot of widgety bits inside it. We put fuel into it, and that gives it the energy to move. Between where we put the fuel in, and where we get the movement out, a lot of energy doesn't get usefully employed, so we have an efficiency question, but that aside, we are basically converting 'Chemical Potential Energy' into 'Thermal Energy', and that into 'Kinetic Energy'.

Power is rate of Energy Transfer, so the more energy we convert from fuel to heat to motion in a set time, or the faster we convert a certain amount of fuel to heat to work, so the more 'power' is being utilised.

Power is NOT the work itself, but the speed that work can be done.



Right, well efficiency is what you get out, divided by what you put in; by this reckoning, the 'useful' energy we get out, the bit shoving our vehicle along, is then 1/4 of the heat energy we put in to get it, so the vehicle is about 25% 'efficient', right? Hmmmmm......... yeah...... sort of....... IF you measure it like that!

If you ignore all the heat that the engine cant 'catch' though, what you put in is only a third of what you could of had. On THAT reckoning, then, while you only get out 1/4 of the THERMAL energy put in, you get out 75% of the MECHANICAL, or kinetic energy that you have to play with. In technical terms, what I'm saying is that your typical motor-vehicle, is only 25% THERMALLY efficient, but it is 75% MECHANICALLY efficient.

There is another efficiency, though, and that is 'Combustion' efficiency. The thermal efficiency, was the amount of useful work you get out, compared to how much heat you put in to get it. The mechanical efficiency, was the amount of useful work you got out compared to how much mechanical energy the engine trapped; BUT if you start at the beginning, you have a fuel, containing chemical potential energy, and it's not just possible, but quite likely, that not ALL of that potential energy is released during combustion.

Different fuels have different calorific values, that is to say, per litre, they contain different amounts of chemical potential energy, and there are tables that tell you what the calorific values are for certain 'reference' fuels.

I have looked at 'Wonder Widgets' in more detail in "Wonder Fuel & Widgets!", which is really a look at the sort of pseudo-science and warped logic used by the snake oil merchants Any way, while the matter of 'efficiency' if often core to a lot of this sales hype, the same faults also crop up in a lot of other debates.

So, if we have a 'combustion efficiency', then a 'thermal efficiency', and then a 'mechanical efficiency', we can introduce another, OVERALL efficiency.

Now, I said, efficiency is pretty simple; how much you get out, compared to how much you put in to get it. Overall efficiency then, lets go back to basics, and look at what we get out, compared to what we put in. Take a calorific value for our fuel, measure the useful work we get that fuel to do shoving the car along, and divide one by the other. Typically, the useful work out, would be about 20% of the potential chemical energy put in.

That doesn't sound that great, but in fact, ISN'T actually all that bad, and if you do the same 'Bottom Line' calculation for other devices, the internal combustion engine actually fares quite well!

For example, a tungsten light-bulb, actually produces more thermal energy than it does useful light energy. A 100W domestic light bulb, is only actually chuckling out about 20-25W of light, the rest is wasted as heat. However, while that light bulb might be drawing 100W of power from the electricity supply, we have to go back to the power plant and look at the amount of 'potential energy' being bunged in to the furnaces to make that electricity to work out the 'Overall' efficiency. Obviously it depends on the type of power generation, and the amount of 'transmission conversion' between the power station and the light-bulb, but if the light-bulb itself is only 20% efficient, the overall efficiency isn't going to be particularly wonderful!

OK, a lot of arguments are made from the notion that a motor-vehicle is SO dreadfully inefficient. Doing a 'Bottom Line' efficiency, comparing the useful work out, to the potential energy in, I've offered an 'Overall' efficiency of around 20%, which doesn't sound that great, but, they say that statistics is the art of lying with numbers! You can make it sound a whole lot worse.

So, here is typical bit of 'perverted logic'; The 'Combustion Efficiency', is only 75%, you only get three quarters of the energy stored in the fuel, out as heat. 'Thermal Efficiency', is only 25%, you only get one quarter of the heat, turned into mechanical effort. a quarter of three quarters, is three sixteenths. But, the 'Mechanical Efficiency' is 75%, so you only actually get three quarters of the mechanical energy you put in, out as useful work. Three quarters of three sixteenths, is nine, sixty-forths, or just under 15%........ Same numbers, but a bit of fudging, and we've managed to LOOSE a quarter of our overall efficiency in the doing!

Did you spot the mistake? Well, if you didn't, the 'Combustion Efficiency' compared the potential energy in, to the thermal energy out. That much was good. The 'Thermal' efficiency, though, compared the thermal energy caught, to the useful work out, which would have been fine, BUT, that is where the sum should have stopped, because at that point it had compared the total in, to the total out. But, it then took the 'Mechanical efficiency', and derated the total an extra time! Easy to be bamboozled by logic, isn't it!

Anyway, the usual purpose of this kind of argument is to suggest that with such a woeful efficiency, there must be HUGE scope to find 'Efficiency Gains', and get so much extra power, or so much extra economy.

Combustion Efficiency & Emissions

One of the more deliberated debates of the moment concerns 'emissions'. According to popular opinion, these are the horrible 'pollutants' caused by burning fuel, chief amongst which is the Carbon-di-Oxide, that's blamed for everything from the melting of the polar ice caps, to the credit crunch!

The usual story is that 'Emissions are Evil', and then it's suggested that using such a woefully 'inefficient' device as a motor-vehicle, 'needlessly' spewing such 'emissions', is more antisocial than running naked down the high-street wielding a Viking war axe, decapitating pregnant women along the way! Yeah! Well, I'm afraid that the staunch enviro-mental's and eco-terrorists, that promote such arguments are NEVER going to let the 'facts' let alone sound logical reasoning interfere with their 'faith' in the cause!

However...... The 'dreadful' inefficiency of a motor-vehicle, is often used at the logical 'lynch-pin' of these argumen



For 'simple' fuels, like methane or propane, experiments can often get to within 2% or 3%  of the chemical potential energy that chemical theory might predict. For more complicated fuels, like Methanol, calorimeter experiments can still be pretty accurate, and within perhaps 5% of predicted theory. But, as you get to larger chemical compounds, so the theory gets more complicated, and the accuracy decreases. For 'reference' fuels, the accuracy is usually within about 10%, but for 'commercial' fuels, it may be a lot lower.

Which, put simply, means that it's almost IMPOSSIBLE to know EXACTLY how much 'energy' you are starting with in your fuel, and at BEST, you can only make an 'educated guess' of what you MIGHT have, within pretty wide limits.


The actual fuels we get to see in day to day life aren't exactly the same; The crude oil used to make the common fuels comes from different 'reserves' and the mixture of compounds in the 'base' crude varies quite widely. Then after they have been refined, the actual fuel you get at the fuel-pump has been blended to the oil companies own recipe, and apart from the stuff in it that burns, there are usually also other additives. The one most people will be familiar with, as it was removed due to the noxious emissions it was claimed to produce, was lead, that was put in as a lubricant and 'knock' suppressant.

Anyway, the tables give calorific values for 'standard' fuels, and to all extents and purposes, most 'commercial' fuels will be close to one or other of the references. But, the calorific values quoted, are derived from burring samples of fuel in a laboratory, in a bit of equipment called a 'Calorimeter', which provides almost 'Ideal' conditions for combustion, and gets out ALMOST every last scrap of chemical potential energy from the sample! Note I say 'almost' all the chemical potential!




For an engine or a motor-vehicle, it's not all THAT difficult to measure how much energy you get 'out'. The 'Useful' energy is motion, or 'Work' which is a force moved through a distance. Measure the Force we are working against, measure the distance we move, and we have the useful 'Work Done'

What is a bit more tricky, is working out how much energy we had to put in to get it! Why? Well, what do you put in? Fuel? Of course, but how much ENERGY is in that fuel?

Conveniently, there are published tables, that suggest the calorific value of most fuels, which is how much thermal energy they should release when burned.

The standards for measuring the calorific value of a fuel are intended to try and get as close as possible to the theoretical calorific value that a chemist can work out from doing the maths on the chemical equations of the combustion reaction, which presumes conditions of 'perfect combustion'. That is, the fuel burns completely, and neatly, according to the equation.

The equation for Propane, one of the simpler hydro-carbon fuels, is: C3H8 + 5x O2 => 3x CO2 + 4x H2O + HEAT. That is actually what is known as a 'Balanced' equation, showing the proportions of combustants and the combustion products. For more complicated fuels, that are often actually mixtures of different compounds, it can get rather complicated, if not down right impossible!

Anyway, significant thing is that the suggested calorific value of a fuel, depends on it being subjected to 'perfect' combustion, in 'ideal' or near ideal conditions....... inside an engine is FAR from ideal!

Basically, inside an engine, your fuel is not burned in pure oxygen, but air, a mixture of gasses, four fifths, or 80% of which is nitrogen, and only 15% oxygen, with about 4% Carbon-di-Oxide, and the remaining 1% other 'stuff'! This is the first departure from 'ideal' conditions.

Next, when the 'charge' of fuel and air is ignited in the combustion chamber, it is already under pretty extreme pressure, having been compressed by the piston in the cylinder to perhaps a tenth of its original volume. The combustion chamber is also hot, and compressing the charge will have made the gasses more so, before they are set on fire.

Now, the 'charge' in an engine shouldn't "explode", it should burn. It's a bit of semantics, but an explosion is instantaneous combustion, the whole charge reacts to form the combustion products all at the same time, while combustion occurs over a longer time, like a match burning from one end to the other.

But, under the conditions in an engine, the burn-time tends to be a lot faster than it would be in 'ideal' conditions






We start with a fuel, be it petrol, diesel, vegetable oil, propane gas, or whatever, and we burn it, turning it into exhaust gasses.

Combustion is a chemical reaction, and it can be written as a chemical equation, but it depends on the actual fuel as to how that looks.

Now, armed with an equation, a chemist can sit down and do some sums and work out the actual quantity of HEAT that should be released by combustion. It's all to do with the bonds in the chemical molecules, and the amount of energy needed to 'glue' them together. You add up the bonding energy in the 'combatants', in this case the propane and oxygen, then you add up the bonding energy in the combustion products, in this case carbon di-oxide and water, and the difference between the two is the amount of energy that should be released. Or at least that is the THEORY!

In practice, it's not always that neat and tidy, especially when you get to more complicated fuels with really big hydro-carbon molecules, and almost impossible when dealing with 'Real' fuels that are more often NOT a nice simple compound, but a mixture of lots of different, often more complicated, ones!

So, in the lab, a chemist will use a device called a 'Calorimeter' to measure the amount of energy released by combustion of a sample of fuel. Basically, they burn the fuel sample in an excess of pure oxygen, to be sure it burns completely, in a chamber surrounded by a water jacket, and they measure the rise in temperature of the water, to determine how much energy is given off.





If you take Einstein's great work literally, e=mc2, or energy is mass times the speed of light squared. Now the speed of light is about 300 million meters per second, so a litre of petrol, weighing just a bit less than 1Kg would contain something like 90,000 GJ of energy. That's a lot. It takes 1J of energy to raise the temperature of 1cc of water by 1 Degree Celsius, so that amount of energy could boil about a billion litres of cold water! That's one cubic Kilometre, or about as much water as is in a reservoir like Lake Vynwry!

But! That is the ENTIRE amount of energy in a litre of fuel, and you would only get it by a nuclear reaction that converts the entire mass of fuel into pure energy, with NO by-products....... As we are well aware these days, even in the nuclear power generation industry, they haven't quite cracked that sub-atomic particle yet, and the best they can do is take heavy radio-active isotopes (Uranium) and decay them to slightly lighter radio-active Isotopes (Plutonium), and release an amount of energy proportional to the DIFFERENCE in the mass they started with, and the mass they finish with.


I mention it though, because energy IS released during combustion, and burn a litre of petrol, and in practice you might boil more like a cubic meter of water.

Atomic phy

Anyway, boiling water isn't all that 'useful' unless you want a cup of tea, or have a steam engine. We are looking at internal combustion engines, and we aren't interested in how much water we can boil, but shoving a vehicle along.






'Work' is the actual 'job' we want an engine to do. Work is 'energy', and as far as engines are concerned, what we are mainly interested in is 'Kinetic Energy', or the energy of motion. Scientifically, Kinetic Energy = Mass x Velocity 2, and a little look at this can be helpful.

At rest, a vehicle has no 'Kinetic Energy', at least as a 'vehicle''. Inside the vehicle, there are probably lots of bits and pieces clattering about; pistons going up and down, con-rods waggling around, shafts turning, valves opening and closing and stuff, but it wont have any Kinetic Energy, as a 'vehicle' until it starts to move.

Moving at a constant speed, the vehicle has 'Kinetic Energy'. It has a mass, it has a speed, so it will have Kinetic Energy. To go from having 'Zero' Kinetic Energy, to having 'some' Kinetic Energy, there must have been an energy transfer, 'work' has been done, to accelerate our vehicle from rest to some speed, so logically, that energy MUST have come from the engine. Remember, energy cannot be created or destroyed, only converted in form, so, the engine is converting 'potential' energy in the fuel, into kinetic energy, ultimately making 'stuff' but most importantly, the vehicle, move.

But, drive a 'real' vehicle, and our engine is delivering 'energy all the time'. If we were holding a nice constant speed along a nice level piece of road, the 'Kinetic Energy' shouldn't change, because neither the mass or speed of the vehicle is changing, BUT, switch the engine off, (or run out of petrol!), and with the engine not delivering any more energy, the vehicle would 'coast' for a while, gradually slowing down, until it came to a halt.

So if the engine is having to deliver some energy to keep the vehicle moving WITHOUT changing the 'Kinetic Energy', it must be having to deliver some energy to do something else, and the clue is, that take that energy away, and the vehicle coasts to a halt, because SOMETHING is trying to stop it moving.

This brings us to the matter of forces, which are things that make things move or hold them still, and brings in lots of 'Laws' conceived by a chap called Isaac Newton, which, for the most part are very simple, but which, wrongly applied CAN be very misleading, and I'll try to explain later, perhaps! Any way, applying Newton's laws relating forces and energy, Work = Force x Distance.

The 'something' that is trying to stop our vehicle moving, is called 'Drag', which I'll look at in more tedious detail in a minute, but the general definition of drag is the overall resistive force acting against motion.

So, our vehicle is moving, and in doing so is subject to some 'drag', which is a force, and If it is moving, it must be covering some 'distance'. Force x Distance is 'work', which is 'energy', so, logically, THAT accounts for the energy our engine is delivering to hold our vehicle at constant speed.

There are a few other places energy can go, and mention of 'stuff' inside the engine whizzing about gives a clue, but for the most part, we are concerned with the 'Useful energy' provided by the engine, that energy which actually does something we want, and what we USUALLY want is the engine to make our vehicle move, so we tend to be concerned with the energy used to over come drag, and the energy used to cause acceleration.

One thing I OUGHT to mention about 'Kinetic Energy' is that idea of 'coasting'. In theory, the science says that energy cannot be destroyed, so, if you have used energy to accelerate your vehicle and give it 'Kinetic Energy', as you slow down, the reduction in Kinetic energy as you slow, is simply being used to overcome Drag, so you effectively get it back again.

And the theory holds, give or take some anomalies, provided you don't use the brakes! The brakes are devices which are designed specifically to put a 'resistive' or 'drag' force on your vehicle, and spend the Kinetic Energy you have put into your vehicle to make it slow down a lot more rapidly than it would otherwise!


Which brings me to look at Drag in more detail, because obviously, Drag is where MOST of the energy that goes into our vehicle gets 'spent'. Energy transferred to 'Kinetic energy' during acceleration, may be a pretty hefty chunk of the energy delivered by the engine during acceleration, but at the end of the day, the vehicle has to stop, so that kinetic energy has to go somewhere, and as said, it gets used over coming drag, either 'natural' drag that the vehicle experiences due to its size shape, weight and stuff, or 'applied' drag, from putting the brakes on!

Remember, Drag is the OVERALL resistance to motion, from ANY source, and there tends to be two MAIN sources of drag, apart from deliberate braking, which is 'Rolling Resistance' and 'Wind Resistance'. Rolling resistance being mainly the 'friction' or resistance to a vehicle rolling over a surface; wind Resistance being mainly the force needed to shift the air over and around the body of a vehicle as it moves through it.

Wind Resistance, is usually of more concern, because it varies a lot, and the faster you go, proportionally, the more you get, as it increases 'exponentially' with speed. Ie; double the speed of a vehicle and you get more than double the wind resistance. But, Wind Resistance is also proportional to the frontal area of your vehicle, and the amount of streamlining it has.

Rolling Resistance, comes mainly from the 'friction' between a vehicle and the road, and in the bearings of its wheels and stuff. If you look at it in detail, there's a LOT of things that can actually all contribute to rolling resistance, and it's not THAT simple. But in a lot of cases the 'rolling resistance' isn't very important to any-one.

Vehicles have it, and the bigger and heavier the vehicle, the more of it they tend to have, but proportionally, it's not usually all that 'significant'; which is to say, a large proportion of the 'Drag' that is of concern to any-one. Dealing with 'real-world' vehicles, the amount of energy used to over come rolling resistance, tends not to be all that big, as at the sort of speeds that most motor-vehicles travel, most energy is being used to over come wind resistance, OR being dumped into the brakes when they are artificially slowed down!

Consequently, Drag is often misleadingly used to refer just to Wind-Resistance, with Rolling Resistance ignored or presumed to be just another 'efficiency-loss', like the energy delivered by the engine, that doesn't get used to do useful work, overcoming resistance in the transmission and 'stuff'.

Power at Work

OK, well, time to look at something a bit more tangible, and consider something 'real', and I'm going to have a drag-race between a Short wheel Base Series III Land-Rover and a Honda 750 'four' motorbike! No contest, really, is it!

OK, well, our Motorbike has a 750cc engine, that according to my spec sheet can deliver about 75bhp, and weighs about 200Kg. The Land-Rover, also has an engine that can deliver about 75bhp, but it weighs in at nearly 1200Kg, and is obviously a LOT bigger.

OK, well, lets get these things on the line and moving. We have a vehicle, each has an engine, and they are going to accelerate, in a straight line, down a nice level bit of tarmac, in the same conditions, and the energy from their engines is going to be used to over come drag, and increase their Kinetic energy. What happens to it after that, we don't care; once they have crossed the trap at the end of 1/4 mile, they'll waste it into the brakes, most likely.

Now, I've bunged some numbers into a spread-sheet, and that tells me that at 30mph the bike, will have attained a Kinetic Energy of 3.5Mj, while the Land Rover will have attained a Kinetic Energy of about 21Mj, 6 times as much, which stands to reason, as the Landy is 6 times the weight, but also, the bike will need to be doing nearly 75mph, before it has gained as much Kinetic Energy as the Landy has getting to 30.

So, I think that it would only be 'fair' to run this as a 'Handicap' and say that the bike has got to get to 75mph BEFORE the Landy gets to 30.....  Bit more of a contest, but, Nah! Bike's barely any bigger than a bloke standing up, where the Landy's the size of a garage door, at LEAST six times the frontal area, bike's STILL going to ace it! OK, lets do it.

The lights change, the Landy driver had his beast in 1st gear ready, lets out the clutch, and floors the accelerator, and the beast shudders..

Bike rider, also ready in first gear, had the throttle open the engine spinning around 4000rpm, the clutch 'just' on the engagement point, held back on the front brake...

The Land Rover JUDDERS into the lead! What IS going on? The bike rider's baulked! It's making all the right noises, but it isn't moving? Is the clutch slipping, or is the back tyre spinning, I cant tell! No, He's off! Looks like he's got his 'seat', backed off the throttle a bit and got some weight over the back-wheel...

The Landy is in trouble now, look, it surged ahead there for a second, but now the drivers got to shift it into second, to keep it going, look at it shake!

There goes the bike, he's just snicked a gear change too, but he's got a LOT more to do yet; and the Landy is already doing almost 25, and the bikes got to get up to almost three times that speed!

It's going to be close! The Landy is giving it all it's got, and he's almost there, but the bikes almost there too, 68mph, looks like he's not sure whether to try and rev the thing out in third or to change up to forth for that last few mph... no he's holding third, and Oh No..... the Landy has pipped him to the post!

Power to Weight Ratio

A bit of fun, but a realistic one. In the 'real-world' the numbers don't always add up the way the science or even common sense says they should. Common sense suggests that the vehicle with the most power SHOULD accelerate the fastest; in our case both vehicles have the same power, so the lighter of the two should be the winner, right?

In the scenario, that 'common sense' held true, the motorbike, accelerated to over twice the speed the heavier Landy did in the same time, which supports the common suggestion that 'Power to Weight' is important, and 'explaining' that 'common sense' one of Newton's laws, Force = Mass x Acceleration, is often quoted..... and SEEMS to make sense. BUT, it's an erroneous application of the science. 'Force' is NOT 'Power'.

Power is what PROVIDES force, but it ISN'T the force itself. In the 'Lore', this often leads to a lot of debate, and fuels the arguments on both sides that it's 'Not the Power, it's the 'Torque', because 'Torque' is force in a rotary situation, and engine's 'torque' figures are often quoted with their power.... Explaining those arguments is going to get complicated, but I'm going to 'back up' to the source of the mistake.

In the scenario, I gave the bike a 'Handi-Cap', based on the 'Kinetic Energy' each vehicle would have. At the same speed, the Landy, six times heavier than the bike, would always have six times the K.E., but because K.E. is proportional to velocity squared, the bike could acquire as much K.E as the Landy just by going a bit faster, and I used a spread-sheet and worked it out, and found that at 75mph, the bike would have as much K.E as the Landy travelling at 30mph.

Power is rate of energy transfer, and the transfer happening is that the engine is turning chemical potential energy into Kinetic Energy, so the bit of science that explains why the lighter vehicle, or the vehicle with the better power to weight ratio, accelerates the fastest ISN'T that Force = Mass x Acceleration, but that Kinetic Energy is Mass x Velocity 2,.

If you ignore the error that Power isn't Force, and presume that Force is in some way directly proportional to Power, yes, it does 'suggest' that the science supports the 'common sense', but, applying Force = Mass x Acceleration would then lead to the prediction that our motorbike, with 1/6th the weight would accelerate 6 times faster, or accelerate to 6 times the speed in the same time; which it didn't; it only accelerated to a little over twice the speed in the same time, SO in practice, THAT explanation cant be completely correct.


If we want to use the equation Force = Mass x Acceleration, and I do, and I will, we have to use it in the right place, and that is where there are actual forces to be looked at.

Scrappy little sketch of a Series Landy for you, with four arrows, representing forces, marked on it.

'Drag', I've already mentioned is the combined force of EVERYTHING resisting our vehicle's motion, so it points in the opposite direction to the way we want to go.

Traction', an important topic dealt with in more detail in Get A Grip. but is basically what we have to 'push against'

'Thrust', then is actually TWO forces, one pushing forwards against drag, the other pushing backwards against 'traction'.

And, it's a convenience of the 'wheel', as a mechanical 'device', that subjected to a rotary 'torque' a wheel translates that torque into a linear 'couple', which is a pair of forces pointing in opposite directions. Works the other way, as well, actually, but any way.

I'll get to the detail of how our 'energy' gets from the engine to the wheel in a bit, and bamboozle you with a lot more ever more vexing equations, mean-while, just trust me, what we have is an engine, delivering 'shaft-power' through whatever mechanics there are, to the driven wheels, where the wheels turn the 'Torque' of that shaft power into a linear couple, we can call 'Thrust'.

Right, a fundamental 'law' of 'couples' is that the two forces in opposing directions are ALWAYS the same size, so our 'Thrust' is pushing as hard against 'traction' as it is against 'Drag'.

'Traction', is the 'grip' we have to push against. Push too hard, and exceed the 'limit of traction', and the wheel will spin, but, provided we have a good surface, and enough weight over the wheel, and we don't push too hard, the wheel will 'grip' and react whatever 'Thrust' we might have, and let us 'push' against 'Drag'. So until we get into a situation where we are at risk of exceeding the limit of traction, we can pretty much ignore that bit of the picture, and worry about the tussle between Thrust & Drag on it's own.

Thrust is a Force, Drag is also a Force, and we know our Landy has mass. So, we can now apply Force = Mass x Acceleration... But remember, forces are the things that make things move or hold them still. If our Landy isn't moving, then the forces are either holding it still, or simply not there, and common sense would suggest that they PROBABLY aren't there.

Could get into a lot of semantics here, remember it depends on which side of the fence you are looking at things from as to whether something is a 'push' or a 'pull', and some-one MIGHT suggest that in a strong wind, our 'Drag' might become a 'Thrust' trying to blow our Landy backwards, while the 'thrust' becomes the 'Drag', as the hand-brake is applying a torque trying to stop it! An interesting diversion, and a valid one, BUT we are worried about engines and power, so it's NOT really a situation I want to explore, so for now, we can PRESUME that if we DON'T deliver any energy from the engine to the driven wheels, we ent got a 'Thrust' there!

So, with our vehicle at rest, our Thrust is zero, and so is our Drag. But, provide some 'shaft-power' to the driven wheels, and we get a Thrust. What happens?

We move! Well, hopefully! We put some force to the wheels, and that shoves against the Drag, and provided we have enough 'Thrust' to overcome 'Drag' we go forwards, at some speed. And this is where we can apply Force = Mass x Acceleration, but it can take a little bit of head scratching.

At CONSTANT speed, there is NO acceleration. Acceleration is the 'rate of change of speed', so IF the speed don't change, we don't have any acceleration, and if we put 'zero' into the equation, Force = Mass x Acceleration, we can multiply it by ANY mass we like, something times nothing is nothing...... we have NO FORCE! Which is curious, because we KNOW have a Thrust Force AND a Drag Force......

Which MEANS that the two forces we have must some-how cancel each other out. It's like two tug of war teams pulling on either end of a piece of rope, the rag in the middle only moves if one team pull harder than the other. Applied to the vehicle as a whole, we only get an 'acceleration' IF the 'Thrust' is bigger then the drag. If the Drag is bigger than the Thrust, we get a 'deceleration', and if they are the same, we get a constant speed.

'Thrust' is the force 'doing the useful work'. And, that 'work' is EITHER overcoming 'Drag' OR increasing the vehicle's 'Kinetic Energy', by exceeding the Drag enough to cause an acceleration.

This is why 'Thrust' is so important, and really the 'thing' that is MOST important when it comes to how a vehicle performs, and so the most significant consideration of 'Power'.

The bottom line is, that the sole purpose of an engine is to deliver 'energy' to do USEFUL work, and that USEFUL work is done by THRUST.


OK, before moving on, I need to explain a bit more thoroughly the concept of 'Torque', which I have so far only mentioned. Torque = Force x leverage, and is often described as a 'Force' in a 'Rotary' situation, which is often good enough, but not always.

Explaining it a bit more fully, it's the 'see-saw' principle; a beam, balanced on a 'fulcrum' push one end down, the beam rotates about the fulcrum, the other end coming up. Put a child on either end, and the heavier child lifts the lighter, but shift the fulcrum towards the heavier child, and you can get the two to balance.

Children apply a 'force' on their end of the see-saw, their weight, (or their mass times the acceleration due to gravity). That 'force' times the leverage provided by the length of beam between them and the fulcrum, generates a 'torque', so small force with long lever, can balance big force with short lever.

BUT, I mentioned when talking about 'Thrust' a 'Torque' is generated by, or produces a 'couple', which is actually TWO forces in opposite directions.

In the see-saw example, with a child on either end, each is generating a torque, the torque one child generates opposing the other; two torques, but only TWO forces..... BUT we should have TWO forces PER 'torque', so IF we have two 'Torque's' we should have FOUR forces. And in the see-saw example, what we have neglected is the 'fulcrum', which is supporting the beam, which is providing the missing force for each 'torque'.

But, you don't always have a 'fulcrum' or something for forces to pivot around, and if you have a 'beam' and apply two forces to it in opposite directions, unless those forces are directly 'head to head', then the beam will twist about a centre between where the two forces are applied.

Any way, the concept of 'torque' is important when it comes to engines, because at the 'coal face', what we want is a linear Force, Thrust, but the engine has been offering force wrapped around a shaft as 'Torque', and for the most part the shafts involved are supported in bearings that do the job of the 'fulcrum' so we only have to worry about one half of the 'couple' involved in a torque... except at the driven wheel.

But, the main thing about 'Torque', as far as engines go, is that it starts as a force in the engine, and ends up as a force at the rear wheel, and the size of any 'torque' in the middle isn't that important, because at the end of the day, what's going to do the 'work' is a force, and we can get almost any force we want from any torque, simply by using a longer or shorter lever. Which is a bit ambiguous, until I say that 'gears' are 'rotary levers'.

Measuring Power

Right, I'm taking the topic off on a bit of a tangent here, to look at how power is measured, because in doing so, it explains an awful lot of other stuff

This is a 'dyno-trace' for a typical car engine, and the sort of chart that some-times gets published in car-makers brochures, magazine's road tests, or 'tuning' people's adverts.

Across the bottom is a scale indicating engine speed, up the side is another scale indicating power and torque, and there are two coloured lines across the chart, one for power, one for torque.

Sometimes the Power trace is shown on its own; sometimes the torque trace is shown on its own, sometimes, as here they are shown on the same chart, but with their scales on different sides. Doesn't really matter, it's a convenience, how the traces are presented.

The important thing is that 'Conventionally', the traces are produced by measuring the Torque an engine makes at a certain engine speed, and plotting that on the chart. Measuring the torque at another engine speed, and plotting that on the chart, and eventually, after a LOT of measurements over the entire range of engine speeds, playing 'dot-to-dot' and joining them all up to make a graph.

First thing, what is actually MEASURED isn't 'Power', but 'Torque' (at least by 'Conventional' Methods). Having measured the torque the engine makes at a certain speed, the 'power' being delivered is CALCULATED, from the torque, using the equation Power = Toque x Revs . Which was why I wanted to explain 'Torque' a bit better.

And I'd better explain why I keep qualifying the way I'm describing power being worked out as the 'conventional' manner; basically it is the 'normal' way, and a fairly easy to understand way of working out power. There ARE others, and I'll mention them in a bit, for now, though, the way I'm going to describe is pretty much the 'normal' way of doing things.

OK, so lets drive a 'Brake', which is the device we are going to use to measure the Torque. It's called a 'Brake', because that is pretty much what it is, a 'brake', just like the drum-brake or disc-brake on your car or motorbike, and it applies a 'load' that resists the 'Shaft-Torque' being delivered by the engine.

What you do is connect your engine to the 'Brake' usually with a short shaft, and 'run the engine up' to whatever speed you want to take a measurement at. You then put the 'brake' on, and that will slow the engine down, so you open the throttle a bit to make it turn a bit faster, and when it goes faster, put the brake on a bit harder, which slows the engine down again....

Eventually, you get the engine turning at pretty much close to the speed you want, with the throttle wide open, so it can't be made to go any faster under the load you are applying, and you make slight adjustments of the brake force, until the engine is turning at JUST the exact speed you want.

Now, at the brake, you have the disk or drum spinning at the engine speed you want, the brake shoes or brake pads gripping it, though will tend to be 'dragged round' by the disk or drum, by the 'Brake Force'.

On a car or motorbike, the pads or shoes are usually held solidly in place, so they cant move, the 'brake-force' reacted by something pretty solid in the vehicle's structure, BUT, you don't need to, you could react it against something 'springy', and watch the brake force flex it...

Which is what an engine brake does. Basically, the 'grippy bit' of the brake is on a bracket that pivots on the shaft that's turning the disc or drum, and is free to turn with it, or at least through a few degrees of rotation. Normally there's travel stops to prevent the mechanism spinning wildly! But, between those two stops, there is nothing to react' the brake-force, except a big lever on the bracket, that can be 'loaded' with balance weights.

So, follow the theory, big lever, and weights to apply a force, and you have a 'torque', and it's back to the 'see-saw'; If you apply enough load to the lever to stop it moving, then that 'holding' torque must be the same as the 'driving' torque, which is what you are getting from the engine. Make sense?

OK, well having said that you measure the 'Torque', that's not STRICTLY true, because if you are using a 'balance beam engine brake' what you are ACTUALLY measuring is 'weight' or balance force, or commonly 'Brake-Force', but given that the lever is a known length, and usually a convenient one, the maths to convert that force to a torque isn't hard.

So, having measured the 'Brake-Force' you can calculate the 'Brake Torque' from it, and from that the 'Brake-Horse-Power'..... which is a long way of explaining why it's called a 'Brake Horse Power', basically a Horse-Power measured on an Engine Brake!

'Available' Power

Right, having explained how a power-trace or 'dyno-chart' is created, a few things to observe about them.

First of all, when 'power' ratings are 'quoted' they tend to be the 'maximum' power the engine made on a 'dyno-run', finding out what the most power it could make was, at all engine speeds. That 'quoted' power is then only available at that ONE engine speed.

Likewise, when a 'torque' rating is quoted, that too will be a 'maximum', and again tend to only be available at ONE engine speed, and usually NOT the same engine speed that maximum power is made! I'll look at that curiosity in a moment; moving on....

The next important thing is that the engine will NOT offer as much power as may be 'quoted' at any other engine speed, so to know how much 'available power' you might expect, you need the full power chart, NOT just the highlights of the 'max-power' and 'peak torque' figures.

But even THEN, you need to be aware of the fact that those figures were recorded under specific circumstances, known as the 'steady state', which is with the engine held at constant speed against a load, with the throttle wide open.

In the real world, where that power gets put to work, the power we WANT will normally be to change the speed of our vehicle. Now most vehicles have some sort of gearbox that means that the 'gear-reduction' between shaft speed and wheel speed can be changed by the driver, BUT in any particular gear, the 'reduction ratio' of the transmission is fixed, so the wheel turns at a speed proportional to the engine speed. If you want to go faster, you have to make the engine go faster.

OK, so once you have accelerated so much that the engine simply wont go any faster you change up a gear, and start again lower down the engine's rev range with a 'taller gear', and yes, there is often a lot of overlap between gears, and you could choose to travel at say 30mph with the engine screaming away in first, or labouring along in fifth, or slightly less tortured in any of the gears in the middle.

But, the point is, in the 'Real-World', when the power is being put to work, it is normally NOT under 'steady state' conditions; the engine is NOT holding a constant speed against a fixed load, it is accelerating between engine speeds.

As far as the mechanics of the engine are concerned, in the 'steady state' the fuel mixture being delivered to the engine and the ignition timing of when its set fire to will normally be pretty close to 'ideal'.

There are actually two 'optimum' settings of fuel and ignition for any 'steady state', one gives best economy, one gives best power; and manufacturers normally find something in-between, that's good compromise, inclined one way or the other, depending on what the engine is to be used for.

Under acceleration, though, those settings are thrown out the window! Engines used to have distributors that had 'vacuum advance' mechanisms in them to pull the ignition timing forwards under acceleration, and carburettors had 'enrichment' devices, like 'accelerator jets' to chuck extra fuel into the engine, when they accelerated. These days the same effects tend to be achieved, by electronic control modules following a 'map', but even so, the ignition and fuelling under acceleration is NOT the same as under the steady state conditions, and you WONT get as much power as the chart suggests you might!

Next you have 'Lag effects' to consider, of which probably the best understood is the phenomena of 'Turbo-Lag' on turbo-charged engines, where the turbine, driven by the exhaust has to 'spool up' making boost to make the exhaust, to make more boost, providing a delay between opening the throttle and actually getting the power asked for.

But there are plenty of 'Lag Effects' and a big one is 'Inertial Lag', or 'Fly-Wheel-Effect'. The purpose of a Fly-wheel is to 'Damp' rotary motion, basically the fly-wheel stores kinetic energy and 'resists' a change in shaft speed, so it saps power under acceleration, and gives it back under braking to try and keep the shaft speed constant. Bigger or heavier the fly-wheel, more fly-wheel effect it has, so the more it resists a change of speed.

Engine's usually have a dedicated fly-wheel, but 'counter-weights' on the crank-shaft also do the same job, and basically ANY bit of spinning mass inside the engine or in the transmission, will also add to the overall 'Fly-Wheel Effect'

There are more 'Lag Effects' in the induction system and exhaust, where acceleration of the engine demands an acceleration of the flow of gasses into and out of the engine. This can get a bit complicated, because even at the 'steady state', the induction and exhaust gasses 'pulse' as they pass the valves controlling their flow, and they are flowing through pipes that change in cross section and volume, making it all rather complicated.

The easiest to explain though is 'stutter' or 'hesitation'. This is more pronounced on engines that are 'over carburetted', little engines fitted with big carburettors for good 'flow' at high revs. At low speed, sudden opening of the throttle would expose a huge cross section of 'port' to the incoming air, and without a lot of 'suck' from the engine to drag it in, the air flow would slow down to almost nothing, and the engine would 'bog' as the air just didn't want to move or drag very much fuel in with it, and it would 'hesitate' and 'stutter' until the air actually started moving at a decent speed again.

So, the 'Quoted Power', the 'max-power' provided by a spec sheet, may give us some idea of how much 'shove' we could expect, but not much. It only tells us how much power there may be at a single engine speed, under 'steady state conditions'.

The Dyno-Chart, gives us a far better picture of how much power might be on-tap, and how it's spread across the range of engine speeds, but again, only under 'steady-state' conditions.

We can only GUESS at how much of that 'potential' power could be available under acceleration, after re-optimising the ignition & fuelling has taken a cut and 'lag' effects played their part, and unfortunately, that is USUALLY what is of most interest to us.

BUT, IF we are aware of how power readings are taken, and what influences them, and don't treat the statistics as cast in stone absolutes, they CAN give us a pretty good idea of how an engine behaves/


OK, baking up a little, there are a few other ways of taking 'Power Readings', and different machines for doing it. I've described the operation of the 'Balanced Beam, Brake Force Dynometer', which is one of the oldest and simplest kinds of Dyno, and is pretty reliable and accurate, but not that quick and easy to use.

A Development of the 'Brake Force' Dynometer is the Hydraulic Load Dynometer, which uses a paddle or pump instead of a friction brake to apply the 'load' to the engine, and another is the 'Electrical Load Dynometer', which basically uses a big electric generator to apply the load.

These CAN be used in the same way, and a 'Balance Beam' used to measure the reacted engine force, but, in the case of the Electric Dyno, electrical Power = Volts x Amps, so you could measure the electrical power being generated, to work out how much power the engine is providing. You can do a similar thing with a Hydraulic Dyno by measuring pressure and flow rates, but that's a bit more complicated to explain, and in either case such 'indirect measurement' isn't as reliable or accurate, though some-times more convenient.

The Electrical Dyno, though does have one advantage, and that is that the electrical generator, can often be 'reversed' and run as a motor to actually 'drive' the engine, and the power needed to do so measured, which can be handy for measuring 'efficiency'.

But, I want to mention the 'Inertial Load Dynometer'. This is a VERY different beast to the others, and uses the 'fly-wheel effect' to measure power. Rather than testing the engine under the 'steady state', the engine is loaded by turning a big fly-wheel, which it accelerates through the engine's rev range. The 'fly-Wheel' stores Kinetic Energy, so to make it turn faster you have to put energy into it, so if you measure the acceleration of the fly-wheel, you can calculate the power being delivered to it.

In the last twenty years, the 'Inertial Load' Dynometer has become probably the most common kind of Dynometer in use, but mainly in garages and speed-shops, not in development houses or research labs. The reason for this is that it's only been since the advent of cheap micro-processors, that we have had the technology to make the speed readings an inertial Dyno relies on, quickly enough and accurately enough. So cheap computers and a pretty simple and easy to make mechanics have made it a very economical Dyno to make and sell, while the necessary computer has also made it pretty 'user freindly' and not a specialised piece of scientific equipment that takes a lot of skilled man-power and time to operate.

Consequently it is the most common kind of dyno used in 'Rolling Roads', where instead of the engine being attached to the dyno directly by a short shaft or similar, the engine is tested in its installation, a car or motorbike, it's wheels driving rollers that in turn drive the dyno via whatever transmission is convenient or necessary.

Which is worth mentioning, because, 'Engine Brakes' or 'Test-Bed-Dynometer' are very good at testing engine's, but they don't give you much clue what you might get at 'the coal face' where the power is put to work, at the driven wheels. Rolling roads, it's suggested would offer a better indication of what you'd see 'in the real world', but with so much mechanical gubbins between the engine and the dyno, they tend to give lower and less reliable and less accurate results.

The 'Inertial Dyno', 'Rolling Road', measuring 'Power at the Wheels' and under the 'dynamic' condition of 'acceleration', fare more closely reflects 'real world' conditions, and you would hope give you a far better idea of what the engine is doing for you 'at the coal face'.

Theoretically, that is true, practically however, it proves other-wise! The accuracy of the Inertial Dyno is not brilliant, and when compounded to the inaccuracy of being driven through a rolling road, can become pretty dire!


Power Test 'recordings', the actual measurements taken from the machine have to be subjected to 'some' maths to calculate the torque and power. Even in the most accurate conditions, there is always some 'experimental error' to account for readings that aren't 100% 'spot-on', and values are often rounded up or down a bit; there has to be a 'tolerance' on the accuracy or recordings, and the more maths you do to those recordings after taking them, so the bigger any 'margin for error' will become.

Next there is what's known 'Correction Factors', which are often applied to the 'results', to make the figures obtained more 'comparable' to others obtained on other machines or in other conditions.

The first set of 'Correction Factors' applied are for Ambient Conditions, where the atmospheric pressure, air-temperature, humidity and 'air quality' all effect the amount of useable oxygen going into an engine allowing fuel to be burned, and so the amount of power it might deliver.

Test an engine in a city full of smoke, on a hot, dry day, half way up a mountain, and it will not give as high a power reading as if you tested it in a forest next to the sea on a cool afternoon!

So, there are tables of 'Correction Factors' to be applied to Dyno-results to 'correct' actual values from those obtained under 'real' conditions, to what would be expected under 'ideal' conditions, and that means more recordings, more maths, and more room for error.

Then there are 'Correction Factors' for the kind of equipment used, so that recordings made on a rolling road can be magnified to be look more like the results you would expect had they been taken on an engine test bed, or to make values obtained on an electric Dyno or Inertial Dyno, likewise look more like those that may have been obtained had they been taken on a balanced beam 'brake'. More Maths, more margin for error!

Which is interesting, BUT, the bottom line is, that whenever you see power charts or power statistics, chances are that they are NOT that reliable and the actual numbers COULD be out by a very big margin.

For manufacturers 'Claimed' figures, recordings are usually made on a Test-Bed Dynometer, and tend NOT to be wildly inaccurate, though the engines actually tested, might NOT be that 'typical' of the ones they fitted to the cars or bikes in the show-room, and could have been specially prepared to perform well on test.

There's then more scope for getting 'flattering' figures by what is known as 'playing the standards game', and choosing a test procedure that has most room for 'interpretation' to allow the engine to be tested in such a way as to get bigger numbers, like running it without an alternator or water-pump, or even 'back-motoring' and adding the power losses inside the engine to the power readings obtained!

And of course, the numbers can then be flattered a bit more, by being a bit 'creative' in the calculations and being a bit 'generous' when rounding readings or applying correction factors!

Doesn't make manufacturers claims entirely 'useless', just means that you have to be aware that they are probably a bit exaggerated, but given most manufacturers will probably be playing the same games, that exaggeration allowed for, they may still be reasonably 'comparable'.

'Independent' Power Claims are another matter, because there is a LOT more scope for exaggeration or 'flattery'. In the 'tuning industry', the people trying to sell performance engines or performance engine parts, like fuel injection 'Chips', exhausts, air-filters or 'ported' cylinder heads, the 'veracity' of claims can vary enormously.

To be fair, the people making the most exaggerated claims aren't all 'crooks' or dishonest, and challenged, many will openly admit to how much they have flattered their claims, and justify them as being within the 'accepted margins of error', grin, and point out that 'every-one' is doing it, and they wouldn't sell anything if they didn't too!

Thing is, the 'accepted margins of error' are just simply HUGE! the 'worst case' is as I alluded to, when recordings are made on an 'Inertial-Load, Rolling Road Dynometer'. There have been a few 'dyno-shoot-outs' covered in the magazines, where vehicles have been run up on different Dyno's up and down the country to find the most powerful. To level the playing field a bit, the organisers took the same 'control' vehicles to each venue to 'calibrate' the machine, and provide a bench mark to apply 'correction values'.

The values obtained for the 'control' vehicles, on the different Dyno's varied by as much a 25% between machines, BUT, the results obtained on the SAME machine could vary by as much as 10% for different dyno-runs taken on the same day, and as much as 15% for runs taken on different days!

So, before debating the correction factors, we have a compound 'margin of error' in the order of over 40%, and overall, the accuracy of 'quoted' power results could be out by as much as 50% or more.

The Shape of Power

OK, so the power claims made by manufacturers, and the power traces or 'gains' that tuning firms suggest might not be that reliable in as much as the numbers bandied about, but we now know how those numbers or graphs were probably created, and have grasped the idea that the 'claimed' power, is only an indication of the power that MIGHT be available.

With a Dyno-Chart, we can see that while our engine might deliver100bhp at a 'peak' of 5,000rpm, it doesn't offer that much power all the time, and lower down the rev range, the most it might provide is an awful lot less. According to dyno-trace I've shown, although it could make 100bhp at 5,000 rpm, at 2,500rpm it offers barely half that, 45bhp, while at 1500 rpm, it offers just 25bhp.

And, those values, recorded at the 'steady state' are probably NOT going to be available to us when we want them, which is to make the vehicle the engine is in accelerate, because of 'Lag-Effects'.

So, the 'Available Power' is not going to be anything like that suggested if we ONLY considered the claimed 'max-power', and if we use the full dyno-trace to give us some idea of what we might expect accross the rev range, we still shouldn't expect to get ALL of what is suggested the engine could offer.

BUT, the 'SHAPE' of the dyno-trace, is still useful, and OK, we might not get ALL the power shown on the chart, we SHOULD still get most of it, and as we go up through the rev-range, the 'available power' if not the full 'potential power' will be a proportion of it. So, lets have a look at the shape of a few Dyno charts.

Chart on the right is NOT a genuine Dyno-Chart for a Series III Land-Rover, in fact none of the Charts I've produced for this article are derived from real dyno-runs with real engines, they are, slightly 'idealised' charts based on real data from a number of engines, of the 'type' I wanted to illustrate. So, forgive me that 'licence' and I'll get on with it.

The 'Landy' has a 'slugger' type engine, which is a way of describing an engine that isn't necessarily all that powerful, but makes a lot of 'grunt' usually low down in the rev-range, often described as 'low down torque' or a 'torque-y' engine

And it's normally explained that a 'slogger' makes it's power from 'Torque not Revs'.

If you look at the shape of the torque trace, it ramps very steeply to a peak very low in the rev range, just 2000rpm, and falls off steadily from there.

The power trace is derived from the torque trace, remember, Power = Torque x Revs, so the revs multiply the torque trace, and low down the rev range, the power builds with the increasing torque and revs.

As the Torque starts to 'drop off' though, the multiplying effect of the increasing revs, keeps the power curve going up, not as fast, but still going up, until it flats, and eventually trails off, as the torque is dropping so much that the increased revs just cant make up the loss.

The engine only offers 75bhp at its 'peak', BUT, it is pulling strongly from less than 2000rpm, with 50bhp or more available until well past it's peak power, a spread of power perhaps 3,000 rpm wide.

In terms of 'Available Power' this engine offers potentially 65% of it's 'maximum power' over 65% of its 'operating range'.

OK, this next chart is again an 'idealised' Dyno-Trace, this time for a 'screamer' type engine, which in contrast to the 'slogger' would be describes as 'all torque, no revs'.

Not entirely fair, 'screamer' type engines often make more torque than a slugger type engine, but it's true its a lot higher in the rev-range, and the big power they make tends to come a long way up, a usually very high rev-range.

This trace actually goes to 104bhp at about 12,500rpm, which is pretty 'heady heights' compared to the Land-Rover engine, but is typical of the sort of Dyno-Trace you'd get from a fairly highly tuned 600cc sports motor-bike.

Now, if you look at the shape of the traces, the torque trace is a lot less severe, and doesn't 'ramp' as quickly as did the slogger's. Instead, it builds gradually, to a 'hump' rather than a peak, between around 10,000 & 13,000 rpm.

The power trace, reflecting that lazy 'hump' takes a while before it starts to do very much, and from tick-over until almost 7,000 rpm, it struggles to deliver 20bhp.

After that, though, the engine 'comes on the cam' or gets into it's 'power-band' and starts pulling hard, the trace ramping steeply to 10,000rpm when the torque starts to flat off, and the rev's start to have to compensate for there being no more torque, but it carries on pulling to the 12,500rpm peak, before dropping off sharply, as the torque curve falls off, and the extra revs cant make up for the falling torque.

This engine is rated at 104bhp, at 'peak', BUT, for 7,000 rpm, it makes less than 20bhp. Only after 7,000 rpm, does it start to 'deliver the goods' but even then, it's only offering more than 50bhp, half its 'max-power' for less than half of its operating range.

The 'slugger' offered better than 2/3 of it's 'peak' power, for 2/3 of its operating range; the screamer, only offered better than 2/3 of it's peak power for barely 1/3 of it's operating range.

'Useful' Power

OK, so, that SORT of suggests that a 'slogger' engine, even though it might offer less 'peak' power, could have more 'available' power spread across the rev-range, so it would be likely it could make as much or more power, more often, than a screamer type engine. And THAT is basically the gist of the 'Torque over Revs' debate, though like much argument on the subject, it's rather simplified.

I've mentioned the 'Power-Band', that region of the engines entire range of operating speeds where it is making a fairly hefty proportion of all the power it can, and the accusation commonly levelled against 'screamer' type engines is that they have a very 'narrow' power-band.

In comparison, 'slogger' type engines, delivering such a significant proportion of their 'peak' power almost from tick-over right across their rev-range, are lauded as not having a 'power-band'. Technically they do, it's just that it's almost their entire operating range!

Now, when it comes to 'power-bands' something I find rather perplexing, is when people try and suggest that a motor-bike engine's 'power-band' from say 10,000rpm to 14,000 rpm is 'narrow', only 4,000rpm. Take a 4x4 diesel engine, and the 'broad tractable' spread of power that is applauded for, is over an entire operating range that doesn't go beyond 4250rpm, and doesn't start until nearly 1000rpm!

It's all relative. 3000rpm of a 4x4's Diesel engine's 'broad spread' of 'available power' is 1000rpm LESS than the 'Narrow' power-band of a motorbike, but it is 100% of the engines range of operating speeds, where  4,000 rpm of an operating range that's 14,000 rpm wide, is less than 30%.

At the end of the day, I understand the debate, and can see where the two sides are trying to get to, but unfortunately, its like trying to explain 'power to weight ratio' with the Force = Mass x Acceleration equation. Right logic, wrong application.

In the real world, 'screamer' type engines are more often found in motorbikes, and light, sporty cars; 'slogger' type engines in 4x4's and trucks, and in those applications, the engine's 'character' suits the vehicle.

Back to the comparison of our Land-rover and our 750cc Motorbike. Land-Rover has a 'slogger' type engine, motorbike, more of a 'screamer', and BOTH have a gear-box.

So, riding the bike, when you want to make it accelerate, you get it into a gear, that puts the engine speed somewhere you can get at the 'useful power'. It may have a 14,000 rpm 'red-line' and that may mark the 'entire' range of operating speeds, BUT when you want to put the thing to work, you ONLY use the helpful 'power-band' and 'work the gears' to keep it 'in the sweet-spot'.

"Ah", say the opponents, "That's it, 'screamers' ONLY work if you have the gearing to be able to get at the power, that's why they need so MANY gears, and why they are so tiring to drive!"

Hmmmmm, yeah! Well, I'm not convinced; my Land-Rover, with it's 'broad flexible' spread of power, that would supposedly demand less gears, and less effort finding one to let me exploit the engine's potential power, would SORT of give lie to the suggestion. It had a four-speed gear-box, with a 'two-speed' transfer box, and an 'over-drive'. OK, so lets ignore the Hi-Low transfer ratio's, as the low range is supposedly for 'off-roading', but still, with the Over-Drive, that gives me EIGHT gears to mess with, and trying to get her up and down the inclines of the rolling Derby Dales, I KNOW I made good use of at LEAST six of them! My 750 Honda has just FIVE gears........

And the exponents of the 'slogger' start spluttering and trying to find fault in the example, and explain that it comes down to 'weight'... and If I am feeling devilish, I might throw the suggestion of 'power-to weight' into the melting pot to fuel their fury....

Ultimately it's about 'matching' the engine to the application. 75bhp Land-rover engine and 75bhp motorbike engine. Bike makes it's 'peak' power at 9000 rpm, Landy at 4250rpm, just under half the speed.

IF you were to fit the bike engine into the Landy, and 'match' the gearing, so that it's peak power co-incited with the same road speed as where the Landy-engine made it's peak power, you would have to roughly halve the overall reduction ratio, so that at ANY road-speed, the bike engine would be turning twice the RPM.

Now, having 'matched' the gearing, forget the numbers, the 'operating range' of engine speeds goes from 0 to 100%, and over that range, where the Landy engine was delivering over 65% of ultimate 'maximum' for 65% of the operating range, the bike engine would only be offering the SAME % of 'Available Power' for 33% of the operating range.

And because that 'Useful' Power was stacked mainly up the top end of the operating range, with so little down the bottom, you WOULD need EVEN more gears to be able to get the engine spinning the rpm that let you get at the power. It comes down to ....... Thrust!, and the acceleration that can provide.


Power = Torque x Revs, and Torque = Force x Leverage.

We want 'Thrust', because THAT is what is going to push us along, muscling us through drag, letting us accelerate, and hauling us up hills, so do some algebra on those equations and you get:- Force =  Power / (Leverage x Revs)

Now, the force we are interested in is 'Thrust', and it's put at the driven wheels by the power coming from the engine, delivered through the transmission, which contains gears, and 'gears' are rotary levers, so to make it a little easier to use, I'm going to swap the term 'Force' for 'Thrust' and the term, 'leverage' for the term 'gearing' and get: Thrust =  Power / (Revs x Gearing)

Now, at any engine speed, there is going to be a certain amount of 'Available Power', we know it's not going to be ALL that the dyno-trace might suggest the engine could offer, but will be a reasonable proportion of it.

The 'Transmission' we know gives us 'gearing' and with a gear-box, probably the choice of a few different ratio's, and if we know what they are, we have the three bits of information to put into our sum and work out roughly how much 'thrust' we might expect to get from the engine.

More than that, if we also know the size of the wheel, knowing how fast it's turning, we can also work out the 'road speed' we would be travelling at, at that engine speed, and in that gear.

However, what we are really interested in is how much MORE thrust we may have than we have 'Drag', so we would have some clue as to how much 'acceleration' we might coax out of the vehicle.

Force = Mass x acceleration, but the in this case Force = Thrust - Drag. So what we really need to know is how much Drag we may have to worry about at any particular road-speed.

Conveniently I have a graph. Difficult to calculate drag, because there are so many variables,  but given some rough data, about how fast 'real' Land Rovers and Range Rovers can with different engine's of different powers, I've worked out APPROXIMATELY what the drag would be at any given speed, for a roughly Landy sized, shaped and streamlined vehicle!

So, for a given road-speed, all we need to do is look-up the corresponding Drag value, and use that in our sums, and we are away.

Now, with four gears to consider, and a range of road-speeds and engine speeds, the sums could get rather tedious, so, I put all the 'data' into a spread-sheet and got that to do the job for me, and make me some more graphs. And they loo complicated!


Well, it LOOKS complicated, but it's not THAT bad. There is a line for each gear ratio, and the acceleration you could expect in that gear plotted against the road speed.

Basically, if Force = Mass x Acceleration the acceleration is going to be the 'Net force' from the thrust, after the drag's been taken off it, divided by the mass of the vehicle.

Only curiosity, really is that the acceleration trace can go negative, which suggests that the Thrust at those speeds where it IS negative isn't enough to over come the drag.

Those places where the acceleration goes negative are at the very beginning of the trace, where the engine is delivering little power, and you'd 'slip' the clutch to pull away from rest, and not use the taller gears to go so slow, or at the end of each trace, where the power has peaked out, and you aren't getting any more go, and either need to change up a gear, or simply don't have the power to go any faster!

This first graph shows the acceleration traces for a Landy shaped sized vehicle, with a Landy type 'slogger' engine, and Series Land-rover gear Ratio's, taken from the Haynes Manual.

This second graph, shows the acceleration traces for the same Landy sized & shaped vehicle, with the same gear ratio's, BUT with the overall reduction ratio doubled (By using the 'low range gears of the transfer box) to match to a 'screamer' type motorbike engine, while the third graph, shows the acceleration traces for a 'screamer' motorbike engine in a more 'motorbike' sized shaped vehicle, and gearing to suit.

Now, whatever the gearing, and whatever the vehicle, if you ignore the values, and look at the shape of the traces, the acceleration trace very closely follows the shape of the power traces in EVERY case. They may be skewed or stretched, BUT the basic 'profile' is there.

The pertinence of the traces, though is that they show the importance of 'Available Power' and the effect of gearing to how the vehicle 'behaves' in the real world, and how 'matching' the engine to its installation and selecting good gearing are REALLY as crucial to getting the sort of performance you want, than any real question of power or torque.

Something that the first chart shows quite clearly, is that the top speed of the Land-Rover with the Land Rover engine, is about 70mph or so. First gear acceleration, is quite 'brisk' up to about 20mph, and second gear offers quite a bit until it runs out of breath at about 40mph, but after that, it just doesn't have the power to overcome the increasing drag, and it lumbers lethargically up to its top whack, pretty slowly, hitting a wall at 70mph or so whether in third or fourth gears, where the traces cross the axis and go negative.

The traces for both third and forth, though, extend well beyond where they cross the axis, and the fourth gear trace extends right up to around 100mph, which denotes that it is 'geared' for 100mph in top, BUT doesn't have the power to pull that gearing and actually go that fast.

This is an important point, given that a lot of people seem to believe that increasing the gearing will make their vehicle's faster. In most cases it wont, as the gearing of most 'standard' vehicles is already actually taller than the engines have the power to pull to max-power revs, let alone the red-line.

The chart for the Land-Rover with the motorbike engine shows the importance of gearing a little more clearly, my rough guestimate that the gearing needed to be halved to roughly 'match' the engine to the vehicle, wasn't quite right fourth gear, is too tall to be of any practical use, at no point does it's trace lie above the axis suggesting it could provide positive acceleration, while third gear is only a little better.

The main 'problem' is that 'lack' of power in the lower rev-range, and the long ramp up to where it does start to make useful power. even with the extra power the engine makes, it doesn't help make the truck very much faster, it tops out at about 75mph or so, and the acceleration on offer is never as impressive as for the standard engine.

If the overall reduction was dropped a bit more, so that all of the gears gave some more reduction, the lower 'first' would offer a bit more useful acceleration low down, where it's lacking, and the engine might be able to rev out a bit, and add a few mph to the top speed.

But, even so 'optimised' the acceleration on offer wouldn't be as brisk as that provided by the standard engine, and the top speed not significantly greater. It WOULD work though, but despite having 25% more 'peak' power, the lack of 'Available Power' would see it actually SLOWER point to point in the real world.

The third chart, showing the acceleration from the motor-bike engine in a motorbike, is just for comparison, trace only shows four gears, really there ought to be one, possibly two more, as most bikes have five or six speed gear-boxes, and those extra gears would be lower then the lowest shown, and offer some pretty amazing rates of acceleration.

Top gear lumbers the machine up to about 140mph, but with no particularly startling acceleration, in fact slightly lowering the gearing in this case might actually see the machine achieve a bit more top speed.

With a couple of extra lower gears, the acceleration on offer at lower speeds would be pretty startling, but even without them, because of the machine's light weight, and drag inducing bulk, the lack of 'Available Power' isn't that much of an impediment, and even without those lower gears, the machine accelerates better than the Land-Rover could over a much wider range of speeds.

'Matching' The Ingredients

Essentially, when it comes to 'power', what is important ISN'T big numbers, and certainly not big 'max-power' figures, and having a lot of power is NO guarantee of a 'quick' vehicle. In the real world, vehicles have to work, and work well, over their entire range of operating speeds.

The 'Dyno-Trace' gives you some clue as to the amount of 'Available Power' an engine might have, and how it's spread about the rev-range, but, it is the 'gearing' that takes 'Available Power' and makes it 'Useable Power', allowing you to select a ratio that will let you get at that power at the road speed its wanted, to get some 'thrust' from it.











If we want to use the equation Force = Mass x Acceleration, and I do, and I will, we have to use it in the right place, and that is where there are actual forces to be looked at.

Scrappy little sketch of a Series Landy for you, with four arrows, representing forces, marked on it.

'Drag', I've already mentioned is the combined force of EVERYTHING resisting our vehicle's motion, so it points in the opposite direction to the way we want to go.

Traction', an important topic dealt with in more detail in Get A Grip. but is basically what we have to 'push against'

'Thrust', then is actually TWO forces, one pushing forwards against drag, the other pushing backwards against 'traction'.

And, it's a convenience of the 'wheel', as a mechanical 'device', that subjected to a rotary 'torque' a wheel translates that torque into a linear 'couple', which is a pair of forces pointing in opposite directions. Works the other way, as well, actually, but any way.

I'll get to the detail of how our 'energy' gets from the engine to the wheel in a bit, and bamboozle y












When it comes to acceleration, the equation Force = Mass x Acceleration is often applied wrongly to the situation, to explain why vehicles with the better power to weight ratio accelerate the fastest.

The bike, is only 1/6th the weight of the Landy, so applying the theory wrongly, the suggestion is that the bike SHOULD accelerate six times as fast as the Landy, given the same power.


Thing IS, that power ISN'T force, and the 'maximum' power an engine might deliver is NOT the 'power' that engine is ACTUALLY delivering constantly throughout acceleration, while of the power the engine MIGHT deliver, it's only 'Useful' power IF it makes the vehicle move!


In a 'rotary' situation, . And the equation Force = Mass x Acceleration, would be correctly applied to the force delivered to the driving wheels.

So, doing a little substitution, and given that 'leverage' is effectively the 'gearing' between the engine and the driven wheels, what we'd get is the equation, Acceleration = Power / Revs X Gear Reduction X Mass, or that for a given power, the acceleration you'd see from it depends as much on the engine revs and the gearing, as it does from the vehicle weight.

Now, our Land-Rover, has a 2.25l engine, it's not very highly tuned, but what it does do is deliver a lot of force, without making too many engine revs, I think it makes its maximum power at about 4250rpm, where a Honda 750, makes it's maximum power at around 9,000 rpm, more than twice as high.

But, evening things out a bit, road-speed is directly proportional to engine speed, times gear reduction, so to some extent, having an engine that makes it's power from twice the engine revs, would be compensated for by having to have twice the gear-reduction, so the 'driving' force delivered to the rear wheel would be about the same.

The matter gets a bit more querulous if we look at the  old argument that it's not 'power' that makes acceleration, but 'torque' and particularly, 'low down' torque or 'mid-range power'.

Now, the Land-Rover is quoted as offering something daft like 110ft-lb of torque at something ridiculously low down like 1,800rpm, where the Honda is quoted as giving something fairly puny like 45ft-lb of torque, a lot closer to it's peak power, at around 7,000rpm.

Back to the sums, and sorry, but it's a complete red-herring! the 'peak torque' an engine can deliver is completely irrelevant. Torque is an expression of force in a rotary situation, and it can be multiplied or divided by the gearing you have.

Power = Torque x Revs, so it doesn't matter whether you have your power as a big torque delivered at low revs, or a small torque delivered at high revs; what you have is a fixed amount of shaft power, put to the road through some gears. So what you'll get is a force which will give you an acceleration, and you'll get the SAME force and the SAME acceleration from the SAME power, however that power is delivered by the engine!

The 'correlation' between 'low down torque' and acceleration DOESN'T come from the numbers being bandied about, but from the inference those numbers have on the shape of the power curve, and THAT has on the 'Power Delivery'.

When an engine seems to make a lot of its power as a big torque a long way down it's rev range from where it makes it's maximum power, it IMPLIES that the engine is in a fairly 'mild' state of tune, with the power spread about the rev-range fairly evenly.

When an engine has a peak torque that is pushed a lot further up it's rev-range, a lot closer to, if not co-incident with it's maximum power, it IMPLIES an engine that is rather more highly tuned, and it doesn't make very much power any-where but at or around it's maximum.

It's NOT always the case, and it is a pretty big 'generality', and the 'state of tune' is only ONE thing that effects the 'power delivery', BUT, what I am getting to is back to the suggestion that it ISN'T the power that the engine MIGHT deliver that gives acceleration, but the power it actually DOES deliver.

The 'power rating', is the 'maximum' power the engine might provide. So, take the Land-Rover engine, which we have inferred has a big, low down 'peak torque' suggesting a good spread of power across the rev-range, and compare that to the Motorbike engine, that has a relatively small, and high 'peak torque' a lot closer to it's maximum power, suggesting a much narrower spread of power, and it's LIKELY that as we accelerate from tick-over to maximum power, the Land-Rover, even though it doesn't give any more power at 'MAXIMUM' its actually Delivering MORE power, for more of the time, as it accelerates through it's rev range to that maximum.....

Which suggests, that the 'Useful' power is described by the Dyno-Trace for an engine, showing how much power it could offer at any given engine speed in its rev-range, not just it's 'maximum power' rating at ONE engine speed. Which is where we'll go in  next.

Measuring Power







When we 'accelerate' a vehicle, it's easy to see the 'energy-transfer' that's happening, and see 'power-in-action', but, hold a vehicle at constant speed, and the Kinetic energy shouldn't change, BUT, if we were on that bike or in that Land-rover, and backed off the throttle, we KNOW that we'd slow down.

So WITHOUT any acceleration, we are burning some fuel and using some energy to JUST keep the vehicle moving without any acceleration. Energy cannot be created or destroyed, energy is 'work' and if we are putting energy IN to our vehicle, and NOT seeing that energy increase the Kinetic energy (making it go faster), it HAS to be going some-where and doing something. And, the answer is, it is being used to over-come 'Drag'.

I'm going to look at drag in a LOT more detail in a bit, but, basically it is the overall 'resistance to motion' that our vehicle experiences, and is made up of lots of different things, that between them all conspire to try and stop us moving. There are two main parts to Drag, one is what's known as 'rolling resistance', the other 'wind resistance', and simplistically, our bike or Land-Rover, trundling down this drag strip are going to be fighting against the friction between their wheels and the tarmac, and the air they have to muscle aside as they go.

Now, Work is the energy needed to move a load. The 'load' in this case is the 'Drag-Force', so we now have TWO parts to the overall 'energy-transfer' going on, when we accelerate our two vehicles.

First part is that SOME of the energy from the engine is having to be used to overcome the 'Drag', and only what's left can be used to increase the Kinetic Energy of the vehicle by making it accelerate.

And, in our example, we have a motorbike, which has just two wheels, and is about as wide as a person, and a Land-Rover, which has four wheels, and is about as wide and tall as a garage door! So, shouldn't take too much debate to convince that the Land-Rover is LIKELY to suffer RATHER a lot more overall 'Drag' than the motorbike!

But, a curiosity of 'Drag' is that it's proportional to speed. Faster you go, proportionally the more Drag you suffer, but, where Kinetic energy is proportional to the square of speed, Drag, unfortunately isn't so neat, and how much more drag you get for how much faster you go depends on a lot of different aspects of the vehicle in question, and the conditions it's moving in, however....

So, back to our Drag Race, and looking at the 'energy transfer' that's happening; we have energy coming from the fuel being burned, and delivered by the engine. SOME of that energy is being used to over come the 'drag' the vehicle experiences, the rest to increase the 'Kinetic Energy' of the vehicle.





Power in Action

When it gets to the 'real-world', though, the subject starts to get a bit more ambiguous, and people start talking about power in different ways; they talk about 'max-power', or 'pulling power', or 'useable-power', or confusing aspects of 'scientific' power, talking about, 'tractability', 'torque', 'willingness', and 'power-bands' and things, usually trying to explain why vehicles 'work' the way they do, confirming or contradicting the 'quoted' power ratings they may have.

Bottom line is, power is power, it has a strict scientific definition, and hence you just CANNOT have 'different kinds' of power!  BUT, there are LOTS of different ways things can 'Work', and it's 'Work' that explains a lot of the debates and arguments.


A 2 tonne Range Rover is 2 times as heavy as a 1 tonne Land-Rover, which is 4 times as heavy as a 250Kg Motorbike. At 30 mph, the motorbike would then have 1/4 the Kinetic energy as a Land-Rover, or 1/8 the Kinetic energy of a Range Rover, travelling at the same speed, But...

Accelerate the bike to 60mph, and because Kinetic Energy is proportional to Speed squared, double the speed, and you quadruple the Kinetic energy, so it would have as much energy as the Land Rover. Accelerate the bike further, to 90mph, triple the speed, and now, three times three is nine, so the kinetic energy the bike has is nine times what it was at 30mph, more than the Range-Rover, eight times as heavy, travelling just a third the speed!

BUT! The Kinetic energy a vehicle HAS, for most purposes ISN'T all that important very often. Important to recognise it, but, it's not very useful until we have to consider a 'change' in the kinetic energy a vehicle has, because it's only THEN that we have any 'energy transfer', or 'Work' going on.

Lets imagine our Land-Rover & Motorbike, lined up in front of the Christmas tree at a Drag strip. Not moving, the vehicles don't have ANY Kinetic Energy. Gubbings whirring about inside them might, but we're not concerned with that just yet, vehicle as a whole doesn't have any Kinetic energy.

So, lights change, and our two vehicles roar off down the strip. And as soon as they start moving, they acquire Kinetic Energy. Energy can't be created or destroyed, only 'converted', so that energy HAS to have come from some-where, and obviously, that energy has come from the fuel, being burned in the engine, and shoved to the wheels of the vehicle, through the transmission.

Now, at 30mph, the Kinetic energy the bike has, is 1/4 that of the Land Rover, so  the 'Work Done' to accelerate the bike is 1/4 that done to accelerate the Land-Rover, but carry on accelerating the bike to 60mph, and the Kinetic energy it has then, is the SAME as the Land-Rover travelling at 30mph.

It takes the same amount of energy, to do the same amount of 'work', and accelerating a Land-Rover to 30mph is as much 'work' as accelerating a motorbike to 60mph.

OK, so that's the 'Energy Transfer', the 'change in Kinetic energy', the 'work-done', what about 'power? Well, power is the rate of energy transfer, so the POWER needed to do that same work, depends on how fast that work is done, right?

So, if the Bike and the Land-Rover both have equally 'powerful' engines, then the bike SHOULD be able to accelerate to 60mph in the same time it takes the Land-Rover to accelerate to 30mph.......

And that little illustration supports a lot of ideas about 'power to weight' ratio's, and why, 'in the real world', it's not just about power, but the weight, or mass you want that power to move, that can be important. BUT, it's still VERY simplistic.









 so, travelling at twice the speed of a Land-Rover, because speed is squared, the motorbike would have as much Kinetic energy.



In engineering, we have LOTS of different 'Kinds' of power, which is one of the reasons that the subject can get so confusing, but to try an de-mystify it a bit.

Quoted Power

The first thing to recognise is the matter of 'Quoted' power ratings. Open a magazine or brochure, or look at a spec sheet on the internet, and most will 'quote' a power rating for a vehicle's engine. It used to be in BHP or 'Brake Horse Power', but with increasing 'standardisation' the number might be annotated, "(DIN) BHP," or just "DIN", sometimes 'PS', but more commonly these days, 'kW'. THIS is just the START of the confusion in "Quoted" power ratings!

To explain the different 'brands' of power, I have to explain that you cant 'measure' power, it's a rate. You have to measure the 'energy transfer' that's going on, and time it, then CALCULATE the implied power. But usually it's a bit more convoluted than that, and you cant easily measure the energy transfer, what you have to do is measure the 'load' on the engine, and the work it does, then do your sums from that, so it's a LOT of maths.

But on top of that, there are also a lot of different 'standards' for how you measure what you do, before doing the maths, and there-in lies the discrepancy in the suffixes.

Two briefly explain two of them, BHP and kW; the first, BHP is the 'old fashioned' unit of measurement for power, under the 'Imperial' system of units, that included inches and pounds and stuff. The kW is the 'Metric' unit of power, from the system of units that include mm and Kg. According to international standards, 1 kW is about 1.3 BHP, or 1 BHP is about 0.75kW.

The other suffixes, or more precisely 'qualifiers' DIN or PS and such, denote the measurement 'standard' that's been used to obtain the figures, and that's a bit more difficult to explain.....

The basic procedure is to run an engine up to speed on a test bed, with a 'load' applied to it's crank shaft. The load is applied by a device called a 'brake', hence 'Brake' horse power, and it's pretty much like a conventional wheel brake, but it's 'reacted' against a spring, or a set of weights, like a pair of weighing scales, measuring the force the engine is making, or the brake 'reacting'..

From the measured force, if you know the speed the engine is turning, and a few other dimensions, you can calculate the work being done, and hence the power being delivered. And the principle is pretty much the same for ANY power measurement, BUT......

First of all, it's not always that convenient to measure an engine's power on a test bed, at the crank-shaft or flywheel. Easy enough if you are an engine maker, developing a new engine, or testing your products as they come off the line, but not so if you are a tuner working on a complete car. Bit of a pain to have to pull the engine out just to see if the new carburettor jetting has given you any improvement.

So, whey back when, we got a discrepancy in 'quoted' power figures between measurements made at the 'fly-wheel', on an 'engine brake', and between measurements made 'at the wheels' on a rolling road ', which is basically the same contraption, but connected by shafts or chains to a set of rollers that the car can sit on.

'Fly-Wheel' BHP figures tended to be higher than 'Rear Wheel' BHP figures, because of a number of discrepancies and efficiencies, which included the 'power-losses' of the vehicle's transmission, and the 'rolling resistance' of the wheels on the rollers, and all the 'gubbins' between between the rollers and the actual 'brake'.

In their favour, Rolling road, power measurements, would tend to offer a better indicator of how the vehicle might perform with an engine 'in the real world', telling you how much power actually gets to where it's used. But, on the down-side, because of all the mechanical widgets between where the power is made and where it's measured, they don't tend to be as accurate.

So, consider, a manufacturer builds a car, and they 'quote' a power rating of 100bhp for the engine, quite legitimately, as that was what they measured at the crank-shaft. A tuning company then takes that car, and re-maps the Engine Management Unit. They reckon that their 'Tuning chip' has liberated an extra 10bhp from the engine. Unfortunately, when they put the car on the rollers, they only actually get 93bhp out of it!

If, in adverts, they 'Quoted' the power figures they obtained on a set of rollers, it would appear that their tuning had actually ROBBED the engine of 7 BHP, not ADDED 10! Which leads me to the matter of 'Correction Factors', which is basically a method for 'fudging' the figures. 

What you do, is a like for like comparison of the same engine, measured by the different methods, and calculate how much they vary by. Our engine delivering 100bhp on the test bed, might have only delivered 85bhp on the rollers. So you could say that the transmission has absorbed 15BHP or 15% of the power

So, when you measured 93BHP from the 'tuned' engine on the rollers, you can then 'Correct' the value you measured either, by adding the 'lost' 15bhp, and getting a 'corrected' value of 108BHP, or you could multiply your 93BHP by 115% and get 107BHP.In this case, there isn't much in it, but selling a 'performance' tuning goodie, you'd obviously choose the 'correction' factor, that made your product look better.....

Which the original manufacture had probably done already! Because when THEY put their engine on the test bed, they probably only got about 90BHP from it, so using a little creative accountancy, stripped it of 'ancillaries' that might have put any load on the engine, above that the brake did.

Anything attached to the crank-shaft by a belt will put a 'load' on the engine, and most engines at least have one belt driving the alternator, the water pump and cooling fan, while these days there may be even more for power steering pumps or air conditioning and things. Together, these 'ancillaries' can put quite a lot of 'load' on an engine.

Once upon a time, it was quite common for engines to be 'bench tested' with the electricity for their ignition system taken from a mains transformer and their cooling water from a hose attached to the tap, and NOTHING else attached, so that the measurements obtained showed the most power possible.

So, what was needed was some sort of 'standard' so that when power ratings were 'quoted' they were all measured the same way... Only, it never really happened, and manufacturers could chose to use standards published by various standards organisations or different versions or revisions of those standards, picking the ones that were easiest to use or which gave the most flattering figures!

To the old SAE (Society of American Engineers) standard, engines could be tested without 'ancillaries', and it was common for them to be run up on test without water-pumps or alternators. The torque an engine made at the fly-wheel, was then measured against a mechanical 'brake', with the crank speed measured via a calibrated 'tacho'.

Power = Toque x Revs  (an equation I'll explain later), so multiplying one reading against another gave you the power the engine made. And being so simple, it's a pretty accurate and reliable way to measure power.

However, an engine tested at sea-level on a cold day, will be sucking in air that is more dense, than an engine tested a few thousand feet above sea-level on a hot one. Denser the air, more oxygen you get in the cylinders on each induction stroke, so the more fuel you can burn in it, hence the more power you can get out.

So, even with the 'simplest' test standard, there are still 'correction factors' in the standard to be applied to account for 'ambient conditions', and those each add experimental error and calculation inaccuracy.

The modern PS standard (published by the International Standards Organisation), is based on the DIN standard, and that is a bit more convoluted. It essentially measures the power at the fly-wheel as the SAE standard, and includes the same sort of 'correction' calculations for ambient conditions.

The biggest difference, though, is that AFTER measuring the power the engine delivers at the crank shaft, the brake is 'motored' to drive the engine with the ignition switched off, and the power needed to turn the engine at the same speed measured, then ADDED to the power measured at the fly-wheel.

This effectively measures the power delivered at the piston tops! Ie; before any is lost in the bearings of the engine, or to drive the valve gear, let alone any 'ancillaries'! But, with two power measurements, one driving and one driven, there is a lot more scope for experimental error or inaccuracy, and twice the need for 'correction', so it is one of the least 'reliable' standards, but usually one of the more 'flattering'!

Cumulatively, the amount by which 'quoted' power figures can vary, depending on the test equipment used, the test 'standard' followed, and the 'correction' factors applied to the results is so HUGE, that without a LOT of qualification, 'Quoted' power figures can be virtually meaningless!

Max Power

The other thing about 'quoted' power, is that the power quoted is the MOST power that who-ever has tested it can get an engine to deliver. It's the 'Maximum' power rating, 'Max-Power', and happens only under very specific conditions.

Back to how its measured. A bit like doing a Hill-Start, you balance the engine's revs against a brake, trying to hold the engine speed constant, then you record the revs and the load, and from the the equation, "Power = Toque x Revs ", you can work out the power.

If you do that across the engine's entire rev-range, at say 250rpm intervals, you can plot a 'Dyno-Trace', which would look a bit like this:->

Basically the blue line, 'torque' is the load you have measured, and you calculate the pink like, 'power' from it, and plot it on the same graph against engine revs

Having done that, you can then look at your traces and find the highest values you recorded, and the engine revs they occurred, and those are the values usually 'quoted' for an engine. For this example, the 'quoted' specs would be something like:-

Power = 100BHP @ 5,ooo rpm, Torque = 115ft-lb @ 4,ooo rpm

So, first of all, 'Quoted' power isn't necessarily all that accurate, the numbers 'quoted' depending on how, where and when it was measured; with what equipment and what 'correction' factors were applied to get from the numbers actually recorded, to those actually 'quoted'.

Next up, the figures 'quoted', however 'reliable', are only the 'high-lights', not the full story. They tend only tell you the biggest numbers that some-one could get an engine to produce. It's the 'Dyno-Trace ' that gives the full story, how much power an engine might make across it's entire operating range of engine speeds that does that.

'Useable' Power

This is where the science starts to loose it's relevance, and some of the old 'lore' gain a bit of credence, and particularly the comments, 'it's not how much power you got, it's how much you can use', and ' it aint what y'got, but how y'use it'.

Bottom line is, engines make cars or motorbikes move, and what we are usually bothered about is NOT the numbers they can achieve on a test bed, but what they can do, in the metal, in the 'installation', in the real world.









Mentioned the . Apply a bit of Algebra to that, and what you get is another equation, "Power = Engine Displacement x Cylinder Pressure x Crank Speed". Basically, the equation has 'substituted' the Engine Displacement and Cylinder Pressure for 'Torque'; because if you look at Torque, that's what makes it! But, I'll look at that in more detail a bit later.

The engine displacement, or capacity, tends not to vary, being fixed by the size of the holes in the engine block the piston goes up and down, and how far up and down inside them they go, which is controlled by the length of the crank-shaft 'throw'. Only things that can vary are the engine speed, or 'revs' and the cylinder pressure.









So, 'Torque' is an expression of cylinder pressure and engine displacement. Engine displacement tends to be fixed by the physical size of the holes in the engine block, and the length of the crank-shaft 'throws', so in operation shouldn't change, which means that any change in 'Torque' is entirely due to change in cylinder pressure, hence, Peak-Torque, will be co-incident with 'peak' cylinder pressure.

Getting simplistic, you are going to get the most cylinder pressure, when you get the biggest 'bang' in it, so it will be when the engine has a full charge of air, which means when you DON'T artificially 'choke' the supply of air into t he engine by using part throttle. Peak-Torque and Max-Power, then are BOTH only achieved when the throttle is fully open.







Next, Engine's have a displacement, and that is usually provided, and some-times they will quote the 'bore' & 'stroke' dimensions, and the number of cylinders the engine has, from which it can be calculated.










Power Charts






Parable of Rubble!

Imagine three pallets of bricks in a builders yard, a lorry for them to be loaded onto, and three blokes to do the loading. Each bloke is asked to load one pallet, so they each have the same amount of work to do. Remember, work is a load shifted a distance.

First bloke is a hefty hoddy, and he stacks 30 bricks at a time onto his hod and walks them to the lorry, up a short ladder onto the lorry's bed, and stacks them on another pallet. 2000 bricks, moved thirty at a time, he takes 67 trips to load them.

Second bloke is the yards 'lad'; last time they had some bricks to load, blokes stuck a hod on his shoulder, and took bets on how many they'd be able to put on it before his knees buckled, he managed eighteen! So, faced with this pallet, he grabs six bricks at a time, and runs between his pallet and the lorry to keep up with Hefty hoddy!

Third bloke is they yard Joker, and a bit canny. He gets a portable scaffold, and a block and tackle; straps his entire pallet, and then, very slowly, lifts it off the floor with the lifting block, then just as slowly, pushes the scaffold and the pallet to the back of the lorry, then lifts the pallet some more with the hoist, until the pallet is level with the lorry's bed, then shoves the scaffold over and lowers the pallet to where it's wanted.

Now Hoddy, did a good job, and slow and steady, he took one and a half hours to shift all the bricks. The lad, started strongly, and with his light load, he was running to and from the lorry, doing two or three trips to Hoddy's one, trouble was, carrying 1/5th the number of bricks he had to do five times the number of trips, so he never really got ahead, and after an hour was flagging, and after Hoddy had finished his pallet and gone had a cuppa, he still had about 1/4 of a pallet left, so Hoddy actually had to help him finish off.

Joker? Well, Joker actually took the longest. It took him 25 minutes to get his scaffold set up, and when he came to try and push it with the bricks swinging beneath, it almost wouldn't move, and when one of the casters encountered some spilled pea-gravel, it didn't! Took him best part of an hour to bunt and shove the 'rig' to the lorry, and a good fifteen minutes swinging on the end of his tackle rope!

But, if you take out his 'setting up' time, and if he'd swept his path before he started though, he might just have pipped hoddy to the post , but in fact, he took almost as long as the 'lad', though he did do it on his own.

RIGHT, three blokes given the same job, all tackling it in slightly different ways. First two use shear man-power, second uses some 'mechanical advantage', but they ALL did the job using their own muscle-power. Who is the 'strongest', who is the 'fastest' and who us the most 'powerful'?

Lessons of the Load

There's a LOT of things to be learned from this example, but the first one is that it doesn't matter HOW the job gets done, only that it does.

Of our three workers, the Hoddy was certainly the 'Strongest' and probably the most 'powerful', in strict scientific terms, BUT he was also the most suited to the task, shifting bricks was his vocation; he had the physique and technique for the job. Which is probably THE most important lesson about power.

Practically, getting a job done, isn't about strength alone, nor about speed, but having the 'right' combination of either to get the job done most expediently! But power is an arbitrary index, a 'function' derived of strength and speed, BUT, depending on the job to be done, one might be more important than the other, AND there may be a few more considerations besides.

In the example, the 'lad' obviously was the least 'powerful', though he was fast, and he could have done the same amount of work, but in smaller bites, over a longer period of time. And where Hoddy probably earned £60 a day, the lad was probably paid £60 a week,  economically, it might have taken him three times as long to do a job, BUT, it would have cost the boss just over half as much to pay the lad to do it than Hoddy!

The Joker, though shows the principle of 'mechanical advantage'. Joker wasn't as powerful as Hoddy, though he did exert more strength in the task than Hoddy did, and, excluding the time he wasted setting up his rig, and messing about shifting pebbles from his wheels, he DID do the job fastest, so demonstrate the greater 'power to do the job'. Which is important to note.

In a direct weight lifting contes, he WOULDN'T have been able to lift as much as the Hoddy, who is certainly the 'stronger'; and he certainly wouldn't have been able to keep up that level of exertion all day, as Hoddy would.  What Joker did was use some mechanical aids to improve his 'efficiency' and direct what 'power' he did have to most advantage.

Hoddy, strictly the most powerful of the three, only lifted thirty bricks at a time, where he had the capability to lift perhaps twice that many in one go; he DIDN'T push himself to the limit on the strength he exerted. Nor did he rush about with them, pacing himself to the task, where Joker, had to put ALL his strength into the hoist rope to get the bricks off the ground in one go, and into trying to make the rig move on it's casters.

Which begs a question, what would have been the outcome if Hoddy had been given the lifting rig, and Joker the hodd? Joker, could lift about 40 bricks in one go, to Hoddy's 70, but Hoddy only lifted 30 at a go, to be able to balance them as he walked and pace himself over the task. If we say that Joker did the same, and carried perhaps 25 bricks at a time, he'd have had to do 80 trips to Hoddy's 67, and if he was that bit 'faster' carrying the lighter load, might have been able to beat Hoddy's time.

Hoddy with the lifting rig, on the other hand, would have been another matter, and where Joker struggled to get the bricks into the air even with the hoist, Hoddy, able to exert nearly twice the force wouldn't have struggled anywhere near as much, either lifting the bricks or making the rig move, so he'd probably have shifted the bricks even faster than Joker did.

So, our Hod carrier is the most powerful of our three contenders when it comes to shifting bricks, whether he carries them a hod full at a time, or whether he has a lifting rig.

But, the 'lad', no-where near as powerful or capable could do the same job, even though it may take longer, and there may be some advantage to that, like him doing it cheaper.

Likewise, the 'Joker' was also capable of doing the same job, and depending on circumstances, doing it just as fast, or faster, though he was working a lot closer to the limitations of his ability.

Point is, there is 'strength' and there is 'speed', and the two together are what we call power. Though strength and speed are still important, and MORE important than all three, is getting the job done! And in 'getting the job done' PRACTICALLY 'power' IS only half the story. The other half is 'What's the Job?'!

In the example, we just had one pallet of bricks that needed loading, and any of our candidates could have done it. So who was 'best' for the job depends on what else was important.

If it was just a matter of loading the lorry as cheaply as possible, then letting the 'lad' take all day to do it, would make him the 'Best' person to do the job.

If it was an urgent task, then getting them loaded as quickly as possible, would mean that the Joker, would probably be the 'best' man for the task.

The 'Hoddy', the person most suited to the task, would only be 'best' for the job, really, if there was more than one lorry to be loaded, and it was a matter of loading lorry after lorry after lorry, all day long.





Nature of the Task

OK, so power is strength and speed. time to translate the ideas to engines. We could have an engine that can deliver a lot of pulling 'force', but doesn't turn very fast, or we could have an engine that turns to heady revs, but doesn't make much force, or we could have something in between, all of which could have the same 'power rating'.




Which begs a definition of 'Work' really, but the FIRST thing to note is that by the definition, Power is an 'ABILITY'. That takes a little to wrap ones head round, but bear with me. Power is an 'imaginary' concept, it is NOT something you can see, touch, taste, hear or smell...(No, what you are smelling is the vapour of hi-octane petrol! That's an aromatic ester, NOT power..... nice though!)

So, it's an ability, a skill or aptitude, like, I don't know, gymnastics. Look at a gymnast, and you can't SEE their skill. Their leotard might give you a clue that they are a gymnast, but like my Gran always said, never judge a sausage by it's skin! I could put on a lion cloth, wouldn't get me to the Olympics though!

You might be able to observe their physique, and infer that it might give them some gymnastic ability, but it would still only be an assumption. You ONLY get a measure of a gymnasts ABILITY when they use it, and then you don't see that 'ability' you see a back-flip, or summersault or what-ever, you see the act, NOT the ability.

Same with power. We NEVER see the raw 'ability', only the outcome, the effect, what that power CAN do, from the WORK it does, and like scoring a gymnast, we measure that, to get some idea of the level of ability.

Engines don't 'Make' power, and you certainly don't 'feel' power; engines make FORCE and you feel the EFFECT of that force. To say that an engine 'Makes' 150bhp, would be like saying that a cup 'makes' 300cc of tea!

An awful lot of people are ignorant of that, or misinformed, or simply follow the common usage, the 'Jargon'. When they talk about power, and say things like 'you can really FEEL the power'. Their engines make SOMETHING, and they feel 'Something', and that 'something'  they erroneously call 'Power'.

Fact of the matter is, that the SOMETHING,  people usually feel, is actually THRUST, which is actually a force, that I'll explain a bit later, because engines DO make force, but THRUST is only the useful part of the force they make.


OK, well having leapt ahead, we need to look at Work. Power is the 'ability' to do work, so what's 'WORK'?

Work is a Load moved over a Distance

Which is still a little ambiguous, BUT, with a little explanation, somewhat more helpful.

A Load, is a force, which is something that we CAN feel and measure, as is distance. 'Real' or 'tangible' quantities, that we can start to get to grips with.

So, if Power is the ability to do work, and work is a load moved over a distance, we can say that power is how hard, or how fast we can shift a load....... Time for some maths!

Basic Power Equations

OK, starting with the definition of WORK, if Work, is a Force moved over a distance, we can put that into algebra, and make a sum out of it, and say:-

Work = Force x Distance

Now, if we do the same with the definition of POWER, we can put that into a sum, and say:-

Power = Work / Time

And if we do a little bit of Algebra on those two sums, and substitute 'Work' for it's equivalent in Power, we get:-

Power = Force x Distance / Time

Which is quite intriguing, BUT,

Speed = Distance / Time

So, we can do a bit more substitution, and distil power a bit further to:-

Power = Force x Speed

Which is probably the most useful definition of the stuff, and starts to explain a lot of what people might be blathering about, and I'll come back to that in a bit.

Thing is, we can do an awful lot of algebra and messing about with equations to get ever more convoluted 'derivatives' if definitions that may or may not be helpful to us, depending on what concerns us. For the moment, though, I want to leave the matter here.

And, having made much mention of the REAL thing of importance, Thrust, and introduced Forces, having mentioned that Thrust is a force, I want to go look at that those for a bit.

Force & Thrust

Chap called Newton devised a whole set of 'laws' about forces a few hundred years ago, that are very useful, of which a couple are of interest to us right now.

Forces are what make things move or hold them still.


Every Action has an Equal & Opposite Re-action

Scrappy little sketch of a Series Landy for you, with four arrows, representing forces, marked on it.

Thrust, is a curious one, because it has two arrows, one pointing forwards at the rear wheel hub, and another pointing backwards at the tyres contact patch.

This is a 'couple' or a pair of forces, of equal magnitude, in opposite directions, which is what we get as the result of, or which cause a 'torque', that I'll explain in more detail later!

However, working against the 'Thrust' forces are two 'reactions', one of which is 'Traction', an important topic dealt with in more detail in Get A Grip. , the other 'Drag'.

Keeping it all simple, THRUST is the useful force we get out of our engine, delivered to where it is used, at the driven wheels, and what we have available to push our vehicle along.

Traction, is it's accomplice, and very simplistically, it's the grip we have at the driven wheels, so that the thrust can push against the 'Drag' and not simply spin the wheel against the surface it's on! So, provided we have more traction than we have thrust, we can usually ignore it, and just worry about Thrust and Drag.

Drag, basically the overall resistive force trying to stop our vehicle moving. Usually mentioned most often in relation to aerodynamics or streamlining, because 'wind resistance' often makes up a big chunk of it, but not all. Drag is EVERY force trying to impede motion, and quite a lot of that comes from the tyres, and what is known as 'rolling resistance', but I'm getting ahead of myself again!

So, if we take Newton's 'Laws' we have a force, Thrust, and we have a 'Reaction' to it, 'Drag'. Those forces are acting on our vehicle, so they can either make the vehicle move, or they can hold it still, but I need to introduce another equation here, and that is:-

Force = Mass x Acceleration

Which is another of Newton's laws, and a very useful one, and the one that gets so many people concerned with power to weight ratio's. Basically, heavier something is, the more force you have to exert to make it accelerate.

Right, if the thrust force and the drag force are equal, then the vehicle will not accelerate; it will move at a steady speed, because the forces are in balance and cancel each other out.

To accelerate, you need more trust than you have drag, and THEN you will start to accelerate; but only by as much as the thrust force exceeds the drag force.

All fairly simple, BUT what we have to have a look at now, is DRAG. Mentioned that drag is the overall resistance working against our vehicle's movement, but a lot of debate on the topic centres around aerodynamics, because that is often blamed for being almost the sole source of drag.

It's not. It can be a very large part of it, but it's not the whole story. But, because it's what every one blathers about, I'm going to look at it first.

Aerodynamics in a nut-shell

Aerodynamics is the study of the resistance different shapes offer against  the flow of a fluid passing around it, or the shape passing through it. The idea at the centre of the subject is that a 'tear-drop' shape offers an awful lot less resistance to flow than does a brick-shape. and a lot of people went in search of the 'perfect' streamline, which is a sort of elongated teardrop with a sharper nose and longer tail.

Fascinating subject, and explains why aeroplanes can get off the ground, spin bowlers can make a cricket ball curve in the air, and how a golf ball can be made to actually 'fly'...... BUT, I'm not going to get into all that, I'm going to try and stick with the basics.

Scrappy sketches again;  this one is a flow diagram for a ball. Pretty simple, perfect sphere, as blunt at it's nose as it is it's tail.

As it moves through the air, the air has to move aside to let it through.

The air shoved aside by the very centre has furthest to 'detour' around the shape, so compresses that striking closer to the edges, and it all has to find a way around, so the area of 'disruption' is greater then the frontal area of the ball..

Bit like kicking a football at a garage door, air hitting the front of our object has mass and momentum, and so exerts a force, but there's a bit more too it, and the air pressing on that air, exerts a force too, and cumulatively THAT is what makes wind resistance.

Thing is though, while its applying a 'pushing' force on the front where it strikes, as it goes round the back, it has to straighten itself out again, and you get a 'slip-stream', a 'dead' region, where the air coming round doesn't want to go, that causes a partial vacuum, which applies a 'suck' on the back end, as well.

OK, so, as far as wind resistance goes, we have a body of air, being muscled aside by our vehicle. Bigger that vehicle, so the more air it's going to have to muscle around it, so the more wind resistance you are going to get. Next, the faster our vehicle is going, the more quickly it's going to have to muscle that air aside, or the more air it's going to have to shift in the same time, so again, faster we go, more wind resistance we are going to get.

Putting that into maths, we can say that wind resistance = frontal area x speed x something. Where the 'something', is all the possible factors we don't know are lurking!

In there, somewhere will be something do do with the shape, the co-Efficient of Drag, and possibly a few other things like the density of the air, which is where aerodynamics starts to get rather 'woolly', and use lots of factors called 'dimensionless groups', because the number of variables in that 'something' is huge! But, to keep it simple, I'm going to ignore all of them, apart from the 'shape' of our vehicle.

At the front, we have positive pressure building up on the 'nose'. At the back, we have that 'suck' in the slip stream, and in the middle we have a bit of a hiatus as the air goes from one zone to the other. Time for another scrappy sketch, this time, air flowing over a barn door.

It looks a bit confusing, but we have the same thing going on, positive pressure at the front, and slip stream 'suck' at the back. Only differences are that at the front we have this area of 'stand off', and around the edges an area of turbulence.

Because the front is flat, there is no angle to direct the air around the edges, so it will tend to hit it square, and bounce straight back into the air behind!

The air behind, tries to push it back again, though, so at some point ahead of the barn door, there's a boundary where the air going one way meets the air going the other, and they both go sideways.

Ahead of that point you have a 'separating' air flow, like we did around the ball.

Behind that point, there is a zone of 'turbulence' or air swirling about trying to find a way out, and which spills past the edges and into the slip stream.

OK, so streamlining then. First of all, we have that 'bluntness', obviously, the sharper the front, the less air will be bounced straight back, and more easily 'spilled' round the edges, and the more easily it's spilled around the shape, so the less air will be disrupted, so the less wind resistance.

Mean while there's that slip stream at the back, and the very sharp corner the air has to go round gives us a big slip stream, so the more suck, so the more wind resistance.

TWO things about that, first is that IF the barn door was thicker, then the air wouldn't have to make SUCH a sudden turn going from the front to the back, so without ANY 'stream-lining', it would give a bit less wind resistance.

Next, if we gave the shape a nice smooth tail, to actually help the air make that turn, we'd get less turbulence, and less slip-stream, and so less suck.

And the 'Co-Efficient of Drag', or cD, takes these three factors into account, and gives an index of 'aerodynamic efficiency, based on the bluntness of the front, the smoothness of the tail, and the length of the overall shape.

However, what it doesn't take into account, is another occurrence of speed. We've used speed once, because the faster we go, the faster we have to muscle air aside, but, when we look at the shape, we find that the faster we go, the faster the air has to make those changes of direction, and that leads to another place where we have to multiply by speed again.

And what we end up with, is a sum that says, Wind Resistance = Frontal Area x Co-efficient of Drag x Speed x Speed x something

But by now, as long as we keep everything else constant, the 'something' shouldn't matter, and what we are left with is:-

Wind Resistance = Frontal Area x Speed2 x cD % x Constant

OK, well to dispel some myths at this point, about streamlining and cD factors. First of all, back in the early 80's, Ford tried defending the early Sierra's 'jelly-mould' shape by pointing out it was 'aerodynamic'. Audi, responded by claiming loudly in their advertising, that their more conventional sharp cornered Audi 80, had the class leading co-efficient of drag.

It probably did, BUT, the cD is pretty irrelevant, and it doesn't tell you which car will be subject to the least drag. That depends mainly on the frontal area. But also, the cD can be either 'calculated' from looking at the basic shapes, and working out the ratio's of the nose sharpness and tail smoothness in relation to the overall length, OR it can be 'equated' from testing a real car in a wind tunnel and actually measuring the drag it generates against a reference shape.

A 'theoretically' aerodynamic shape, like a truck, with a barn door front and back, CAN actually give a very poor 'calculated' cD BUT a very good 'equated' cD in the wind-tunnel. Conversely, a tear-drop 'stream-liner' can prove the contrary, it all depends.

Explanations of the anomalies were suggested by a Professor Kamm, I believe in the late 1930's, gaining wider recognition in the 1950's & 60's. He looked very closely at them, and pointed out, that around the 'stand-off' of a 'barn-door' shape, you got a separation of the air-flow ahead of the shape, that followed almost a natural stream-line.

He also found a similar thing at the back, where the air flowing around the slip-stream, without a long tear-drop tail, naturally followed the 'stream-line', and the benefit of 'tear-drop' stream-lining was at best, marginal. More important was controlling or minimising turbulence, which lead him into the science of 'spoilers', which technically are 'flow correction devices'.

Intriguingly, what we tend, on cars, to call spoilers, are often believed to produce 'down-force'. If they DID, that would make them an 'inverted aerofoil', not a spoiler........ most don't, so we refer to them correctly, for the wrong reason! But there you go!

Point is, that aerodynamics is an imprecise science, and 'quoted' cD figures can be pretty meaningless, and certainly shouldn't be used as any indication of which vehicle might be subject to more or less drag. (certainly without a LOT of qualification, and a more detailed understanding of what that qualification means!)

The actual shape of a vehicle, and how stream-lined it is, DOES effect how much drag it's going to be subject to, but again, looks can be deceiving, and 'Kamm' effects can show shapes to perform contrary to how their 'aesthetics' might suggest. So.


Right, going back,

Drag is the OVERALL resistance to motion

Of which, wind-resistance is often a fairly hefty chunk, but we've discovered that wind resistance is directly proportional to frontal area, exponentially proportional to speed, and influenced to some degree by stream-lining.

OK, walking about, wind resistance tends NOT to bother us! 3 or 4mph, we hardly feel ANY wind resistance. Get on a push bike, do maybe 8 - 10mph, and the wind flowing round us feels like a gentle breeze; it's noticeable, but not really significant. Get on a moped, 30mph, and you might just about start to appreciate a bit of wind resistance.

On 'Dawg', my un-faired 750 Honda, and, 40mph? Definitely beginning to get a little bit of a tug from the wind. 50-60ish, and it's definitely is. 70+ and you can really feel the wind resistance, and on a bike might be looking to get your head down behind a fairing out of the blast, because it IS starting to pull that hard. Much over 100mph, and wind resistance is starting to become pretty hefty, and the exponential is starting to get pretty steep.

I bunged some numbers into a spreadsheet, and drew a graph. It shows the drag force you get for a certain speed, from a vehicle sort of roughly Land-rover sized and shaped. It's very much a 'rough-reckoner', but it's not unrealistic, and the numbers were based on 'real' data taken from a selection of Land-Rovers.

Basically, up to about 30mph, wind resistance is on the 'flat' of the ramp, it doesn't increase very much with speed, and the major part of the drag you get is from rolling resistance, and it's all pretty 'linear'. After 30mph, the wind resistance starts to become the larger part of drag, but, it's still not 'ramping' too quickly, up to about 60mph-ish.

After 60mph though, the multiplier of speed x speed starts to kick in with a vengeance, an the wind resistance starts to increase quite dramatically to around 110-120mph, and beyond that, the drag is increasing so much for so little extra speed, it's getting very much more difficult to go much faster.

Power & Speed

OK, this is where it's starting to get interesting. We know what power is, and we know that it is power that gives us thrust. We know what thrust is, it's the force that shoves us along. We know also, that Thrust is used to over-come drag, and we now know what drag is, and how it increases with speed.

Leaping ahead again, and with the numbers in my spread-sheet, I turned the 'drag vs speed' graph, into a 'power vs speed' graph. again, its for a Land-rover sized / shaped vehicle, a bit rough and ready, but based on 'real' examples.

It's not that accurate, but it gives you an idea. A Land-rover 4-cylinder engine, with around 50-60bhp, will get a Land-rover up to around 60mph or so. Double the power, though, fit a 100-120bhp V8, and it wont make your Landy go twice as fast, just half as fast again, around 90mph-ish.

It's a law of diminishing returns. More power lets you go faster, but the faster you go, so you need more and more power, to go just that little bit faster. And I mention this now, because it is an immutable law:

Power = Speed









'Power', at least as far as people so frequently use the word, simply doesn't exist. It is a convenient imaginary commodity, or index we can use in engineering to help us understand things doing work, we talk of it as 'real' because in maths it IS real; we can give it a value move it around and use it to work out loads of interesting or useful stuff.

But in the real, physical world, of rusty nuts and bolts, skinned knuckles, and puddles of EP90..... at no point is there any 'Power' to be seen, touched, tasted or smelled! (No, what you are smelling is the vapour of hi-octane petrol! That's an aromatic ester, NOT power..... nice though!)

It is simply an index or rating of potential; How much effect you might get from something, by way of forces, distances & time. Power, exists in one place, and one place only, and THAT is in the imaginary world of maths.

Recognise THAT, and you are a long way to better understanding the subject, what people are actually talking about, what they MIGHT be talking about, and WHICH of them might actually be trying to tell you something useful. And I am now going to try and tell you something 'useful' about Power......








THIS article, is going to look at some of the myths, legend and lore of internal combustion derived motion..... (engines shoving cars or motorbikes along roads or tracks!), and try and sift the wheat from the chaff as far as which bits of common wisdom have some seeds of scientific truth or basis and which are, well, pretty much rubbish.

What I want to do REALLY with this article is to get you to rethink your ideas of automotive 'power', and to consider not what 'every-one' says about things like torque and revs and cylinders and stuff, but look at the science of what an engine is doing, and be able to work it out better for yourself. (Footnote 1)

And my starting point is to make three pretty bold statements;

Power does NOT exist

Well, THAT'S a curious place to start! Writing an article about 'Power' and just about the FIRST thing I say is, 'it doesn't exist'! Like THAT makes any sense?!? Yeah, I know, but bear with me. It WILL become clear.......... maybe.

Basically, 'Power' is an abstract commodity, an idea, NOT something tangible, and in THAT sense, the statement is entirely valid. Power, certainly power as most people talk about it, simply doesn't actually exist.

It doesn't Matter HOW and engine does what it does, only that it DOES IT!

The engine is a pretty complicated widget, but at the end of the day, what counts is that it shoves your vehicle along. A lot of the myths, legend and lore about engines stem from observation or experience of how vehicles behave, and people then trying to attribute that behaviour or 'character' to facets of the engine.

Thing is, that, there is so much going on, and when you are experiencing or observing the 'engine' second hand, from how it makes a vehicle behave, there is a lot of room for confusion or error.' Which leads me to my third statement:

The Engine Does Not live in isolation, it MUST work not JUST as an engine, but as a part of the VEHICLE it propels

Which is a sentiment I have expressed, I'm sure in plenty of other articles on the site, and SHOULD again seem simple common sense; but over the years I have been astounded at how many people, and so often not just 'enthusiasts' but knowledgeable ones, have lost sight of that, and become obsessed with the parts attached to or in their engine, and or the numbers they have got on rolling road runs, that they have almost ignored how the VEHICLE performs doing what it was intended! (See: You can do ANYTHING to a Landy! )

Power the Imaginary Commodity

Power has a strict definition in international standards, it is, the 'Rate of Work'. Work, similarly has a standard definition, it's the net energy transfer of a process. Sounds tedious, but it'll make more sense, translated into maths:-

Power = Work / Time

Work = Force x Distance

Now, with a bit of head scratching and a lot of algebra, you can take these equations and a few other basic ones, and derive a whole shed load of new ones. And one such important derivative is this:-

Power = Force x Speed

Running with these ideas; power, is a ratio, a rate. It isn't a real commodity it is strictly an index of 'potential', which is an imaginary one.

The 'Potential', is 'Energy', which can take many many forms, but in this situation is usually 'work', itself a ratio; the REAL commodities, the things we can see, feel, or measure are Force, Distance and Time.

So, engines don't 'Make' power, and you certainly don't 'feel' power; engines make FORCE and you feel the EFFECT of that force.

To say that an engine 'Makes' 150bhp, would be like saying that a cup 'makes' 300cc of tea!

An awful lot of people are ignorant of that, or misinformed, or simply follow the common usage, the 'Jargon'. When they talk about power, and say things like 'you can really FEEL the power'. Their engines make SOMETHING, and they feel 'Something', and that 'something'  they erroneously call 'Power'.

'Power', at least as far as people so frequently use the word, simply doesn't exist. It is a convenient imaginary commodity, or index we can use in engineering to help us understand things doing work, we talk of it as 'real' because in maths it IS real; we can give it a value move it around and use it to work out loads of interesting or useful stuff.

But in the real, physical world, of rusty nuts and bolts, skinned knuckles, and puddles of EP90..... at no point is there any 'Power' to be seen, touched, tasted or smelled! (No, what you are smelling is the vapour of hi-octane petrol! That's an aromatic ester, NOT power..... nice though!)

It is simply an index or rating of potential; How much effect you might get from something, by way of forces, distances & time. Power, exists in one place, and one place only, and THAT is in the imaginary world of maths.

Recognise THAT, and you are a long way to better understanding the subject, what people are actually talking about, what they MIGHT be talking about, and WHICH of them might actually be trying to tell you something useful. And I am now going to try and tell you something 'useful' about Power......

The Balance of Power

Right, moving on a little, I want to look at forces, and a real vehicle 'on the move'.  Doesn't matter, could be any wheeled vehicle from a mining locomotive to a quarry dumper truck; as far as we are concerned, its a box on wheels, with an engine in it somewhere turning some of them! But because I have data to hand, I'm going to use the example of a Series III Land Rover.

Power = Force x Speed, and Work = Force x Distance. These are important, but only after we've found out what forces we are playing with, and to do that I need to introduce a few more sums, sorry!

Force = Mass x Acceleration

Torque = Force x Radius

First is one of Newton's immutables, while the definition of Torque, is something that is VERY important to the topic, and could do with a closer look, but I'll come back to it. This last equation though may take some to wrap your head round; but look at it; it's basically, Power = Force x Speed, only applied to something turning rather than going in a straight line.

Power = Torque x Revs

Back to our SIII, we know a few things about it, I got them out of the book, but they were unfortunately in English, not science. In Science we use what is known as the SI system of 'units', which is basically 'Metric', so we have to convert the inches and Tons and stuff to Kg and mm. (footnote 2) So:-

Scrappy Sketch time;  I've put on a few arrows, and inclusion of 'Drag' and 'Thrust' kind of imply that it's not stationary; it is, those values are just 'zero', but we might as well get them in there straight away.

You'll also have noticed that the Thrust force, or motive force, is a bit weird; two arrows, one pointing forwards at the rear axle centre, the other pointing backwards at the tyres contact patch with the road....

Yes; that's the way it works, its called a 'couple' and is the force you get from a torque.

Intriguing thing to note about couples, is that divorce is so much cheaper.... SORRY! that the two opposite facing forces are always the same size!

Every Action, Has an Equal & Opposite,  Reaction

One of those often quoted 'sayings', its actually another one of Newton's Laws. But, you'll have noticed that pointing against the bottom arrow of the Thrust couple is another arrow I have marked 'Traction'.

This is VERY important to note, it is another topic in it's own right, and subject of another article on here; Get A Grip.

In short, it is, as I have shown, the resistive force, or 'reaction', that anchors the Thrust couple. But it's not the only 'reaction' to the Thrust couple, there's another one, and that is 'Drag', which isn't shown head to head with the motive force arrow, its shown acting somewhere on our Landy's windscreen.

This is because that is where the force TENDS to be centred. It's actually spread about, like weight, but it gets complicated, so we work out the effective 'centre' of action and put it there. And it tends to be about the bottom of the windscreen on most vehicles.

I've shown the Weight (footnote 3) and Support forces, weight acts down through the Centre of gravity, which on a Series Shorty is somewhere approximately where I have shown it, about in the middle, and a tad beneath the waist line. Quite low actually for an off-road vehicle. A bit approximate, but close enough, and we can presume that the weight distribution is equal on front and rear axles.

The Weight then, is reacted by the support forces, one on each wheel, half that of the overall weight acting through the CofG. As far as our 'Power Balance' is concerned, the weight isn't important, but it gives an example of a simple 'Force Balence'.

Distilling some Newtonian Fizzix, Forces are the things that make things move or hold them still. If opposing forces are equal, then we have a force balance and, by the Force = Mass x Acceleration rule, they wont accelerate, which means they wont change speed, which means that IF they are stationary, they stay stationary, and if they are moving, they keep moving.

Landy in the sketch is sat on a level surface, it has a weight acting down, and some support force acting up. Landy isn't sinking into the ground, so the weight isn't overcoming the support. It isn't floating off into the clouds either though, so the support isn't over coming the weight, so logic says its staying still (at least vertically), and hence the forces must be in balance.

Right, apply that same dint of logic to the motive forces and we have, a thrust couple, resisted by traction and drag. I said the thing is at rest, so its NOT moving, so we can presume that the forces are in balance, and because we know its not moving, add that the magnitude of those forces is Zero.

OK, so lets get this thing moving! Engage drive, release clutch, bit of throttle and we are away. The engine is now giving us some Thrust, a force, so the Landy is going to start moving, in fact, by Force = Mass x Acceleration, it will accelerate, at some rate proportional to it's mass.. And, if this science stuff works, it will keep accelerating until some sort of force balance is reached between the Drag and the Thrust; when, it will stop accelerating and keep going at constant speed.

POWER = Force x Speed

From our data sheet; we have a finite power, which in SI units is 60KW, delivered at the engine's crankshaft at 4,250rpm. We can now do some sums and work out how fast our wheel will turn at that engine speed.

Overall Gear Reduction: is 5.4:1 reduction, or we need 5.4 turns of the crank to get one turn of the wheel; apply a bit of maths and that means that at 4,250rpm engine speed, our wheel will be turning at, 790 rpm.

Our Wheel, had a diameter of 0.75m; circumference of wheel is Pi (3.142) x Diameter, so, each turn of the wheel will move the vehicle, 2.4m.

If we multiply the circumference by the wheel speed, we then get, 1,900m/minute. Or, 113Km/h. That is, 70mph! Which is pretty close to  the 'Real World' top speed of a well fettled Series III.

But what we have NOT taken into consideration is those forces; or 'Power'. So, lets have a look at them; Power = Torque x Revs or Power = Force x Speed.

We have a Thrust at the rear wheel, and we have a Drag force on the windscreen. When the vehicle is travelling at constant speed, they MUST be equal, or it wouldn't be travelling at constant speed, it would be accelerating or slowing down.

A new Rule for you, derived from another bit of Fizzics that says energy cant be created or destroyed, only converted in form So;

Energy IN = Energy OUT

It's not STRICTLY true; depends on what we are looking at; there could be some energy storage involved, but for our purposes there isn't so it's good enough and keeps things simple. Work is energy, and Power is rate of Work, SO, with a bit of algebra, I can derive that:-

Power In = Power Out

Our 'Power In' is coming from the engine; and the data sheet says that it's 60Kw, delivered at a crank speed of 445 rads/s, and if we put those numbers into the equation Power = Torque x Revs, we get, 60Kw = 134Nm x 445 rads/s

That power is then shoved through the 'transmission', which has an overall reduction ratio of 5.4:1; and we worked out before, when the crank turns at 4250rpm, the rear wheel turns at 790rpm, which converted to SI is 83 rads/s

Power = Torque x Revs; energy cant be created or destroyed, remember, so Power will still be 60Kw; but the transmission has reduced the revs, so to keep the accountants happy, must have increased the torque; so doing the sums; 60Kw = 723Nm x 83 rads/s.

(If you look at those two sums, you may notice that the torque has been multiplied by a factor of 5.4, the speed reduced by a factor of 5.4, the overall reduction ratio of the transmission;)

Right, we now have the torque on the rear wheel, which is 0.75m in diameter. Torque = Force x Radius; so the Thrust = Axle Torque / Radius of Wheel, which works out at:- 2000N or 2Kn. Thrust MUST equal Drag force, SO, we can say that our Drag Force, is also 2Kn.

But, we can work it the other way around too; Power = Force x Speed. Our Landy is moving at constant speed, we worked out, 113Km/h; our engine is offering 60Kw of power, and Power In = Power Out. So, again correcting Km/h to m/s, we can drop some numbers into Power = Force x Speed. and get 60Kw = 2Kn x 31m/s

Either which way, we get the same thing, the Drag Force acting on the vehicle at that speed!

So, wrapping this bit up; the Balance of Power, is the four way balance between The Torque and Crank speed of Power IN from the engine, against the Drag Force and Road Speed of the vehicle in action.

Power In


Power Out

Crank Shaft Speed x Engine Torque = Road Speed x Drag Force

What's usually important to us is speed; how fast we can go; and the limit to that is basically provided by how much power we have to play with; more power we have, the more force we can get to the driven wheels, the more drag we can over come.

Back to my bold comments in the introduction; "It doesn't Matter HOW and engine does what it does, only that it DOES IT!";

Power = Torque x Revs, so it doesn't really matter whether the power the engine can provide is made as a small torque and lots of crank revolutions, or as a big torque with very few crank revolutions; what matters is the torque and speed of the driven wheels... and even THEN, actually what matters is the thrust force and the speed of the vehicle.

Again; "The Engine Does Not live in isolation, it MUST work not JUST as an engine, but as a part of the VEHICLE it propels", and that vehicle will have a transmission, that contains gears, which can change the speeds of the shafts and multiply the force by reducing the speed or vica verca.

Which is important to note, bearing in mind my comments about 'power delivery' and 'engine character', because we have just identified ONE pretty significant variable that can quite dramatically alter the power delivery from the engine, and make the vehicle behave quite differently to the way may be implied by power traces or figures.

But the bottom line is, you can ONLY go as fast as you have enough power to give you the THRUST to over come DRAG.

Lot of 'lore' concerns gearing, and an awful lot of people believe that raising the gearing will make their cars or motorbikes faster; it might, but you can gear up a vehicle as much as you like; if you don't have the power to pull that gearing, it aint going to go that fast!

What a Drag!

Right, Drag crops up a lot in myth, legend and lore, and things like wind resistance, streamlining, and the 'co-efficient ' of Drag, gets a lot of mention and debate; so, the first thing to do is to define what drag actually is.

Look at the picture we used in 'The Balance of Power'; it is a SINGLE force, that specifically resists thrust.

I mentioned that I had shown it on the picture pointing at the windscreen, and said that that was because it was sort of spread about a bit, but we simplified matters by gathering all the 'bits' of drag together and considered them as a single force acting through a centre.


Drag is the sum total of ALL forces resisting motion

Now there can be a lot more to this, BUT, normally, at least for motor-vehicles, two forces make up the most part of 'Drag'

'Rolling Resistance' + 'Wind Resistance' = Drag

Looking at Rolling Resistance first then, as the name suggests, its the resistance offered against the wheels turning. The smoother the road surface, the narrower they tyres and the 'harder' or better pumped up they are, the less rolling resistance you'll tend to get.

Trying not to get TOO bogged down, remember, Energy = Work, and Work = Force x Distance.  After a long drive, you might have noticed that your tyres will be quite warm; certainly if you have come to change a flat, you'll probably have noticed it was a bit hot when you came to take it off, and they talk a lot about warming their tyres or tyres 'over heating' in racing commentary and stuff.

Not that important, but Tyres DO get hot, and the reason is that there has been some energy conversion going on. Tyres have a flat bit at the bottom, as they go round the tyre carcass has to distort as the flat bit moves around the tread. The tyres are rubber, and pumped up with air, so they act like a spring and deform a little, so we have a spring force and a distance of deflection..... or Force x Distance... work, energy.

That energy has to go somewhere, and most of it gets wasted as 'heat' which is what warms the tyres up, and we see some of the force involved in that as 'rolling resistance' or a part of Drag.

There are lots of other bits of Rolling Resistance; but the largest part is that resistance offered by the tyres and the friction and deformation they experience as they go round.

There are a couple more sources of rolling resistance, and I should mention that we measure our 'Thrust' at the driven wheels.

Any resistance to motion in transmission, like the oil being stirred up by the gears in the gearbox, or the friction in the wheel bearings is over come before we measure the thrust, so doesn't get included in the rolling resistance.

But, if we have non-driven wheels, then any resistance in the mechanics of the vehicle to their turning WILL be included in the rolling resistance, because it's the THRUST being used to turn those wheels and so over come any resistance in them.

Another bit of rolling resistance, particularly for cars, is down to camber and geometry effects, and basically wheels don't always point in the direction they are rolling, the geometry has them steering slightly against one another, and that gives rise to 'scrub'. Which is friction, and the thing which wears your tyres out so quickly, if the car's tracking is out of true or something like that, but is there in some measure even if all the geometry is set up to the factory settings.

I'm not going to get bogged down in it; but it DOES effect motorbikes as well, even though they don't have as many wheels or the complicated geometry of cars. Motorbikes get it because they 'lean' and they have tyres with a rounded profile, rather than a square one, to let them do so......  like I said, just acknowledge that you get rolling resistance from the friction of 'scrub' from geometry effects!

So, onto Wind Resistance; name SORT of narrows it down, it's the resistance of the wind or air being muscled aside or around the vehicle as it travels through it.

Much of the reason that people talk about streamlining and the Co-Efficient of drag so much when talking about drag, and don't tend to look very much at rolling resistance, is that first, it is the more mysterious and 'exiting' part of the topic, and second, because wind resistance, certainly in the sort of situations most people are concerned with, is often the largest part of the overall drag.

At 'high speed', which I'll define now as anything really over about 50mph, wind resistance is SUCH a big bit of overall drag that any rolling resistance is usually such an insignificant fraction, as to not be worth even worrying about.

OK, just said that Wind Resistance is the more mysterious and 'exiting' element, for which you can read, 'complicated' and tedious.

The subject is mainly governed by a branch of science called 'Fluid Dynamics', which is all about fluids moving, or things moving in fluids, and what makes it so tortuous is that because of all the variables involved, rather than dealing with 'proper' values, like time, distance, force, or even derivatives, like 'power', it uses a whole new set of units called 'dimensionless groups', which are ratio's of ratios.

If you though it took a leap of the imagination to believe that 'Power' wasn't a 'Real' thing, then this lot will REALLY fry your mind! So I am going to try and avoid getting into the technical detail of the subject as best I can.

One technical detail I cant avoid though is the 'Co-Efficient' of Drag, for the simple reason, that it is so often quoted and talked about. The Co-Efficient of Drag, or cD is one of those 'dimensionless groups' of Fluid Dynamics, and it is a 'correction factor', and not much use to any one but aerodynamicists and marketing men!

If we put a box into a wind tunnel and blow some air over it, the air will try and 'drag' the box along with the air-flow; so if we attach a spring balance to the box we can measure the actual drag force for a certain wind speed.

Force = Mass x Acceleration; and you probably think that there isn't much accelerating in our wind tunnel, but actually there is. You see, the air, as it hits our box, has to change direction; so its speed in the direction it WAS going reduces, and it accelerates in an entirely new one. Air, has mass, therefore, accelerate it, and it gives us a force.

And treating the stuff in bulk, what we actually see, is a 'pressure', and Pressure x Area = Force. So, bigger the frontal area of our box, the more force that pressure will give us. Which gives us a fairly fundamental rule of 'Drag'

Wind Resistance is DIRECTLY proportional to Frontal Area

Bigger the frontal area, more material there is pushing against the wind, more wind that's got to be shifted, so the more drag there's going to be, make sense?

But; we then get into the question of streamlining; for a GIVEN frontal area, a bullet shape offers less resistance than a brick, doesn't it? And if you've been exposed to any of the Legend & Lore, you'll probably have some idea about a 'teardrop' being almost a 'perfect stream-line'.

OK, explaining the cD then; if you put a box, an almost imperfect streamline, in the wind tunnel, you could measure the drag force that shape gives. Do the same for a car shape of the same frontal area, and you could measure the drag force that would give. Now divide one by the other, so you have the drag the car shape gave as a percentage of the drag the box shape gave, and you have a 'Co-Efficient' of drag, or an index of how much better the car shape is compared to a box.

Not QUITE that simplistic, like I said, they use lots of ratio's and the standards for measuring the cD aren't simply dependent on the frontal Area, the actual maths uses the ratio of frontal area to length.... and I'm NOT going to take the matter any further.

Point is, quoted cD's are an index of aerodynamic efficiency, and in the real world don't tell you MUCH more than how brick shaped your car is............ A Lotus 7 does NOT have a very good cD factor, an Audi 80 does; but the Audi has a far bigger frontal area, and so will be subjected to more wind resistance. I'll let you decide which is the more brick shaped!

OK, backing up a little, the relevance of the cD diminished, there WAS a nugget of useful information in there, and that was about how the air, hitting our vehicle, is slowed in the direction of travel, and accelerated in a new direction, as it is shoved aside or pushed around whatever shape we have, and THAT acceleration, times the mass of air, gives us the force we experience as wind resistance.

Now; there's a LOT of direction changing going on; and the faster the vehicle is travelling, so the more quickly the air has to change direction. Just like going round a sharp corner in a car, faster you go, the harder you are thrown side ways. So, the higher the speed, the greater the acceleration, so the more force the wind resistance offers.

But, it gets worse; faster you are going, the more MASS of air you have to shift in the same time; so not only do you have to make the air accelerate harder round your vehicle, you also have more of it to accelerate. This gives us another fundamental rule of 'Drag'

Wind Resistance is EXPONENTIALLY proportional to SPEED

And I need to explain an 'exponential'..... awkward ruddy things, but basically in the maths, something is multiplied by itself or some fraction of itself over and over.

The relationship between wind resistance and frontal area is a nice straight linear proportionality; you have a pressure, which is force times area, so double the area, you double the force.

The relationship between speed and force though isn't so straight forward; double the speed, you get MORE than double the drag. Best way to explain it is with a graph, so here you are:-

This Shows the relationship between speed and wind resistance, and speed and rolling resistance, between zero and 240km/k or 150mph, for a 'typical' vehicle. Numbers are based loosely on what you'd expect for something with the frontal area and aerodynamics of a Land Rover.

And you will notice it is a sort of curve shape ramping up, getting ever steeper, the higher up the speed scale you go.

If you read off the numbers for for a speed of 110Km/h, you'll find that the graph gives rolling resistance as 0.25Kn, and the wind resistance as 1.25Kn, and the overall 'Drag' as the sum of those; 1.5KN.

If you remember, when we looked at it earlier, we worked out that the Drag was about 2Kn, so our graph and our sums are out by about 25%...... There are lots of possible reasons for this, and hopefully they'll become a bit clearer in a bit. But three short answers;

First, the graph is NOT very accurate, it's based on pretty vague data, through which a 'line of best fit' has been drawn.

Second; for simplicity, there were a few 'presumptions' in the calculation of Drag, one of which was that we used ALL of the engines power to travel at the speed it would go at the engine revs that power was available. Standard gearing gives 70mph at 'Max Power' engine revs; a 'good' Landy will possibly go a little bit faster, so that suggests there is a little bit of 'spare power'.

Lastly, I'm also a bit sceptical of the 'quoted' power figure of 80bhp; seems a bit on the high side, 70 or 75bhp would be more 'reasonable'. Which introduces a question I'm going to look at somewhere, how reliable are the 'Power Figures' that get bandied about, and answer, with lots of boring detail, as 'Not Very'!

Any way, that chart is pretty useful, basically it tells you how much force you are going to have acting against you at any particular road speed. Force x Speed = Power, so you can work out how much power you will need to have in order to go that fast, or at least for a Land Rover sized and shaped vehicle; others will have similar shaped charts with different numbers on the scale, but the principle follows.

So, you see how the exponential works; up to 50mph or so, the relationship between speed and drag is very linear, mainly because up to about 50mph MOST of the overall drag is coming from the rolling resistance.

Over 50mph, the wind resistance starts to over take the rolling resistance, and so become the more significant factor in the overall drag, and while the rolling resistance keeps increasing, it doesn't increase very much, unlike wind resistance, which starts going ballistic.

Now, since I had the numbers in the spreadsheet, and it only took a little jigging to produce, I have converted the 'speed / drag' chart to a 'Speed / Power' one.

Bit iffy up the top, like I said, it's not THAT accurate, but, in the middle its not THAT far out, and you can use it to find out the intriguing bits of knowledge that:-

Not too sure about the idea that 600bhp would get you up to 150mph; don't think it's been tried. Seen a few Landy bodied dragsters; one had a around 800bhp of Ford big block V8 in it, but they only quoted the elapsed time and terminal velocities it achieved over a quarter mile! Irrelevant any way, it had been dropped so far it only had about half the frontal area! But, I have seen a few Landies with 350bhp-ish V8is in them, and the idea that that could take them to around 140mph seems about right.

Take note; that top end is getting VERY silly indeed; 75mph only needs 75bhp or so, but to go 90mph, needs 100bhp. So, to increase the speed by 1/5 you have to increase the power by 1/3 or you get a 20% speed increase for a 33% power boost, and the higher up the scale you go, the more and more power you need to find to get ever smaller speed increases.

And THAT is all due to this exponential due to wind resistance.

But, remember, when we looked at the balance of power, we mentioned the effect of the gearbox, and I just mentioned that a bit of the discrepancy, when we did those sums to what the chart suggests, may be because the gearing wasn't exactly matched to the peak power revs, so we probably didn't use all the available power to go that speed.

This introduces the subject of matching the gearing to the power delivery; and having an engine powerful enough to go the top speed you desire is only half of the story; to actually be able to exploit that power, you need to have the gearing between the engine and driven wheels matched so that top speed and max power coincide at the same engine revs.

Back again to what I said about the engine having to work as a part of the vehicle as a whole, and the first caution against the common 'lore' that more power will make your car faster; it may make it accelerate harder, but unless you have the gearing to exploit it, might not do a thing for top speed.

But then that MIGHT not be what you want........

Sloggers & Screamers!

Back in the introduction, I mentioned that often what was actually more important than 'Power' was Power Delivery, or the engine's 'character'.

It's NOT what you do; it's the WAY that you do it!

Corny line; but hey! Power = Torque x Revs, and I've already alluded to it; doesn't matter whether you have an engine making a lot of force very slowly, or an engine making only a little force very quickly; they'll deliver the same amount of power, and ultimately could do the same thing.

BUT; in the real world, these engines sit in vehicles, that they are expected to propel; and not just at constant velocity, or at top speed on a straight and level road; they have to accelerate and brake, turn corners and hold constant speeds other than their maximum. This is where they WAY that the engines deliver what they do starts to become important.

I'm getting a bit ahead of myself here; to describe sloggers and screamers, and look at the 'lore' that I am going to, really needs some explanation of actual engine design, a look at 'Power Curves' or 'Dyno-Charts', and the effect of gearing. But, I wanted to break the monotony of maths a bit, and illustrate power in action, because it will help when I DO come to talk about power curves and gearing.

So, to describe two types of engine 'character'; on the one hand we have the 'slogger' and on the other we have the 'screamer'. Sloggers, make their power from making a big bang in their cylinders, but not doing it very often, (High Torque / Low RPM); Screamers, make their power by making lots and lots of bangs, but not very big ones. (Low Torque / High RPM)

Now the best way to illustrate the difference in 'character' between the two types of engine is in relation to motorbikes. Because  motorbikes tend to exaggerate the subtleties of a lot of automotive dynamics, and for illustration, I'm going to look at two motorcycles, the 'Manx' Norton, and Honda's 'Hailwood Six'.(footnote 5)

Back in the 1950's, Norton built a bike called 'The Manx'. I could critique this machine for hours, but essentially it was a pretty rudimentary machine, even by the standards of the day. Norton would have described it as 'Simple and Effective', though, and I guess that's reasonably apt.

Basically, it won races because it rarely broke. Even for the era it wasn't a fast machine, but it worked well as a package. It's simple single cylinder engine made it light and nimble, and relatively reliable. On the track, it didn't make huge amounts of power, or rev to heady heights. What it did was make a modest amount of power from a modest rpm limit, that gave the rider power he could use easily.

At the Isle of Man, during the TT races (from where 'the Manx' got it's name), spectators commented that coming through Douglas, on the closed streets, the Manx Norton's would be coming through at about seventy miles an hour and the engines revving so slowly, that you could almost hear each individual power stroke. Lore would have you believe that they fired every lamp post! (footnote 6)

At the other end of the spectrum, the 'screamer'. About ten years after the Manx Nortons, Honda went to the Isle of Man, and they took with them a motorcycle that was just simply outrageous for it's era. It was a 250cc machine, with six cylinders, and it revved to a reported 20,000rpm.

Now, the Honda 'Six' (or the 'Hailwood Six' to aficionados, as it was ridden to victory by the late great Mike 'the bike' Hailwood) was the complete opposite of the Manx Norton. It was the pinnacle of technology for the day. It was EXTREMELY complex and won races through being technically more advanced than the competition, and piloted by a great rider.

Where the Nortons might have been treated to a new spark plug before each race, the Honda was completely stripped down and rebuilt, and where even average club riders could get results from a Norton, only the superstars could get the Honda to work well.

But, for the minute, we can forget the complexities and subtleties of the engine design, and just look at what it was like to ride, and how it behaved on the track.

So lets see how these things are to ride. Get on a Norton, and engage first gear (no, that's the brake, gear lever's on the other side on old brit bikes remember!), slowly release the clutch, and instantly, you feel the torque trying to push you along.

Get on the Honda, engage first gear, slowly release the clutch, and .... you stall.

Reason being that at or close to tick over, the Honda was hardly making any torque at all, and the force it could put to the rear wheel, even in first gear was less than the drag, despite it being geared probably three times lower than the Norton!

OK, lets start again. Get on the Honda, engage first gear, OPEN THE THROTTLE, and make the thing rev to 6,000rpm, and slowly engage the clutch.

Sorry, I should have said 'Slip' the clutch, and feed the power in. Careful! These things are collectors items; worth a fortune they are! You haven't broken anything have you; mudguard? Fairing? Your neck? No good. It's OK, that bruise on your chin where the handle bars reared up will go down in a day or two. Would you like to have another go?

Right. Bit of satire, but you get the idea. You could stick your Granny (footnote 7) on a machine with the easy power delivery of the Norton but the Honda was a bit of a handful!

If you got too few revs off the line or were in the wrong gear coming out of a corner, then it would bog, as you dropped into a portion of the rev range where it just simply didn't make enough motive force for the speed you wanted to go.

Go the other way, and the gearing was such that you could easily be in to low a gear at to low a speed, and find it making such ferocious motive force from that gearing, that rather than pushing you forward, it would lift the front end and smack you on the chin.

And hopefully you have got the idea; both machines were designed to do pretty much the same thing; win road races; the 'approach' each of the manufacturers took to solving that problem, though was a bit different; and an important ingredient in that approach was the engine.

Norton elected to stick with a single cylinder, for simplicity and lightness; They didn't go over the top with the technical features of the machine in the search for race winning power; they aimed for something that had a good overall balance of power, handling and reliability.

Honda, though, elected to try and push back the boundaries a bit; speed needs power, so they used every bit of technology they could muster in the search for speed. The bike they built then, did exactly what the Norton did, win races, but did it differently.

The difference then, between a 'slogger' and a 'screamer,  is not so much the actual revs that these engines turn or the power they actually make, but the 'character' of how they deliver the power they do; whether they do it in an easy, forgiving or lazy manner or whether they give it in a bit of a frenzied, frenetic and 'busy' one.

There is an AWFUL lot of myth, legend and lore surrounding the two types of engine, and lots of facets of engines and design features are associated with either type, and the actual descriptions, 'slogger' and 'Screamer' suggest types of engine that love or loath high crank speeds, which re-inforce technologies associated with them that favour or hinder high engine speeds.

But; get into real examples, and there are just SO many anomalies and contradictions, you cant be that simplistic, and try and define the types, by the engine speeds they turn, OR to try and guess at whether an engine may be more inclined to a type by the engine speeds it turns.

I'm going to look at some Power Traces next, these put some more flesh on the bones, and they CAN give a bit more of a clue as to whether an engine is more inclined to be a slogger or a screamer; BUT, end of the day, as looking at the traces should hopefully show, question is really only decided from a subjective appraisal of how the vehicle the engine's installed in, performs in action.

Soft, Lazy and easy to live with, slogger; or a frenetic, frenzied and more demanding 'screamer'.

The Shape of  What's to Come

What's to come, is 'Power', and the shape of it, is that of the Power Trace, or Dyno-Chart.

I had a devil of a job trying to find reasonable examples of the different traces I needed to illustrate this bit of the article, so in the end I made it up! Well, sort of! These three power traces are NOT 'real' power curves for real engines. They have been derived from taking Dyno data from varicose engines, and scaling it and moving it about a bit to get graphs that are a bit more representative of the 'type'.

The one above, I have titled as the Dyno Trace for a 'Typical' car engine; It provides a peak power of 100bhp at 5000rpm, and peak torque of 116ft-lb at 4000rpm.

The actual data used to derive this chart was taken from a few charts and some data-sheets I had lying around, and is not far from what you would expect to get from a  1600cc, four cylinder, two valve per cylinder, carburetted, saloon car engine in the sort of state of tune you'd find it in the 'base' model variant.

The features to note are that the Torque Curve is a dome shape; curves up at the beginning, and down at the end, a nice wide crest in between.

The Power Curve, is also a nice neat shape, bit like a ski slope; climbs in an almost straight line from the bottom, then rounds off at the top.

In a minute, I'll hopefully put this engine to work, and you should see how it gives a nice progressive and 'tractable' power delivery.

This next graph, is another 'theoretical' one; but the data used to massage it into shape was based on the Land Rover 2.25l four cylinder petrol and diesel engines; BOTH are definitely 'sloggers'!

Maximum Power of 72BHP is made at 4250rpm; Maximum Torque, of 125ft-lb is made at 2250rpm. Note the shapes of the curves.

The torque trace ramps very quickly, like a cliff; over the rim, though it drops away like a ski slope; unlike the 'typical' engine, the torque curve is actually pretty 'sharp'.

Power Curve on the other hand is more like the flat top hill shape of the 'typical' engine's torque curve. YES I have got the lines labelled the right way round! goes up a bit steep to begin with, but then flats off and, well, Peak power is quoted at 4250rpm, but its SO flat, it could be any where between 4000 & 4500rpm!

Predicting the power delivery of this engine, in action, you'd probably expect that lazy power curve to make the power delivery even more progressive and tractable, than a typical engine..... but we'll see.

Last in the line up; another 'theoretical' power trace, this time for a 'Screamer' type engine. Modelled mainly on date for various motorbikes, where the 'type' is more normal, it's the sort of shape you'd expect to get for something like a 600cc, four cylinder engine, with double over-head cam valve actuation, four valves per cylinder, and multiple carburettors or electronic fuel injection.

This engine makes 104BHP, at 12,250rpm, and peak torque of 46ft-lb, at 11,250rpm.

The curves, curiously are not a LOT different in shape from the 'typical' engine, only thing is that the rpm scale is twice as long, and the hill and ski slope are half way up it, a shallow ramp leading to each one..... Main difference is that the Torque Curve is underneath the Power Curve and their peaks are almost at the same RPM.

This is worth noting, because one bit of 'lore' suggests that the closer in the rpm range the peaks of power and torque are, and the higher up the rev range they are, so the more 'peaky' or screamer like the engine will tend to be.

This would be a good juncture for me to describe how Dyno-Charts are made, or how engine power is measured; to explain why 'quoted' power figures are so unreliable, and why there can be so much variation in them...... but I'm not, I want to get on and look at how these power curves would work in a vehicle.

Power To the Wheels

To give you some idea of how these engines would work in the real world, what we have to do, is get away from the 'data' and do some modelling on it; torque and rpm at the crank are all well and good, but we have a vehicle and where the trust gets put to use is at the wheels.

Since we have the power/speed chart, lets drop these engines into the Engine Bay of a Series III Land Rover; THIS is going to be FUN! So, let me explain the 'theory' of what I'm about to do!

We aren't the slightest bit interested in the crankshaft rpm or the available power or even the available torque; what we are interested in is what that will make the vehicle do; which is........ move! (Said this was going to get technical, didn't I!)

Now, we said earlier, that the vehicle, subjected to a 'Thrust' would accelerate until that thrust was equal to the drag, then it would maintain a constant speed. When I said that, I was only concerned with the thing in top gear and it reaching top speed. But now, I'm going to bring into play the real gearbox..... this COULD get complicated.

Lets have a look at ANOTHER, chart!

Looks, err.... technical; but let me explain what's going on:-

Now the interesting bit; Thrust is there to over come Drag; When the Thrust force = the Drag force then we have a balance of power, and we go at constant speed, right? But, if we have MORE available Thrust than there is Drag to be beaten, what's left over, after we've over come the Drag, can be used to accelerate the vehicle; make sense?

Conversely, if we have more drag than we have available thrust, then the thing ISN'T going to accelerate, it's going to slow down.

And, our chart, has an axis showing acceleration; which can be positive; above the axis; Making our vehicle go faster; or negative, beneath the axis, making our vehicle go slower.

So, lets drive this thing on the chart. And first of all we come across a problem, because the chart tells us that even in 1st gear, until we are doing about 5km/h, we don't have enough power to accelerate; which is quite true.

Remember your driving lessons; you were thought to select a gear, and feed the power in with the clutch. If you didn't, the engine stalled, right?

Well, that's what the chart is telling you; get the revs up, get the engine spinning to make a bit of extra force, and feed it in with the clutch to get the car rolling; 3mph, and you are away; engine should be making enough force by then that you don't have to worry about it.

So, first gear, and the amount of thrust the thing can make is ferocious; gives twice the rate of acceleration of second gear, right the way up to about 20km/k, when it will feel like you have driven in to a rubber wall, and the thing will carry on accelerating, but the more you try, the less you'll get, as the ramp is heading down wards. Cue jerky change into second.

If you want this mother to move anything like 'smoothly' then you should 'short shift' into second about 15Kph, because that will drop you down onto the Ramp of the Second Gear curve, just as it's starting to give the most 'oomph', and it will avoid the rubber wall thing. likewise, you'd better get ready to shift into third at about 25Km/h, if you want to avoid the rubber wall second has, just after.

Forth, you might as well forget, until you are doing at least 40Kmh, because the line is hardly of the bottom; and you may wonder whether its even worth using, because if you look at the chart, the 4th gear curve actually crosses the axis and goes negative just before the 3rd gear line.....

Yup; according to our chart; with this engine top speed is going to be about 88Km/h, just a tad over 55mph, whether in 3rd or 4th....

But remember, this is a low powered, high-torque engine; Maximum Power of 72BHP is made at 4250rpm; Maximum Torque, of 125ft-lb is made at 2250rpm.

So, lets look at another, and compare them;

This is the Joe Average, 1600 Rep-Mobile engine, and it provides a peak power of 100bhp at 5000rpm, and peak torque of 116ft-lb at 4000rpm. That peak torque, is almost 10% down on the 'Slogger' engine, BUT the thing is offering nearly 30% more power.

Despite the power up, it's not actually making much more acceleration; at least it's not offering the ferocious rate of acceleration that the slogger did.

Curves are about the same height, but longer; so get in and drive this one, and you'd not be having to make snatch changes so quickly, it would actually be a bit more 'forgiving' to drive, and because the acceleration ramps ramp for longer before dropping off, it would actually feel more progressive, and be easier to control. Basically, those soft curves give a soft power delivery.

Interesting to note that 4th gear acceleration goes negative at about 88Km/h again, but this time, the available thrust in 3rd gear would keep you accelerating up to over 100Km/h.

That suggests that 4th gear is too tall for this vehicle; and to get the most out of it it would need the gearing better optimising,  which is something I'll look at in a second, after looking at the 'screamer' type engine; but it's worth noting that we've looked two engines on a Standard Land Rover Series III transmission, and they are both over geared in 4th, and both are typical of the sort of power and power delivery of engines that could be fitted to a Series III.

So, onto the screamer; this is going to be a laugh!

Should have predicted that really; only gear that does anything useful is 1st, and even then you'd be slipping the clutch up to about 20-30Km/h, to get the thing moving! 2nd, 3rd & 4th gears, look at the curves, the thing just doesn't make enough thrust to allow it to haul gearing that tall.

But, between 50 & 100 Km/h, at lest in 1st gear, the thing has a pretty useful looking 'hump' of acceleration; granted, its only a third of what the Slogger or Typical engines offered in first gear, but if we lowered the gearing? I mean this thing revs to about double what our 'typical' engine did, so if we doubled the amount of reduction so that the rev range sort of matched that of a normal engine.....

Why not; Series III conveniently has a Transfer box in it, with a 'Low Range' that just HAPPENS to double the overall gear reduction; lets give it a try and see what we get:-

Now that's interesting; 4ths still as much use as a fishing rod to a polar bear; 3rd's a little high, it runs out at 100Km/h and never offers much acceleration; 1st Hi-Range actually took us faster, until we ran out of revs, But, 1st & 2nd look marginally useful.

1st a bit tall and spikey, BUT, if we played around with the gearing a bit more; maybe used different cogs in the gearbox to get slightly different ratio's, pulling 4th back to something just a bit higher than 1st Hi-Range, and dropping first back to something a bit lower than it is, and spacing 2nd & 3rd about where 1st & 2nd are........

Be a bit tight, the ramps would be a bit spiky, as they were for the 'slogger' engine, but you COULD get this thing to work, and haul a Land Rover sized truck along at a respectable rate; A five speed gearbox, to over lap the ratio's a bit might help, and it wouldn't exactly be 'easy', but it COULD be done.

If I had used a model based on something a bit more aerodynamic, a little smaller, and rather a lot lighter, what would the acceleration traces have looked like? Lets have a look!

The Spread-sheet 'model' I created only had four gears, so messing with the parameters to optimise the gearing I've had to space them a bit wide; a 'Real' motorbike, would almost certainly have five or six gears, not four, so there'd be one or two extra lines and there's be more overlap between the humps.

But, in a vehicle, probably a fifth the size and weight, that 'peaky' power delivery isn't any where near the problem it was in a bigger heavier one.

Because the engine needed to be made to rev to get ay useful force out of it, in a heavy vehicle, it needed really low gears, which meant that it ran out of revs very early in terms of road speed. In a much lighter vehicle though, the lack of weight it has to shift means that even with the little power it has low down in the rev range, it can be geared high and still make some useful force before it 'comes on the cam'.

Matching the Components

OK, lets have a think about all this; the 'typical' engine, had nice long smooth acceleration ramps, with a lot of overlap between gears; it's easy driving; the thrust builds up nice and progressively; you can accelerate a long way in any gear, the rate of acceleration increasing the faster you go.

If you don't want to 'press on', though, you can short shift and let the engine lumber up the ramp of the next gear up, or you can hold a constant speed, at part throttle and have a fair bit in reserve for going up hills or accelerating round corners and stuff.

The slogger engine, on the other hand is no where near as 'forgiving'; it can make a lot of acceleration, but only low down or by using the lower gears. Towards the top, it has little left in reserve and it's struggling to hold a decent road speed, so hills are going to be a problem, and you better not slow down too much for corners of you'll be an age trying to get the road speed back up.

But, it has a few advantages; all that  low down acceleration, with very little change in road speed means that at low-ish speeds you have an awful lot of 'grunt' in reserve on the throttle; at 20-30Km/h, as you'd be doing off-road, in 1st or 2nd gear, you would need hardly any throttle to hold a constant road speed; BUT, if you wanted to push the thing up a rock step, there would be the force available to let you do it, AND without much altering your speed, so you could maintain good directional control and make small corrections as you 'felt' your way over the obstacle.

There's also lee-way there for adding a lot of load to the vehicle; we estimated that our Land Rover weighed about 1.5 tonnes; that's not exactly light; but all that low down acceleration could be used to get a much heavier vehicle moving; so a Landy towing a stock box with a ton of horse in it, perhaps. Wouldn't move it very quickly, but it WOULD move it.

Ultimately, the engine makes a lot of very little; it does almost as much by way of speed, and possibly more by way of acceleration, than an engine with 30% more power. But. the 'Character' of the power delivery is eminently more suitable to a vehicle that has to haul loads for a living, like a truck or locomotive, or in our case a Land Rover (where the 'load' is its own weight up and down extreme inclines!)

As for the 'Screamer', well, hampered by gearing in this case that was just NOT suitable for it. If that was sorted though, just like the slogger, it could do the same job, but those peaky acceleration ramps would make it just as demanding to drive; it would accelerate, BUT you'd have to be dang sure to be in the right gear at the right time if you didn't want the thing to bog or stall!

And unlike the Slogger, which had those peaks well down the speed range, the screamer had them in the middle and up the top; so add any weight or a steep hill, and that thing is going to struggle.

Which is why you find 'screamer' type engines in motorbikes and sports-scars, which just don't have so much weight to lug about, and where the demands of keeping the engine spinning in the 'sweet-spot' wouldn't be an impediment, but part of the thrill of driving it.

Power Provides Thrust - Thrust over comes Drag & makes Acceleration

If you look at the acceleration curves, and it's most noticeable on the traces for 1st gear, but the shape of the acceleration curve is almost identical to the shape of the power curve.

It's not a co-incidence; doesn't really matter what the torque or engine speed are at the crank-shaft, what we feel is the thrust from the driven wheels, which gets there as 'power' however the torque and revs have been altered by the transmission.

So POWER is the defining commodity, the important 'Thing', as far as what the vehicle does, and how it behaves in action; BUT, it isn't the ONLY thing.

The shape of the power curve, or the 'character' of the engine, gives you some idea of what kind of application it would be most useful for.

The 'Character' though is translated to where its used by the gears; so they have to be 'matched' to get the power from the engine to where its going to be used, and put it in the right place at the right time and deliver 'thrust'.


I have not, so far mentioned economy (See Eco-Drive); a lot of 'lore' is spouted about economy, and very little of it is all that useful, I'm afraid. There is a bottom line, though, and this is it:

Power = Fuel Consumption

Maths behind this is pretty simple; earlier we said that Energy In = Energy Out; we were looking at the energy coming from the engine, and going to over come drag, but we could equally look at the energy going into the engine to make that power.

The 'energy' used to make power in the engine comes from our fuel; it's originally contained as 'calorific value' or chemical potential energy, and by burning the stuff, we get that energy out as heat. (See The Chemistry of Combustion  )

So, we bung in 'Fuel' and we get out 'Work'; rate of work is Power, 'rate of fuel' is fuel consumption; it's THAT simple. More Power we use, so, the more fuel we are going to have to burn to get it, and the worse our economy is going to be.

Power is used to do two things; over come drag, and provide acceleration.

Now, acceleration, we can control via the throttle and the gears; drag, we cant do much about, BUT, we do know that the stuff increases exponentially with speed. In our model, 50mph, used about 25bhp, half as fast again, 70mph, took twice the power, 50bhp, less than quarter as fast again, 90mph, took double the power again, 100bhp!

if you want to go further for your gallon, SLOW DOWN!

Amazing how many people want to see big fuel savings but aren't prepared to slow down to get it! But it's the surest way to save fuel, and it doesn't cost a thing to achieve, or need any particular effort or dexterity with the spanners! (though a bit of maintenance and servicing will often go a long way too).

That exponential is a bugger, BUT, making it work for you, rather than against you, slowing down by as little as 5mph can make a very big difference. If you were to 'cruise' at say 68mph on the motorway, rather than 73mph, difference on your journey would be, less than 7%, that's just four minutes in an hour, but the fuel saving is likely to be a 15% or more.

Mentioned earlier; power, hence fuel, is used to provide thrust; that thrust is used to over come drag, and what's left to provide acceleration. So, it's not just speed that's a killer of economy, but so is acceleration.

In fact any harsh attitude changes, like heavy breaking and steering too; anything that adds 'load' or results in high forces are detrimental to economy; but it's a big contributor to 'wear and tear' as well, and that adds to running costs just as surely as fuel.

"Smooth and Progressive"

It's a catechism that's oft repeated, but a smooth flowing driving style is a great thing to acquire. Depends a lot on reading the road, thinking ahead and 'planning' your driving, rather than just responding to circumstance, but............. (See:-Better-Driver)

A 'smooth' drive, gives passengers an easy time, to begin with. It gives your wallet an easy time, too, with less fuel used, and less wear and tear, and hence less maintenance.

But it NEEDN'T be 'slow'; any racing driver will tell you, going fast is all about making the most of your available traction; load the tyres and suspension 'hard' and you are wasting effort to react cornering and breaking forces, or traction to provide accelerating thrust, to get back the road speed you COULD have held if you had planned your drive and driven it as smoothly as you could.

On the road, same principles apply; less traction you waste with hard braking, cornering or accelerating, so the faster you will go, but with more control and a far greater margin of 'safety' between you and the point where the vehicle will skid!

And, that 'observation' necessary to plan your drive and be smooth and progressive, ALSO makes you more aware of hazards; so not ONLY will you have better vehicle control, and be driving faster, further within your vehicles limitations, and so be less likely to have an accident; you'll ALSO be far more likely to spot an accident coming an NOT get into it in the first place!

AND you'll be saving money, from less fuel, and less wear & tear! You KNOW it makes sense!

Series Land Rover Drivers, will, as a matter of course, have to acquire a smooth and progressive driving style; Series Landies are almost incapable of ANY sudden movement! As an aid to 'planning' your driving, if you are a Series Sufferer, I understand WH-Smiths now stock 18month calendars so you can pencil in when you ought to start braking for that roundabout you can see ahead............. (sorry! couldn't resist)

Beating Drag

Looking for economy gains in other areas is normally an awful lot of effort for very small gain. Having gone through all the science, there isn't an awful lot you can do.

Drag is your main enemy, and that is exponentially dependent on speed; proportionally dependent on frontal area, and slightly effected by aerodynamics.

If you don't want to alter the speed; then that leaves you frontal area and aerodynamics to play with. Frontal area, is normally pretty well fixed by the body of the vehicle in question, so not normally something you can do much about; and aerodynamics? Well in MOST cases, again, pretty well fixed by the vehicle shape and structure.

The only vehicles where there is much scope for playing with either frontal area or aerodynamics are motorcycles; motorcycles have the 'soft bit' (rider) on the outside, so to a large extent the size and shape of the 'vehicle' is dependent on the rider.

If you ride, you'll know how that those oversized water-proofs you bought, because they were easier to put on in a hurry, and you thought would give room for a few extra woolly jumpers come the cold weather, flap about in the wind, and tug at the handlebars!

The 'typical' street bike, though is a bit like 'Black Dawg'. It has little bodywork, and the headlamp attached to the handlebars.

'Touring' bikes, though, will often have a big 'Wind Jammer' fairing on the front; a large screen that the rider can sit upright behind fairly comfortably; makes things comfortable for the rider, but adds a huge amount of frontal area, and the streamlining of a barn door. To get better MPG, these or 'touring screens', can be removed.

Sports-bikes on the other hand, tend to have sharp pointed 'full-fairings' with a bubble over the handlebars that the rider can duck down behind. Adding little to the frontal area, they do offer some better streamlining, and even on relatively low powered machines can add quite a lot of useful extra speed, or economy, even the little 'bikini' fairings that cover little more than the headlamp and clocks can add quite a bit of useful wind 'deflection'.

Land Rovers? F-O-R-G-E-T IT! You might gain a bit by removing a roof-rack; but you are up against the odds. They have external rivets, brackets poking out hither and dither, and a shape that's, well.... like a shed!

Other cars, you might find a small gain from removing sporty spoilers, that don't normally do much by way of providing stability at speed through air correction or down force, but do look pretty in the sales brochure; wheel arch extensions, and any other unnecessary paraphernalia, but even then, possible gains are likely to be small.

The Search for Economy, I think is probably going to have to be another article in it's own right when I get round to it; and I'll include all the 'Teflon's-Tips' for economy in it. A lot of it though, comes down to little more than good servicing and maintenance and a bit of common sense. Here, to keep things brief if not entirely to the point, which is the 'science' of the subject, I want to look at a couple of bits of 'lore'.

Efficiency Gains

Fuel Widgets; (See Wonder Fuel & Widgets!) The claims their makers, or sellers make in the magazines for the sort of fuel saving widgets or gadgets advertised, often defy physics. I have yet to find one that even SOUNDS like it could plausibly work! The sort of things I'm talking about here are 'fuel cats' or 'petrol ionisers', magic 'filters' and magnetic modifiers.... bunkum, snake oil, talisman & charms!

Typical sort of advert shows something that you drop in your fuel tank, clamp around the fuel line or splice into the petrol plumbing. Fits in minutes! they claim. They then go on to suggest lots of science about ions or magnetic flux or particle polarisation, and, well they use lots of scientific speak you might be vaguely aware of but probably don't have a degree in rocket science to be able to challenge; sounds pretty plausible most of the time, BUT. There is ONE fundamental principle that they avoid considering;

Energy Cannot be Created or Destroyed'

These things suggest that you can travel as fast and use as much power as you have always done, but SOME-HOW, their gizmo lets you do the same amount of work from less fuel. Which kind of begs the question, where does the 'saving' come from?

Sales blurb normally goes on about 'efficiency' and tells you how utterly inefficient your internal combustion engine is, suggesting that there is a HUGE reserve of 'wasted' energy that their widget can tap into.

Now, the 'thermal efficiency' of your engine IS pretty dire; engines only manage to capture about a quarter of the heat energy released when a fuel is burned, and deliver it as 'power' at the crank shaft.

Overall efficiency is even worse, because you don't get all of the energy you could out of a fuel when you burn it in an engine; then you have to take off all the 'losses' of the energy used inside the engine making things go up and down and round and round and stuff; and down-stream of all that, through all the widgery bits of the vehicle until the power eventually appears as 'Thrust' at the driven wheels.

But, the calorific value of your fuel wont change, NO widget or gizmo can increase the amount of energy stored in your fuel!

Next; whatever the inefficiencies of your vehicle, there doesn't seem to be MUCH that can be done about any of them by doing 'something' mystical to the fuel supply!

The thermal efficiency of an engine is fixed within relatively narrow limits by some laws of thermodynamics, which I could explain by talking about heat sources and heat soaks and entropy, that I REALLY don't want to get into. But, the essence is that it's an inefficient process, because combustion is so effoff HOT.

Heat escapes at a rate proportional to temperature difference; bigger the difference, quicker it escapes. You know how fast a room will cool down when you turn the gas fire off. Inside it's what, 25oC; outside, chilly day, what, 5oC. That's a temperature difference of 20oC.

Inside an engine, we have fuel burning in a cylinder, at a few THOUSAND oC! The engine itself is normally held at a constant operating temperature of about 85oC so we have a HUGE temperature difference! Like 100x ambient temperature, rather than 1/10x it!.

Rate of energy loss is proportional to temperature difference, but it's another exponential one. Works on a 'half life' principle. But, either which way, you are looking at a 'thermal gradient' 1000 times steeper than the one one your room with the fire turned off has, so that energy is going to try and escape from your engine at LEAST 1000 times faster.

BUT, because of another bit of thermodynamics, all to do with internal energy, you NEED that temperature difference to be able to turn some of it into 'pressure' which is what gives us the force the engine captures and shoved to the driven wheels!

Energy = Pressure x Temperature x Volume

It's an equation based on the law that says, energy can't be created or destroyed. When we set fire to our charge with the piston at the top of it's power stroke, we have an initial volume, an initial temperature and an initial pressure; when it gets to the bottom, we have a new volume, a new temperature and a new pressure.

Energy = Pressure x Volume x Temperature in each case; and 'Energy In = Energy Out'. We get SOME energy Out, by way of the work done pushing the piston down the bore; that's the only 'useful' bit; the rest is left in the exhaust gasses, and mostly disappears down the exhaust pipe with it; but another 'chunk' disappears through the cylinder head and engine block due to that temperature difference, and winds up being wasted into the atmosphere by the cooling system.

So, WHATEVER claims that the snake-oil merchants make about the thermal efficiencies of your engine, they are pretty much fixed by the temperature the fuel burns at at the pressure the combustion chamber is put under.

And the temperature that the fuel burns at, it's 'flash-point', is a chemical property of the fuel, like it's boiling point. You can chant as many incantations as you like, wave wands, magnets or special alloys around it, you aint going to change the fuel's molecular structure by any of it, and THAT is the only way that you'll change either its boiling point or 'flash-point'!

So, the only way that you might find any 'efficiency' gain, messing with the fuel, is in the 'combustion efficiency', which is a very different thing to 'thermal efficiency'.

Combustion efficiency (See The Chemistry of Combustion  ) is how much of the fuel actually gets properly burned to release its energy. 'Perfect' combustion, is defined by a chemical equation, which for a simple fuel like methane or propane, would be Fuel + Air => Carbon Di Oxide + Water; the proportions of each 'combustant' and each 'product' defined by the mass of each needed to balance the number of molecules of carbon, oxygen & hydrogen on either side of the '=> symbol.

When they measure a fuel's 'calorific' value, they do it in a device called a calorimeter, and the conditions provided for combustion are near 'ideal' so that you get pretty much 'perfect' combustion, in accordance with the chemical equation.

In real life, in a real engine, you don't get 'perfect' combustion; the conditions are far from 'ideal', and you get some mix of combustion 'products'; that aren't predicted by the chemical equation; which for burning petrol or diesel in an engine tend to be a proportion of Carbon-Mon-Oxide, rather than Carbon Di Oxide; some Nitrides, or compounds that include nitrogen from the air, that get dragged into the deal by the 'extreme' conditions in the cylinder, and some 'carbon' compounds, and particularly for diseasels, compounds of sulphur. These are the 'Emissions' that have been the subject of so much attention in recent years.

Again, the bottom line is that the 'emissions' are essentially determined by the combustion 'conditions', which are fixed by the engine; the compression ratio, combustion chamber shape, ignition timing, stuff like that, NOT by the condition of the fuel!

Yes, the 'emissions' are fairly closely related to the chemical composition of the fuel, but short of chemically 'cracking' the fuel, you aren't going to change that, and even if you DID, you'd still have the same molecules of carbon, hydrogen, oxygen, nitrogen and whatever other 'ingredients' are in there to dispose of some how....... probably as 'emissions' of similar sort to just burning the stuff straight!

There is only one thing upstream of the engine that can in ANY way effect the efficiency of combustion, and that is 'fuel atomisation'; which is the spray or droplet size of the fuel 'mist' as it's introduced into the air stream by the carburettor or fuel injector.

And the Snake oil merchants tend to hint that their product does something that effects fuel atomisation, because it is the ONLY claim that might bear even passing scientific scrutiny.... but no more!

Spent a lot of time at college doing experiments on this one in the lab. Lots of pictures of 'spray patterns' & having to measure average droplet diameter, then correlating that with data taken on the dyno using different compression ratio's, ignition & mixture settings, and comparing the resultant power graphs, emissions results and specific fuel consumptions!

Very tedious job indeed, but one that the motor makers would pay a lot of money for gimps like us to do, over, and over and OVER again Because.... with the imperative to get ever lower emissions and ever higher MPG figures, optimising the atomisation, is a VERY important thing for them to do.

Basically, big droplets of fuel more easily 'drop out' or 'dew' from the air stream; they also burn more slowly, so are a bit more tolerant of lots of ignition advance, higher compression ratio's and higher combustion chamber temperatures; BUT they tend not to burn so completely, so give less favourable emissions. So, ideal is to get the droplet size as small as possible, without loosing power or getting chronic pre-ignition.

So, mess with your fuel atomisation by bolting a widget somewhere, and if it really worked, the BEST it could do is nothing at all! Because IF it 'improved' fuel atomisation', by which it made the droplet size smaller; chances are it would make your engine start to knock, loose power and possibly melt holes in things like valves or pistons!

Only explanation for the thing finding any demonstrate able 'gain' would be if the atomisation you were getting from your carburettor or fuel injection was not what it should be, as in your carburettor was worn or out of tune, or your fuel injectors were 'gummed up'.

In which case, the thing isn't 'finding' you any 'gain', just making up, MAYBE for a fundamental deficit! Fixing the carb or injector would be a far better way to get improvement, probably give more, and certainly more safely! Which brings me onto carburettors and fuel injection; which often work or don't work for pretty similar reason as widgets!.

Economy Carburettors

Mentioned earlier; power, hence fuel, is used to provide thrust; that thrust is used to over come drag, and what's left to provide acceleration. So, it's not just speed that's a killer of economy, but so is acceleration.

All carburettors are in some way inherently compromised. Their function may be simple, all they have to do is put the fuel into the air stream in the right proportion (See:- "'Metering' The 'Charge' "), but, they also have to vary that proportion according to the load on the engine, and well as maintain it over a wide variety of engine operating  speeds & conditions.

But one feature they all share is they have some mechanism for 'enrichment' of the mixture under acceleration; in 'performance' carburettors, and particularly the 'venturi' type, like Webbers or Solex's, they often have 'accelerator pumps'. A little piston on the throttle linkage, and a nozzle in the carburettors mouth, that squirts neat fuel into the maw when the loud pedal is pressed!

Great for acceleration, NOT so great for economy! 'squirter' carbs, are NOT the most efficient, and when they 'squirt' any notion of 'metering' goes out the window; they basically just chuck an excess of fuel down the ports to be sure. Obviously, having a carburettor that DOESN'T feature such a horrendously inefficient device is likely to aid economy.

So, 'economy' carburettors, wont have such features to 'promote' acceleration, but will also have such features as will 'deter' acceleration.

Basically, they will often have a smaller 'choke' size so that they don't flow as much air, limiting available power; but also, they are arranged so that they wont ALLOW the engine to accelerate as hard as it might, by having smaller 'jets' NOT giving as much enrichment as they could; hence saving fuel from a rich mixture PLUS saving fuel from limiting acceleration. Basically, they will limit power and limit throttle response.

Back to Teflon's Top Tips; if you know THAT, then you probably don't need a fancy 'economy' carburettor! Simply DON'T accelerate so hard with the carburettor you have! Back to the idea of driving "Smooth & Progressive".

Idea that 'Fuel Injection' is the panacea to all carburetion worries is another legend; Fuel injection is digital carburetion. Some of the compromises in a carburettor are in the fact that it normally uses pressure effects to meter the fuel into the air stream, which is quite difficult over such a wide range of operating conditions.

Fuel Injection makes things a BIT simpler, by having a 'fuelling map' in the systems electronic brain that tells the injector how much fuel to squirt in for any given engine speed and load.

In theory it's the 'perfect' carburettor, and can get the mixture strength spot on right across the rev range and load range. Practically it's not THAT brilliant. There are still compromises in there, they are just different ones.

For the most part, though, electronic fuel injection can work a bit better over a wider range of loads and speeds than a 'typical' carburettor, but the difference that gives in terms of economy isn't always that huge.

Just as a point of note; lots of 'chippers' out there offering performance 'upgrades' to engine management systems; these things are often marketed rather like 'widgets', which makes me sceptical of a lot of them to begin with; but I do know how they achieve what they do.

One clue, is in the fact that an awful lot of them come with a 'power switch', that allows the driver to choose between the 'factory' fuelling map, and the 'power upgrade' one. Take a 'chipped' car in for it's MOT, just make sure you are on the 'factory' setting, or it's likely to fail the emissions test!

A lot of the gains that the performance chippers find are from where the manufacturers have mapped the fuel and ignition system to restrict power and acceleration, to aid economy or improve emissions, and all that the after market people have done is up the fuelling rates a bit in those areas.

Its actually quite interesting looking at some modern engine's dyno traces, to note that there are often quite gaping 'flat spots' in the power curve, which seem quite remarkable, until you compare with the various standards for measuring emissions or fuel economy, and increasingly, noise, and realise that they have actually 'choked' the engine quite deliberately over very narrow portions of the rev-range, where the engine will be run to do those tests!

'Tuning For Economy'

Another bit of lore, suggests that you can 'port' an engine for economy, the same as you can for power. Again to some extent true. Some tuning practices are good for economy and power. Porting though is a questionable one.

Principle of 'porting' is to reshape or remodel the ports in the cylinder head for better flow. Normally that is for higher 'peak' flow. Again, more detail in Pots, Pans & Cams!, more still in Tuning & Super Tuning , But, a bit like economy carbs, there's usually not a lot of economy to be found in porting, unless there is some peculiarity or gaping inefficiency in your engine.

There is a notable one; 'precipitation', or 'dewing' that I've already touched on, which is where fuel held in suspension in the air stream gets condensed out and 'puddles' like dew on the port walls, or just gets globbed together into huge droplets, so that it wont burn completely, and the carburettor has to be set rich, to compensate for the fuel that 'doesn't quite make it'.

Its the same thing as I mentioned earlier about atomisation of the fuel in to the air stream, but rather than actually getting the optimum droplet size, holding those droplets in the air stream until they get to the combustion chamber and set on fire!

Depending on the port design, it may be that the air flow is too slow to keep the fuel in suspension, it may be that sharp corners cause turbulence that centrifuges fuel out of the air stream, or mixes it into the bigger droplets, or splits it up into smaller ones.

This is heading into the realms of the 'dark arts' of engine tuning. A really GOOD artisan of the art might be able to do something to make a head flow better for economy, but the demand for 'porting' tends to be from people looking for more power, so it's not something many tuners will have a huge amount of experience of or empathy with.

The only generalisation that could be said is that 'small' ports favour economy; big ports are good for flowing large amounts of air into an engine, which is what you need for good power, when the throttle is wide open; but when you are driving for economy, you are using a part throttle far more often, and then having smaller ports, that give a higher air stream speed for the same volume of air flowed, can help prevent fuel 'drop out' and give the sort of 'inertial effects' that help cylinder filling that a larger port wouldn't allow unless the throttle was wider open or the engine turning a lot faster.

After market 'tuning' of cylinder heads from a mainstream manufacture would tend to only be able to tale metal off what's already there, so easy to make a port, or hole bigger; bit more difficult to make it smaller! Though, it CAN be done, and I have had experience of 'extreme' tuning, where the tuner has actually filled both the port and combustion chamber with weld and cut both to the shape and size they want from scratch.(Footnote 10) This is NOT the sort of practice that you'll find people offering to do in 'Max Power' or 'Fast Ford' magazines!

Some 'Tuning' practices though are good for economy, and some of them are also good for power. Probably the most significant thing that's good for both is the Compression Ratio.

In Pots, Pans & Cams!, I introduce and make a lot of use of yet another equation, I'm not going to give the algebra behind here:

Power = Cylinder Capacity x Cylinder Pressure x Engine Revs

It's a derivative of the sum we've used a lot here, Power = Torque x Revs, but looking at where the torque comes from by way of the cylinder pressure.

Cylinder Pressure, obviously comes from burning fuel and air, so the more of it you burn, the more pressure you are going to get; but, it's also a function of the compression ratio, which sets the 'initial' pressure in the cylinder, which is sort of multiplied by the heat we get out of combustion.

The compression ratio, then, is set by the volume of the combustion chamber, in relation to the swept volume, of the cylinder, or capacity of the engine, and reducing the combustion chamber volume, normally by 'skimming' the cylinder head, is a good modification to make to most engines; though bear in mind my comments in You can do ANYTHING to a Landy!, there's probably good reasons that the cylinder head doesn't already have a higher compression ratio, you MAY want to think about!

High compression ratio's are good for power and economy, BUT they also promote 'knock' or pre-ignition, and you run into a sort of self defeating spiral with higher compression ratio's, having to retard the ignition, robbing you of any gain.

However, driving for economy, implication is that you'll be driving a lot on part throttle; which means you wont be getting complete cylinder filling, which means that the 'effective' compression ratio in your engine wont be as high as the theoretical one.

Which means that you COULD increase the compression ratio to something ridiculously high, so that you saw the sort of actual compression in the engine at part throttle as you'd normally only get at full throttle. But, the down side would be that under full throttle, you would have to seriously retard the ignition and enrich the mixture to avoid 'knock'.

Great idea, but would mean that on full throttle, you'd probably loose more than you gained! Which is where we get back to this idea of things not living in isolation, and messing with one aspect, element or component of an engine and expecting it, on its own, to unlock the power or economy you would like, is simplistic & futile.

If you are dead set on building a really economical engine, it is possible to get some pretty remarkable results; but, only if everything is 'matched' to get best effect, and ultimately, a large proportion of the economy achieved would come as much from restricted performance as from any great efficiency gain.

And ultimately, the manufacturers aren't entirely stupid, and they have considered these sorts of things very carefully, and optimised the engine as best they can to get the best economy and power they possibly can, leaving little room for 'improvement'.

Note I say 'Little' room......See You can do ANYTHING to a Landy! There IS some room, depending on what's important to YOU, and where YOU might be prepared to make DIFFERENT compromises to the ones the manufacturer elected upon!

Economical Sloggers

Right, back to the sloggers & screamers, and some explanation of why I sort of left the character of engines hanging to talk about economy.

Another bit of 'lore' suggests that 'sloggers' are 'efficient' and 'ecconomical', and 'screamers' are powerful and thirsty.

Doing a bit of myth busting; yes and no! Just made a lot about the fact that using a lot of power uses a lot of fuel; sloggers, for the same capacity of engine TEND not to offer so much power.

Simple bit of logic, you cant use what you aint got!, So basic limitation of the engine SORT of tends to restrict you a bit, so what you'd see is reasonable economy. However! Back to that power trace;

Deriving a road speed vs acceleration chart from the power curves, we got this, which SORT of implied that the available power from a slogger engine offered some pretty astounding acceleration rates.....

BUT, sort of depends; chart was derived from a simple model that considered only the power graph and the weight of the vehicle and PRESUMED that the engine would accelerate between engine speeds without any sort of lag, the only 'load' being that of the vehicle it was shifting.

Real world, unfortunately not so. Put an engine into neutral, and rev the engine, and it WONT reach max revs as soon as you have the throttle wide open; even with no other load, there is some lag.

This is down to a thing called 'reciprocating inertia'; inside our engine, we have LOTS of things going round and round and up and down; all have mass and so are going to be subjected to acceleration, which will all put SOME 'load' on the engine, to rob us of of the force it might make, before we get it to the driven wheels and can use it as thrust.

When 'Power' is measured, it is usually measured at what is known as the 'steady state'; which is when the engine is held at a constant engine speed and load.

Power charts tell us NOTHING about how much 'reciprocating inertia' there is, and how reluctant it is going to be to change engine speeds. Because SOME of the power it is providing, when accelerating between engine speeds, has got to go to accelerate the wirly stuff inside. THEN some MORE has to be used to accelerate all the stuff its attached to between the crank-shaft and the driven wheels.

There can be a LOT of weight in all that gubbins, and an awful lot of it is turning a heck of a lot faster than our wheels, so it will have to accelerate a heck of a lot more quickly, so give us a heck of a lot more 'resistive force'.

Oh-Kay. So there is this 'reciprocating Inertia' that when we accelerate the engine, gives us a resistive force that robs us of SOME of the acceleration the engine COULD offer.

Next, there is what is known as 'throttle response'; another 'lag', but mentioned it in 'economy', because to make an engine change speed, you need to throw in a bit of extra fuel and enrich the mixture, advance the ignition a bit, and get a bit more air flowing through the ports.

So, lets have a think about this; we have a 'slogger' engine, and we said earlier, that it could be a bit tiring to drive, because it would accelerate like bilio on the low down ramp of the power curve, needing a snatch change very quickly to avoid a rubber wall effect as it stopped pulling.

BUT, we have some 'lag' effects in the real world that would to some degree damp that out a bit; so IF we were to fit a really heavy fly wheel, that would increase the reciprocating inertia, and the consequent lag; wouldn't accelerate so hard, BUT that wouldn't be a bad thing, would it?

If we used an 'economy' carburettor, again, would inhibit acceleration a bit by not giving so much enrichment, so we are getting a double or triple whammy.

Thing makes lots of force, which COULD give us plenty of acceleration, BUT because its over such a short rev range, that would make the engine a bit tiresome to drive.

So, IF we were to contentiously tune this engine for economy; the character that gives us the potential to make lots of low down force  could compensate for the fact that we are limiting it's responsiveness, and we'd get something that in installation, didn't accelerate AS harshly as it could, but still accelerated well, and still had plenty of force available to pull really heavy loads; BUT, we'd be able to make it a bit more 'driveable', and a lot more economical.

Considering the 'Lore' then; it's not that 'slogger' engines have anything that particularly endows them with any huge efficiency advantage that makes them inherently more 'economical' than a typical engine or a screamer; it's just that the character of the power delivery is such that it's well suited to being tuned for economy.

Looking at 'real' engines, and the sort of features, facets and technology that tends to go into 'slogger' type engines, by way of the 'topography' that favours the type; there are often a few more facets that can again compound advantages and tend towards efficiency and economy; but, I don't want to get bogged down in it, as ever, see Pots, Pans & Cams!, and Tuning & Super Tuning. so, onto the converse situation....

Thirsty Screamers

We can basically flip the logic for why 'slogger' type engines are more inclined to being tuned for economy here; The 'character' of that power delivery is to begin with much more suitable for something light and sporty, with a predisposition for performance, where economy is probably NOT going to be of enormous concern.

We'd probably want to get as much acceleration out of the thing as we could, so throttle response and lack of lag effects would be more important than saving fuel; so tuning to exploit the performance the engine could offer, in it's intended installation, light fly wheel and things to keep the reciprocating inertia down would be paramount; and throwing bucket loads of 'excess' fuel down the inlet ports to promote acceleration wouldn't be any great sacrifice.

Especially in that lower zone of the rev range where the thing isn't making much useful force or acceleration; in installation, the pilot of whatever this thing was propelling wouldn't want to spend much time with the engine turning down at those lower speeds any way, they'd want to get the thing revving and delivering 'serious' thrust.

Pick 'n' Mix on the 'Typical' engine

Right, well, I THINK this about wraps up the article; we've looked at what power is, and what it's used for; we've discovered how it get's 'translated' from pressure in the cylinder to 'thrust' at the rear wheels; how the 'power curve' gives us some idea of the engine's 'charecter', and the two extremes of the range of character engines might have;

How 'sloggers' tend to be better for heavy vehicles or hauling heavy loads, and are more responsive to being tuned for economy; while screamers are more suitable for lighter or higher performance applications, and are more responsive to being tuned for performance rather than economy.

BUT; between where the power originates inside the engine, there is an awful lot of gubbins that can influence it on it's way to where it's seen and used as thrust at the driven wheels; and how 'tuning' the engine can make a marked difference to how it actually 'works' in the real world in a real vehicle. THIS is where I started; saying:-

The engine is a pretty complicated widget, but at the end of the day, what counts is that it shoves your vehicle along. A lot of the myths, legend and lore about engines stem from observation or experience of how vehicles behave, and people then trying to attribute that behaviour or 'character' to facets of the engine.

Thing is, that, there is so much going on, and when you are experiencing or observing the 'engine' second hand, from how it makes a vehicle behave, there is a lot of room for confusion or error.'

So, lets have another look at our 'typical' engine, and mess about with it; lets say that we 'tuned' it to be more for economy, with heavier flywheel, and restrictive carburettor, small ports and that kind of thing; on the road, would feel more like a 'slogger'; conversely, open it up a bit, fit a less restrictive and more responsive carburettor, lighten the fly wheel, and it would behave more like a screamer.

Permutations are myriad; and common; and quite often you get contradictory tuning practices going on, so an engine might be given a low reciprocating inertia, and tuned to rev out a bit, like a screamer, BUT get fitted with economy type carburettors, holding it back a bit, to make it more 'grunty' low down, and a bit less frenetic.

Which is where the lore starts to break down and you have to look a bit harder at what is actually going on, and why, and NOT rely on lore, which for the large part, is derived of pretty big 'generalises', ideas because there is ALWAYS going to be a contradiction SOME-WHERE;

The 'Exception' to disprove the 'Lore'

So, wrapping it up; hopefully you'll have a LOT better understanding of 'Power', and can now listen, and maybe even talk more confidently on the subject, and know when people are spouting rubbish, or when they are trying to tell you something useful.

And, taking it a bit further; you might like to have a look at the more detailed articles I've written on the subject; where I hope to explain why over head camshafts promote revs, and side valves don't, why 'methanol' isn't used as a 'high performance' fuel much any more, and other stuff like that!


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Footnote 1:-

I said, I wanted to help you explore the 'Science' of motive power; trying to précis this article down to something manageable, I have constantly been having to revise references to scientific theory or laws, and it's got so tedious, I have simply expunged them all!

Most of the principles, theories, and stuff that have provided the definitions and equations and stuff I've proffered as scientific 'fact' comes from a VERY large set of books called 'The Principia', written nearly five hundred years ago by a chap called  Sir Isaac Newton, which remains to this day pretty much the ten commandments for engineers.

'Newtonian Physics' isn't the be-all and end-all; a lot of it is built on earlier works, notably the ancient Greek, Archimedes, who provided a lot of the fundamentals of mechanics. Later scientists have challenged a lot of Newtonian Wisdom, and if not proved him 'wrong' at least suggested his ideas might be a bit 'simplistic', particularly one Albert Einstein!

Any way, MUCH as I love these little bits of scientific semantics, and find a lot of the debates fascinating....... I've had to concede that THIS article is NOT the place to expound upon them. So, the references have gone, and you'll have to take my word for things, or go google Newton & Co!

Another difficulty I have had in compiling this article was trying to make it as accessible as possible to a wide audience of people who may or may not have a vast existing understanding or vehicles and their mechanics.

So I have tried as best I can NOT to get bogged  in explaining how engines or cars or motorbikes actually work, or trying to clarify where different vehicles or engines may exist or throw up anomalies.

In the 'Theory' Section of the site, "Part B - How things Work", contains a number of articles describing the technology of vehicles. If you are not familiar with engines or how they work,  the following articles are worthwhile back-ground or complimentary reading:-

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Footnote 2

From Conversion Factors; 1 BHP = 0.754Kw or 1 Kw = 1.34BHP

From Conversion Factors; 1 revolution = 2 x Pi Radians; 1 minute = 60 seconds, hence; 1 rpm = (2 x 3.142) / 60

Hence;  1 rpm = 0.1047 rads/s or 1 rads/s = 9.55 rpm

An Imperial Ton, is approximate the same as a Metric Tonne.

From Conversion Factors: 1 Inch = 25.4mm


4th 'Gear' has 1:1 ratio. The input shaft of the gearbox turns once for each turn of the crankshaft, and so then will the output shaft

The Transfer box, in High Range, has a Reduction Ratio of 1.148, which means that it provides ONE turn of the output for one turn of the input. Turning that upside down gives a ratio of 1:0.871, or 0.871 turns of the output shaft for one turn of the input.

The differential crown wheel, has a reduction ratio of 4.7:1, so 4.7 turns of the input shaft, or prop are needed to turn the crown wheel one turn. again, turning that upside down, gives a ratio of 1:0.212, or 0.212 turns of the diff, for each turn of the prop.

Having got the individual ratio's they can then be multiplied out to work out the number of turns of the diff, or wheels, for each turn of the crankshaft, or the number of crank shaft turns needed to get one turn of the wheel; 0.871 x 0.212 = 0.185, or 0.185 turns of the wheel per crank revolution, or turned upside down, 5.4 turns of the crank per revolution of the wheel.

Topic is covered in greater detail in The Gear Box - the basics of a 'gear ratio', and working from there through the various mechanisms used to provide different ratio's.

I have mentioned the SI standard of units and measures; this is the generic reference for scientific and industrial research and engineering, and the definitive publication in which all the units are defined and ascribed their unique symbols and stuff is maintained by the International standards Organisation, or ISO, but don't ask me the publication number or title, I'm not THAT much of an anorak!

Abridged versions of the SI system, are variously published on the internet, usually covering those units and measures relevant to certain subjects. Various conversion tables, and calculators are similarly common on the 'net, though the conversion factors I used were taken from the Haynes Manual for the Land Rover SIII Petrol, or that bit of the academic crib sheets that stayed lodged in my head after college!.

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Footnote 3

A bit of Semantics on the subject of weight; it's another term that common usage has adopted confusingly. Go into the supermarket and they mark packets with the weight of the contents; since the EEC has told them they have to mark everything in metric units, I now have to buy 250g of Tomatoes, rather than half a pound! Thing is, that 250g is NOT the 'Weight', it is the 'Mass' of the tomatoes......

'Weight' strictly is a force, remember Force = Mass x Acceleration, but specifically a force due to the acceleration due to gravity.

On earth, gravity is pretty much constant. (though I do suspect it may have surges and spikes which are the things that make me fall off when I'm competing on my trials bike, and account for those strange happenings like bottles that fall out of the fridge when you open the door and stuff!) So in most peoples experience weight and mass are entirely proportional and common usage has confused the terms, which generally doesn't matter much, as the relationship remains the same, it just gets confusing when scientists come along!

Hence my packet of Tomatoes claims that the 'Weight' is 250g, when in fact it is the MASS that is 250g, the WEIGHT would be 250g times acceleration due to gravity, 9.81m/s2, or near enough, 2.5KN.

Not a problem when buying tomatoes, I know what I'm buying, they know what they are selling, good enough; BUT, when it comes to doing maths with it, not that helpful when you have to 'correct' the quoted terms to actually get the weight, rather than the mass!

Just as a bit of interest; also explains the business of 'weightlessness' and how you 'loose weight' if you are on the moon.

In space, the earth's gravitational pull is negligible; its still there, it just gets less significant the further away from the planet you are, and the pull of other celestial objects becomes more significant, but in other directions, net effect being that you don't ACTUALLY break free of 'gravity' it's just that you are being tugged in all different directions by so many gravities that none of them get to pull you in one specific direction, like the earth does as long as you are within a couple of miles of its surface!

On the moon, the gravitational pull it exerts on its surface, is approximately one third that earth affords; so do the maths and a 'mass' of 100Kg, on earth has a 'Weight' of about 1000KN, on the moon, 100Kg of 'Mass' would weigh approximately 333Kn.

'Mass' the bulk of 'stuff'; molecules, atoms, vacant spaces between the ears; hasn't changed, only the strength of gravity!

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Footnote 4

The example of the Norton 'Manx' vs Honda 'Six' came from my first version of this article, written about 2003, when it was simply called 'Power Theory'. The article has been revised, edited, and re-written and re-titled, numerous times since then, but this comparison was one bit I really wanted to keep.

Comment on the 'Manx' Norton being 'rudimentary' was made with a little 'Poetic Licence'; as is it's presentation as having an engine 'typical' of a 'Slogger'.

It actually boasted a couple of 'relatively' advanced features for it's era; first, it had bevel driven over-head-cam valve actuation; and had pretty 'compact' bore & stroke dimensions, that in many guises, (The bike was produced for over fifteen years with annual specification changes), were actually 'Over-Square'.

These 'features', are more commonly associated with 'screamer' type engines, and, for the era it was produced and compared to it's peers, the Manx's peak power at about 6,500rpm, with not very much beneath 4,500rpm, would have made it a 'screamer' and, comparatively, a bit 'peaky'.

I could have sited a more 'typical' example of a 'slogger' engine, by considering a single cylinder road bike of the same era; and to mind come such machines as the 'Enfield Bullet'; which proved quite 'useful' in Trials competition, where slow speed control is far more important than speed. Or the Phelon & Moore, 'Panther', a machine that SO typified the 'slogger' type that it was so described even against other 'slogger' type engines!

But, these were not road racing machines, and I wanted to illustrate the differences in 'character' of two machines, pressed into service in the same environment.

I did consider the example of a BSA Gold-Star, as that machine was a little more 'typical' in some ways, it didn't have over-head-cam operated valves, and was incredibly successful in all manner of competition, from trials to scrambles to road racing. But, because of that, I decided it would probably throw up more questions than it answered. But, watch this space, I'm sure that will give me opportunity to use it as example for something else later!

To offer a bit more about the comparison then; the 'Manx', though intended as a dedicated 'competition' model, was a 'production' motorcycle in that paying public could order one in the dealers.

Most 'Over The Counter' examples, were actually supplied in 'International' specification; but that was the same bike, just with the minimum necessary equipment to meet the, then, fairly relaxed 'construction and use' regulations for use on public roads. And Norton described the bike in sales literature, as a 'machine for the dedicated enthusiast'.

It is a little anomalous then, because it wasn't 'quite' a 'works' racer, or 'factory special'; but it wasn't 'quite' a true 'production' machine either. It would probably be best to describe it as what would, these days, be called a 'Homologation-Special', but either way, it was a bike that was available to the general public, and had to make some compromises to what that demanded.

The Honda 'Six', I commented, was as mechanical marvel, with six cylinders, over-head camshafts, and multiple valves per cylinder, that demanded an awful lot of maintenance and attention to keep it all working properly, and an incredibly talented rider to get the best out of it.

But it was a works machine, a  'Factory Special'. They only made a hand full of examples, expressly for their contracted works riders; they were NOT available to the general public, and not expected to be maintained or ridden by any-one not employed by the factory.

The Manx, in comparison to say, an Arial 'Red Hunter', a mildly 'sporting' mid '50's single; intended for road use, was certainly 'advanced' in its design, and a lot more demanding of both maintenance and rider skill.

Basically, it's all relative, and very subjective, as I say later in the text on the topic.

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Footnote 5

One of the reasons I REALLY wanted to include the example of the 'Manx' Norton in this article, was this idea that you could actually hear each individual power stroke, and the legend of the Manx's firing every lamp post is one of those quaint bits of 'lore' that has always fascinated me.

It's a direct observation, that's become legend; this article is all about 'testing' the legend, so is an ideal opportunity to do a little analysis and see whether the theory and science would support the suggestion, or whether its just one of those things that has been exaggerated SO much over the years as it's just utter tosh!

In my original iteration of this article, I looked at the matter in the main text, where I was comparing the Manx Norton to the Honda 'Six'; but, on revising, trying to get such a lengthy and tortoise article to 'flow' and not get too bogged down with superfluous detail and example, I've had to relegate looking at the 'firing every lamp post' legend to the footnotes.

So, to get on with it; The Max Norton had a low revving single cylinder engine, at least in comparison to more exotic racers of its era, like the Gillera 'Four'. All gets a bit woolly from here; they made the 'Manx' for so long, that the actual specs of earlier bikes are a lot different from those of later ones; and in between times, actual race machines were built using a 'Pick & Mix' of available parts to give different states of tune, and different gear ratio's and stuff to suit different riders and different courses etc.

But putting some numbers to it, it peaked at about 7,500rpm, coming 'on the cam' at about 5,000 rpm, and making it's 'peak' power around 6,500rpm,; while the top speed, depending on gearing, and rider imposed rev-limit, was between 110 & 130mph.

OK, first of all, that "you could actually hear each individual power stroke"

The range of sound frequencies, audible to the human ear are around 22Hz to 22Khz, the lower 'bass' frequencies being the ones we can pick out more easily as actual 'bangs' or thumps, the higher 'treble' frequencies, being ones we hear as tones or 'notes'.

So, to be able to hear, or perceive we could hear, an individual 'bang' going on in the cylinder, it would HAVE to be well down in the lower frequency range, and be happening at the sort of frequency that a drummer would rattle out in a piece of music.

A machine gun will have a rate of fire something like 200 rounds per minute, something like 3 rounds a second. Not going to start looking up lots of data, its in THAT sort of order. We don't QUITE hear the report of each round leaving the barrel of the gun, what we here is the 'clatter' of one report rapidly followed by the next.

That is a frequency of just 3hz, well out of the lower spectrum of the audible frequency, so when we 'hear' the report of a gun being fired, we are actually hearing the 'over tones' or higher frequencies of air disruption created when the bullet leaves the barrel.

It's the same sort of 'percussive' frequency as a drummer in a music band, who could tap out the rhythm at up to maybe ten beats a second; and again, where we might be able to pick out the thump of his bass drum once a bar or so, we don't often hear each tap on the snare, just the 'beat' of one tap over laid on the harmonics of the last.

Our Manx Norton, has a four stroke engine, with just one cylinder. So it will make one 'bang' in it, every other crank revolution. Turning, for easy reckoning at 6,000rpm, it will make 100 bangs a second, or the bangs are at a frequency of 100Hz.

That's well within the range of audible frequencies, but from the example of the machine gun or music drummer, FAR to high a frequency to be able to actually distinguish individual power strokes.

Like the machine gun, we would hear the report of one bang so quickly followed by the report of the next, that what we would actually hear would be a 'clatter' or low frequency 'tone'.....

AND; it gets more convoluted; because, the noise of an engine is coming from at least two different sources; there is the 'bang' going on in the cylinder itself, there is the 'report' from the exhaust, as that 'bang' eventually gets discharged from the engine into the atmosphere, which depending on the 'harmonics' of the exhaust, will certainly be after some delay from the actual 'bang' itself.

So what COULD explain people's 'observation'? Is it PURE exaggeration, or could some-one have heard SOMETHING, that could easily be presumed to be an individual power pulse?

Tough one, that; but, err..... yeah, there's PROBABLY something to it; the frequency of power pulses is certainly down in the lower bass spectrum, not the 0 - 10 beats a second we could EASILY distinguish individual pulses, but not too far off.

What would the bike 'tick-over' at? It apparently didn't LIKE to tick over; but, I guess it must have, if somewhat roughly. Would have been under 1000rpm; lets say for easy maths, 600rpm.

Do the sums again, and Whey-Hey! YES, the frequency of power pulses WOULD be down to around 10 beats a second; the sort of frequency of a music drummer......

So, a spectator might not have been able to hear individual power strokes as one came over Bray Hill at 80mph, turning 6000rpm in top BUT, stat in the grand-stands in Douglas, watching the bikes waiting for the marshal to flag them away, yes, possibly COULD have been ticking over low enough to pick out each firing. But the frequency would still be a LITTLE bit high to genuinely claim to be able to hear each power stroke.

Though, if their reputation for not liking to idle was true, and there's little reason to believe it wasn't, a 'rough' idle, would have the engine 'hesitating' or mis-firing a little, and those irregularities could certainly have been infrequent enough to individually distinguish.

Another thought, is that in the era of the 'Manx', race regulations called for what is known as a 'dead engine start'. Basically, the machine and rider line up on the starting grid, the rider can stand beside his machine or astride it, BUT the engine must NOT be started until the starter's flag has dropped.

It was an impediment contrived to make the machines a bit more 'competitive' and representative of road-going motorcycles. Ie; the rider had to prove he could start the machine unaided as part of the competition.

If the bikes came to the line already started, they could have really high compression ratio's and 'all or nothing' carburetion, that would see them only start with the aid of a a tow up to 30 or 40mph or so, on 'rollers', driven by a convenient race transporter, or other auxiliary 'device'.

Which, as an aside, was frequently the long suffering mechanics, like wot I was, once upon a time! LOTS of fun and games were caused by that particular 'rule'.

Bikes that didn't like to start from cold, would have to be properly warmed up before being presented to the line. The job delegated to the mechanics, who'd be running around the paddock like marauding wilder beast on a cold march morning, spraying ether down the bell-mouths or putting blow lamps to spark plugs, or cylinder heads, or even up exhaust ports, until the thing finally coughed into life!

Bikes that didn't like to start when hot, were even worse; and you could GUARANTEE that if your bike was one of them, there would be a first lap crash and a delayed restart, ten minutes later, giving maybe five minutes for the mechanics to try and get the darn thing cooled off; full oil changes and flushing lots and lots of fuel through the carbs after parking the bike in the windiest spot in the paddock was normally the order of the day in that situation!

Any way; on the line, rider would most often be stood next to his bike, in a sprinter's crouch, hands out stretched on the handlebars; second or third gear engaged, as the mechanical advantage of the gearbox works backwards when 'bump' starting, clutch in, and when the flag drops, wild head down run, then, when a bit of momentum had been achieved, a bounce into the saddle, the clutch dropped with a bit of deft timing as the suspension 'loaded' from the rider's backside landing on the saddle so the rear wheel didn't skid, and......

With a bit of luck, the bike would splutter into life..... and with the throttle opened to get it to carburate cleanly, the rider would haul the clutch in again, and try and find a more suitable gear, before starting for 'proper'!...... Or at least that was the 'Theory'!

Wasn't so bad at the Island where there would only be two bikes starting off at a time, but in circuit racing, with a 'massed start', maybe thirty riders all legging it, head down, and jumping about onto bikes that might or might NOT actually 'catch'..... Well, you can imagine the chaos! Which is why they don't do it any more as far as I know!

Any way, that little performance, as the bikes were convinced to 'catch', or the rider was desperately slipping the clutch and fiddling with the throttle, trying to get the bike to carburate as he tried to take it off load and not stall at such low revs, could see these things popping and farting and sputtering like a the major told his teenage daughter was up the duff by the regimental drummer boy!

To wit; an irregular engine 'beat', with the occasional decent power stroke quite discernable and distinct from the fluffs and wuffs of misfires as more charge disappeared down open exhaust valves during their long 'over-lap' at such low engine speeds, than was ignited in the pot, and quite possibly those few full firings, being every lamp post.... perhaps

But there is another possible reason for the observation, and that is the exhaust. Most of the noise an engine makes isn't the actual bang inside the engine, its the 'report' of that bang as the spent, high pressure gasses leave the exhaust pipe.

The Manx, was normally run without a silencer, so first of all, it would have been rather loud; but, common for the era was the use of megaphone exhausts.

Basically, the megaphone was a tapered cone on the end of the exhaust pipe; worked a bit like the 'horn' on musical instrument, or the 'megaphone' used by a rowing cox or sporting coach, amplifying the sound coming out of it, making the exhaust EVEN louder!

But, helped engines make a little more power, because as the exhaust gas rushing down the pipe hit the megaphone, it had to expand to fill the section of pipe; which meant it slowed down, which meant that it caused a bit of a pressure drop behind it, helping to 'suck' more exhaust gas out of the cylinder.

Later refinement of the idea, and still within the era of the 'Manx' was the 'inverted' Megaphone, which has a second, shorter cone on the end of the first, that had the effect of doing the opposite, and after causing a low pressure wave to suck exhaust gasses out of the cylinder, it then caused a high pressure wave to stop fresh charge escaping, during the period of 'valve over lap'.

Its all to do with 'Harmonic Tuning', which I think I'll eventually explain in detail in Tuning & Super Tuning.

Interesting to note though; picture of the 'Manx' shown in the text, sees it actually fitted with an 'Inverted Megaphone' exhaust; The Honda 6 , in the picture looks to have plain megaphone exhausts, with a much longer taper, and no 'inverted' section on the end.

Any way, one possible effect of those 'harmonics' could easily have been, that at certain rpm, most likely some simple fraction of where peak power was made, 'harmonics' conspired of the positive and negative pulses caused by the exhaust shape, to significantly amplify one in say, four, eight or even sixteen, pulses......  

Or, as the engine was accelerated through the rev-range, an amplified 'bang' would be heard at each 'harmonic' frequency that the exhaust was tuned to; so you'd get a bigger 'pop' from the pipe as the engine reached, say, 1200rpm, 1800rpm, 2400rpm, etc, every engine speed divisible by 600, or whatever the 'harmonic' was.

Those amplified 'bangs' from the exhaust then, could easily be down in the sort of frequency range where they would be discernable as individual 'events', and be easily mistaken for individual power strokes inside the engine.

All right then, so we probably couldn't QUITE tell individual power strokes, even at tick over, engine speeds. We MIGHT have been able to pick out the 'wuff' on certain strokes of a rough idle, or the cracks provided by exhaust harmonics, so in this case, the lore is most likely to have come from observations of something wrongly attributed to something else, and even then, probably subjected to a bit of exaggeration.

So, what about the 'firing every lamp-post' notion?

OK, well, data I have for one example of a Manx, gives a speed of 90mph for 5000rpm in top. Not too accurate, as mentioned, gearing was often varied. But, gives us a 'ball-park' to work in.

We want to know, how far apart in distance on the road, engine firings would be, so, 90mph, is 1.5miles a minute, or we'd be doing about 18" per crank revolution.............

NO! They did NOT fire every lamp post!

In top gear, where they would have covered the most distance between firings, they would have fired about once every yard; my street has lamp posts about every 50 yards or so on either side of the street, so that they are staggered, giving a lamp post about every 25 yards.....

Dang this 'Science' stuff! I LIKED that legend! Which is probably another reason a lot of legends and lore linger; people actually like it; even if it ISN'T very true!

Could there be anything behind it? any explanation for why any-one might mistakenly believe it?

Plenty; as the look at hearing individual power strokes; if the 'perceived' frequency of firing strokes is 10 times or more lower than the actual frequency of firing strokes, then that 'report' or noise that they are hearing and believing to be a power stroke, would be occurring not every three feet, but, every thirty or fifty feet....... THAT is far more a reasonable distance to have between lamp posts, isn't it?

There's also a quirk in that racing on the Isle of Man, takes place on public roads, public roads with shops, pubs and houses fronting them, and its only in those 'built up' parts of the course, that the streets actually have lamp posts, or at least would have HAD lamp posts back in the days the Manx was racing there.

So we might presume that any observer who made this remark was standing in one of the built up parts of the course. In which case, the noise that observer heard could easily have been being reflected back at them off buildings. Buildings have gaps between them, the roads don't always run exactly parallel to the buildings, there could have been any number of echo or harmonics effects modifying the sound heard.......

ASIDE to Footnote:-

Cant really put a footnote to a footnote; so I'm going to plough on with it; The Isle of Man TT races, if you have never been, are DEFINITELY something to experience. If I can find it, I'll add a link to the Isle of Man TT Races official Web-site.

The meeting lasts for a fortnight, split into 'Practice Week' and 'Race Week'; with the roads that constitute the course closed to traffic on alternate days, to allow races. They aren't closed all day, marshals run round moving barriers between races so that you can get around the island in the breaks between them, which sort of implies it's NOT the non-stop action packed 'event' of say a Grand Prix on a closed circuit.

But it IS; there is SOMETHING going on on the island all the time; on non race days there are beach races for moto-cross bikes, trails competition, and at Ramsey, a 'run what y'brung' drag race any one can take part in. And that's just the 'competition' events; there's also custom & classic bike displays or concourse events, as well as various parades or cavalcades, going on around the island whenever there's no racing scheduled.

And, beyond THAT, there's all manner of meets and parties, and THINGS going on, and if THAT isn't enough, the array of bikes of all kinds down on Douglas Prom on an evening is a spectacle in its own right; and there'll be more than the odd loon trying to show off their latest stunt along the sea front!

All in it IS an amazing experience, but the racing itself, on real roads, is quite unique.

It's not strictly a 'race' for a start, it's a 'Time Trial, 'competitors start at timed intervals in Douglas, and are notionally racing against the clock, so you don't ACTUALLY know who's won a race until after the last competitor has crossed the finish line, and the results collated.

(It's not called the TT, because it's a 'Time Trial', though, The TT stands for 'Tourist Trophy', which was the prize offered to competitors taking part. First convened in 1907, there was no international 'championships', so to attract the best riders and machines from around the world, the prize was offered to 'Tourists', or visiting competitors; basically entry was not restricted to members of the organising club.)

But, the course is what makes it so demanding. To begin with, it's 37 1/2 miles long, compared to maybe two or three for a typical 'closed circuit' track, and where at a GP, they might race for thirty or so laps, over 45minutes or an hour, on the island, they'll race for perhaps five laps, taking maybe an hour and a half to complete..

And because the course is so long, its varied, difficult to master, with little repetition, and its a harsh mistress. No wide, graded, super grippy sports compound surface, wide run off areas and gravel traps. The island is on REAL roads, with pot-holes, cambers, kerb-stones, and lamp-posts! It's a narrow, and with little margin to get it wrong.

And spectators, stand on the pavement; sit on garden walls, or watch from the hedgerows, with little but a straw bale wall between them and the racing; its up close and personal, with the noise echoing of the shop fronts, the smell of Castrol-R, warm in the nostrils, and the wind of the bikes passing ruffling your hair!

And as I write this, (November 2007), I am STILL smarting, that I DIDN'T manage to make it to this years, 'centenary' calibration, marking the 100th anniversary of the first event; a bugger this being disabled lark! But, hey, as they say in racing, 'there's always NEXT year!

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Footnote 6

It's co-incident, that whilst working on this latest revision (November 2007), that sat on my coffee table are some old magazines I dug out of the attic a while back and have been browsing through in what we might refer to in polite company as 'personal time'.

On the cover of one of them; 'Classic Bike' August 1992, is a picture of a Manx Norton, with the title, "The Real Thing"

It's a feature article, road-testing NOT Norton's seminal competition motorcycle, but a modern reproduction made in small numbers, as faithfully as possible to the factory blue prints. A feat made possible by most of the parts being still available to support machines in classic competition. (Or at least in 1992, they were!)

Interesting thing about the actual article in that 1992 edition of Classic Bike, though was their conclusion of the road test.

The 'Manx', had a formidable reputation as an uncompromising sporting machine; it was criticised for a cramped riding position, restricted steering lock, difficult starting, a fierce and heavy clutch, gears that were too high for slow speed riding, and carburetion that saw the machine refuse to idle, and not happy unless the throttle was wide open!

This sort of denies my comments or presumptions that the 'Manx' was the 'soft-slogger' I suggested, and that it had a broad tractable 'power band'........ (See comments above in footnote 5)

Thing is; that 1992 Classic-Bike article, tested the bike on real public roads, in the Cotswolds, in fact. And OK, the bike was a reproduction, but built to Factory drawings; it wasn't built by stuffing a lot of modern electronic ignition and fuel injection in or around old engine cases, it was, pretty much, a 'Manx,' as you could have bought back in the 1950's, down to the Amal GP carb responsible for that erratic idle and rough running on anything less than 'wide open'.

Yet; they reckoned that it wasn't THAT bad; it DID have a few quirks and foibles, BUT, it could trickle down Camden high street, dodging tourists walking out of the Edinburgh Woollen Mill shop, or whatever.

So, yeah, actually, you probably COULD stick your granny on a 'Manx', provided the rear set foot-pegs didn't trouble her arthritis too much!

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Footnote 7

A little 'off-topic' chat about the 'clique' of engineers in & around Coventry, in the Hey Day of the British Motor Industry, with a LITTLE comment about porting!

Some where in my collection of 'Junk' is an Austin A Series engine cylinder head, that was a candidate for use on my Challenge Metro street racer. It came with a lot of the usual kind of reported 'providence' that it had been cut by 'el maestro' Les Ryder, and that that meant it was simply the BEST A-Series cylinder head, like EVER! Yeah, right!

Never got to try it out though; offered it up, and the combustion chambers had been cut to suit a 1275GT block bored to 1430, and give a 12:1 compression ratio. I was running a 1275 A+ block, so the combustion chambers over hung the cylinder walls; BUT, mentioning the thing, did raise a few eye-brows, and I did discover a few things about this chap, Les Ryder.

It was in the days before flow benches, when cylinder head porting was mainly done 'by eye'; but apparently this chap Les Ryder was GOOD, and got results by eye that few could match even WITH the aid of a calibrated flow bench, he had 'the touch'.

After being introduced to the name in association with a mini cylinder head, no-one in the world of hot street cars knew anything about him; BUT, I had a mate who with his Dad built and races Ford grass trackers; mostly Mk1 & 2 escorts, though the lad drove a 2.0l Capri on the road built from a scrapper with 'left over' parts from the competition bin!

When I mentioned Les Ryder's name to his Dad, his eyes lit up, and I was provided with the nugget that the chap had a reputation in the '70's & early '80's as THE man to go for for a 'blue print' cylinder head for a formula ford 1600 cross-flow or 2.0l pinto engine, for Formula Ford open wheeler racing, or 'standard saloons' for circuit or rally use. That he worked out of a shed in the back garden of his terraced house in Stoke, Coventry, and only did a hand full of heads a year.

But, it was in association with motorbikes that I started to discover something a bit more interesting. The 'Heron' company was a chain of petrol stations, in the days before the petrol majors franchised them all, and it was founded in my home town of Stratford upon Avon, with the first filling station on the Birmingham Road, next to, 'Heron House' but on the other side of the canal!

In the 1970's, Heron took on the Suzuki Motorcycle Import Franchise, and Barry Sheene's two world championships on the Suzuki 'square four', were won whilst contracted to 'Heron Suzuki'.

Any way; talking 'bikes' as you do; chap was telling me about his Moto-Morini 3 1/2; lovely little Italian bike of late '70's vintage, and a V-Twin, like a Ducati or Moto-Guzzi; but unlike those it was a 45 Degree longitudinal one, like a Harley, with push rod operated valves, but in a decidedly 'cafe racer' frame, and displacing a mere 350cc's. Lairy little things though, and for its day, pretty quick; a good one could match a Yamaha RD350LC two stroke for speed, and with an enthusiastic rider (they HAD to be to have a Morini!) aboard, would loose one in the twisties. A definite 'feat' given that a 350LC had the power to match 550cc four stroke 'fours' of the era, and unlike them, was considered to have 'handling'!

The 'feature' of the 3&1/2 that made it a bit special though, was it's 'Heron Heads', that had 'twin swirl combustion chambers'.

These were apparently the subject of a hotly contested patent dispute; Heron having apparently sold the design to both Moto-Morini and Suzuki, who used it on the last GSX's, and in the early GSXR's as 'TSCC' (Twin Swirl Combustion Chamber!), but with interest also owned by one Harry Weslake.

Historical Note time; Sir William Lyons was the founder of the Jaguar car marque. Harry Weslake was, for many years, his chief engineer. Legend has it, that Lyons & Weslake penned the renowned Jaguar 'straight six' on the roof of the Jaguar factory at Browns Lane in Coventry, whilst on air raid watch! Any way, later, Harry Weslake was to design a 'twin swirl' combustion chamber for the Jaguar six, and the later V12.

Now, another engine of renown is the Chrysler 'Hemi', so called because it had hemispherical combustion chambers. This is supposed to be ALMOST as good as a perfectly spherical combustion chamber, in which the flame front from the ignition source (spark plug) radiates out, reaching the edges of the chamber at the same time, without meeting any disruption or intrusion.

Only problem with a spherical combustion chamber, is that the shape isn't conducive to a high compression ratio. To get a sphere small enough to get a decent compression ratio, it needs to have a diameter a LOT smaller than the bore of the cylinder, which sort of limits the space you have to put valves into; so making filling the sphere a bit difficult. a 'Hemi' then halves the combustion chamber volume, and so makes getting a decent compression ratio an awful lot easier; though most "hemi's" aren't strictly true "Hemi's" they don't have a full semi-circle of 'dome'. they use a crescent, like the top 1/2 of a hemisphere, for the same reason... but any way.

The Weslake 'Twin Swirl' then, was a bit clever, because it used a sort of kidney shape combustion chamber, around the two valves; this meant that a lot higher compression ratio could be achieved; but to get a nice clean combustion, the two zones were arranges so that they sort of mimicked a pair of hemi-spherical combustion chambers, with the sizing such that the flame front caused a two swirls of flame that sort of mixed the charge in the chambers aiding a clean combustion.

Any way, just down the road from Browns Lane, is Meriden, home of the post war Triumph motorcycle factory, up to the demise of the 'Meriden Co-Operative' about 1981.

This was where Les Ryder worked; originally one of Edward Turner's Apprentice Technicians, he worked under Bert Hopwood, in the engine shop.

Edward Turner and Bert Hopwood had both moved on; Hopwood ending up at BL in Long bridge and working in the Austin competition department on the 'Mini' I believe. Edward Turner, had moved around a bit too, but had I seem to recall 'retired', only to keep popping back up in the annals as a 'consultant' on various projects.

But, in the late 1960's, Turner was called back to Triumph in a consultancy capacity, to give his opinion on what should be done about the threat posed by the Japanese motorcycle manufacturers, and in particular, the Honda 750 'four'.

Legend and Lore suggests that it was actually Turner who was to 'blame' for the British manufacturers, of whom Triumph was the foremost, not responding to the challenge of the Honda 'four', in act the reports are a bit unfair; some ten years earlier he had actually pointed out the threat that they posed to BSA's management, and was ridiculed for his opinion!

In his capacity of consultant, though, he was responsible for the Triumph Trident / BSA Rocket 3 project; a triple cylinder machine, of 750cc's, that was basically the old Triumph 500cc 'speed Twin' Turner had first penned in 1938! with three cylinders instead of two! they called it a 'Bonnie & a half'!

To be fair, it was a suggestion that wasn't entirely Turners own, and he endorsed it as a 'stop gap' model, that they could easily develop, since they had been building speed twin's for thirty odd years! until they could get a new engine into production, that had overhead cam shafts, and a 'modular' design, that would allow a range of engines based on common components from 350cc up to 1000cc, i two, three and four cylinder capacities......

Curiously the very same concept, updated a little, and put into metal, co-incidentally by Ricardo for the 'reborn' Bloore industries, 'Hinckley' Triumph company in 1991. Ricardo, being a contemporary of Turner, who had designed a competitor to Norton's 'Manx' in the 1920's with four a four valve overhead cam cylinder head, affectionately known as 'the Ricky'!

Any way; Turner was called in on the Rocket 3/ Trident project; BUT, the lucrative American market needed something to keep the Triumph name in the headlines, and Triumph twins, at the time, were doing pretty well, in the USA where the AMA national championship favoured bikes that were good on dirt ovals as well as tarmac. Consequently, 'Something' was needed to give the old Bonneville a bit more competitive life.

Turner was 'consulted', but he had never been keen on tuning the twins, and had apparently baulked at the Speed-Twin being bored out from his original 500cc to 650, and even MORE unhappy about it being fitted with hot camshafts and twin carburettors to become the seminal Bonneville. So he made his excuses that the Tripple project was taking all his time and attention, and introduced.... Harry Weslake, to the team working on the twin.

Weslake, looked at the twin, apparently in the car-park of the Meriden Arms, just down the road from the factory, and over lunch, sketched out a twin swirl combustion chamber for it on the back of a bar-mat!

OH! Aren't these legends so 'kitch' - trouble is, I've attended far too many high level design meetings in pub lounges, and carried far to many detail drawings around on torn open fag packets to dismiss the notion as 'they just DON'T do that kind of thing', they do!

Any way; Weslake, 'consulted' and from it, a Twin Swirl Combustion Chamber cylinder head was cast for the Bonneville engine, and made in small numbers, released by the American Importers to favoured or promising racers in the AMA championship.

THAT little lot, shouldn't be 'news', it's reported in a lot of books and magazine articles about Triumph, and their 'hey-day'. and that 'legend' will tell you that the Bonneville 'Weslake' head is almost a holy grail for Trident builders.

So, onto the revelation of my 'digging'  about this Les Ryder mini head; Les Ryder was by then the engine shop supervisor. And the Weslake heads, for cost and expedience, were made to Weslake's drawings by the foundry, without any input from the factory.

When they arrived, Les, built one up into a development engine, and ran it up on the dyno, and was apparently singularly unimpressed. Apparently it made no more power than a mildly 'ported' standard head. Complaints, discussions, meetings and arguments followed, and ultimately, Ryder was told "just bloody use them!" Again, from my experience of design debates, that is probably NOT unlikely!

So, he did. But, first, he filled the combustion chambers with aluminium weld, filled the ports up with aluminium weld, and re-cut them to his own satisfaction, and something that looked NOTHING like what Weslake had drawn!

This was admitted to me independently by two different people, both ex-meriden of competition department, and both I came across through needing stuff done to my Cota trials bike!

One was a chap called Ian Booth, who augmented his pension by spoking wheels for competition & classic motorbikes and cars in his shed, at the bottom of his garden, just round the corner from Les Ryder in Stoke, Coventry!

The other, was a chap called Brian Ashe, who when I met him, was trying to eek a living building motorcycle grass-track frames, sidecar trials, and sidecar scrambles outfits in a workshop built under an old railway arch in Leamington Spa.

Yes; Ian straightened my wheels, and Brian straightened my cota frame for me!

Any way; Brian, apparently had been responsible for making the frames for the American dirt track Triumphs, and Ian had been one of the technicians that put the bikes together, and both reported how Les would work late into the night, when no-one was around, modifying the Weslake heads, before they got built into complete bikes and shipped to the 'states, where they won races with abandon.

And no-one would have been any the wiser, EXCEPT, 1972, BSA, who owned Triumph, went Bankrupt, and Les was one of the blokes who took redundancy. Brian, did likewise at the time of the Meriden Co-Operative, 1977, I think. I think Ian stuck it out to the last through the Co-Op years.

So, devoid of Les's talents, 'Westlake' Bonneville's were being built with standard Weslake heads, and unsurprisingly, the riders of these machines were NOT happy that they didn't go as well as they expected!

The US importer, wanted answers. actually, no, he wanted bikes that won races! So the matter was looked into. Harry Weslake, looked at the cylinder heads in question, and insisted that there was 'nothing wrong with them', they were, after all exactly to the drawings he had provided for them! And the matter would have remained a mystery, except, that while the factory and US importers were arguing, a couple of American racers, tried getting hold of Les, to 'help them out' like he had when he had worked for the factory.

What ensued, was Weslake heads were being despatched from the factory, to the USA, distributed to the selected riders, who then sent them BACK to Coventry, where Les, in his shed, filled the ports and chambers with aluminium, and re-cut them, and sent them back!

Apparently, originally, Les had taken heads from the factiory, weaved his magic on them, and taken them back to the factory for shipping, but during the shenanigan's at the factory, a Ryder head had popped up, that no-one was prepared to explain, and Harry Weslake had, "Hit the EFFING roof!" and issued strict instructions that Ryder was under NO CIRCUMSTANCES to be provided with Weslake heads either by the foundry OR the Triumph factory!

Enter Heron! Who on the demise of the BSA group, I believe bought up some of the 'intelectual property', which I think included the licences to the Weslake heads, that they sold first to Moto-Morini, then to Suzuki.

Only, Morini, were, for SOME strange reason NOT that impressed with them, and despatched some-one from Italy to Coventry to get them 'sorted'!

And Les Ryder was again tracked down, and entrusted with the 'development' of the 'Heron Head' for the Morini V-Twin.

According to Brian Ashe, he watched the Italian engineer etch the blue prints onto a solid block of blue Perspex to make a sort of 3-d image of the design, and had Les cut it into metal for him, THEN after Les had cut HIS version, copied THAT onto a block of blue perspex, from which he made paper drawings!

I never actually met Les Ryder himself, nor Harry Weslake, Bert Hopwood, sir William Lyons or any of the other 'Names' of the British automotive industry; but, I suppose it's NOT actually THAT surprising how many people I have met that did, or who witnessed if not were a part of that great era of the British industry.

I mean, Britain's 'Induistry' was centred around Coventry and South Birmingham; within 15 miles of where I have spent all but a few years of my life.

It's all very 'close' to me, personally. The people I grew up with, the people I have met have all had SOMETHING to do with the business, either first hand, or by association.

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Right power theory. A lot of 'Lore' is spouted on forums, in pubs and car-parks and placed like that about Power and Torque and carburettors and cam shafts and bore and stroke ratios and all that kind of thing.

Some of it is has some scientific basis. Some has some truth, but not for the alleged reasons, and a lot of it is complete rubbish. So to cut through all the 'lore' we have to go back to first principles. Basic physics.

So where to start. This is quite a task, and I've found creating this page pretty challenging, because its a huge and diverse subject, and there is so much opportunity for my to wander off on a tangent.

Any way, I think that the best place to start from is to look not at how power is made, but where its used. First, however, we need to define it.

Power = Work Done / Time

Which is a marginally useful equation, but I guess it means we need to define Work Done. So here goes.

Work Done = Force x Distance Moved

Right, this is getting more helpful. Forces, everything in physics seems to be about forces. We can find out a lot by looking at forces, so this could prove useful. Bit of algebra then and we get:-

Power = (Force x Distance Moved) / Time

This could be even more helpful, because, Distance / Time is speed, which basically means that power is a force multiplied by speed. Great.

Getting a bit more technical, there is another equation that defines Power.

Power = Torque x Revs

Actually, that is merely a derivative of the last equation, but applied to a rotating situation, rather than a simple linear one, but without getting into all sorts of complicated algebra with radii and angles and sines or cosines and stuff. The important thing to know though is that Torque is a force multiplied by an amount of leverage.

Torque = Force x Lever Length

Now, we know that a force is what makes something move. Isaac Newton gave us a whole load of laws about this of which his law of gravity was one. The important one to us right now is:-

"Every Action Has an Equal & Opposite Reaction"

And another equation for you coming from Newton's Laws, is:-

Force = Mass x Acceleration

OK. So lets start putting some of this into practice. We have a vehicle, it could be a Land Rover, it could be a Transit van, it could be a Hino tipper truck or a Ferrari sports car, doesn't matter. Its a box on wheels.

The vehicle is pushed forewords by a motive force generated by the engine and delivered to the wheels via the transmission. Resisting this motive force is 'Drag'. Drag is important, and we'll look at it in closer detail in a minute.

But for now, we have a box, there is a force pushing forwards, and another pushing back. OK, not too complicated, is it? If the motive force is bigger than the drag force, the box will accelerate. If the drag force is bigger than the motive force, then the vehicle slows down. If they are both equal, then the vehicle stays at a constant speed, or stays still. Got it?

Okey-dokey. Lets look at that with regard to power.

We have an engine that is providing the motive force. We get the force from burning fuel, in air in a cylinder. The heat released causes the unburned fuel and air and then the combustion, or exhaust, gasses, to expand. That expansion generates a pressure that pushes a piston down the cylinder. Mechanical bits and bobs then gather all that force and re-direct it through the transmission to the wheels where it's can shove the vehicle along.

So, we have a vehicle moving at constant speed, and that is subject to a constant drag force. The force we need to move the vehicle is equal to the drag. The power we need is equal to the drag, times the speed, yeah? Power = Force x Speed, right.

OK, so that's the power we need to keep the vehicle moving at that speed. So if we work back from the driving wheels, we have a motive force and a wheel speed.

If we know the wheel size, we can work out three things:

The constant Pi x Diameter = Circumference. So if we divide the vehicle's road speed by the circumference of the wheel (and tyre), we will get the rotational speed or rpm of the axle.
Torque is Force x Leverage. If we halve the diameter of the wheel (and tyre), we get the radius between the axle shaft and where the motive force is acting, so we can multiply the two to give us the torque on the shaft.
If we take the answers from the two sums above, we have the axle speed, and the torque on the axle. Multiply them together, and we have the power delivered by the engine.
But what does this actually tell us?

Well, not a lot. But its the first step in recognising what power is all about, because the important thing is force, as that is what actually makes things move, and power tells us how fast we can go for a given force.

I'll give you a little demonstration. We tend to work in Brake Horse Power, but standards scientific units are Watts, just like the power of a light bulb. So, conversion factor for you:-

1Kw = 1.34BHP or 1BHP = 0.745Kw

A bit off the point, but worth thinking about, as I've mentioned lamps. 1BHP is 0.745Kw or 745w. The usual rating for a headlamp bulb is 55w. Now allow 15w for your side lamps and tail lamps, and that gives you something like 170w used by your main lights. Add a pair of spot-lamps, or fog lamps, again with 55w bulbs and you will be using about 280w of electricity. Add windscreen wipers, and the heater fan to keep the window de-misted, and you can quite quickly use a horsepower of electrical equipment. Typical car alternator, is rated at something like 45Amps at 14volts. Electrically speaking, Power = Current x Voltage, so that is about 630w, That's over 3/4 of a BHP.

Any way, returning to the subject - we have a Land Rover, and it's making a pretty healthy 75BHP, that's 55Kw. It's making those Kw at a pretty heady crank speed of 4700rpm. Now with a bit of maths, that gives us a shaft torque of 110Nm.

OK, well this thing is a Series 3. That has a 1:1 drive in forth gear, and a 4.7:1 reduction in the rear differential, and is fitted with 33" wheels.

Now, because we have direct drive through the gearbox, there is no change of the ratio between speed and torque. I'm ignoring the transfer box for a minute and going straight to the diff. Here we have a 4.7:1 reduction ratio. That means that the prop-shaft has to turn 4.7 times to get the half shafts and wheels to turn once.

So, power will remain constant, as for the minute we'll ignore any transmission losses. So, Speed in = 4800rpm. Speed out = 4700/4.7 gives us 1000rpm. Now the power has stayed the same, so if the speed has been divided by 4.7, then the torque will be multiplied by 4.7. that means that the torque must increase from 110Nm to about 525Nm.

Now, the wheel is 33" in diameter. If we convert that to meters, we get 0.83m Multiply that by the constant Pi, we get the circumference, 2.6m.

So, our wheel is turning at 1000rpm, which means that the wheel will travel 1000 x 2.6m every minute, or 2.6Km. Multiply that by 60, gives us a speed of 158Km/h, or near enough 100mph.

Right, well that's the speed, what about the force? Well, the force is the shaft torque divided by the wheel diameter. We have 525Nm of shaft torque, and a wheel diameter of 0.83m. That gives us a possible motive force of 625N.

Now a couple of things might strike you here. The first is that 75bhp is pretty good for a Series 3 Land Rover. The second is that 100mph is pretty exceptional. And certainly not the sort of speed that most would expect to get from a 75bhp engine.

But, the thing to look at here is the motive force we can make. We have 625Nm or force, and to be honest, that's not a lot. But remember, I omitted to take any consideration of the transfer box. That gives a reduction in the order of 1.3:1 from the gearbox to the prop-shaft, so in reality, the speed would fall from 100mph to about 75mph, and the motive force would increase from 625N, to 815N

Which is important. If we go back to our box on wheels, the drag force is coming from friction and wind resistance. If we ignore friction for a second and look at wind resistance, what you have is pressure that builds up in front of the vehicle.

The faster you go, so the more pressure you have to over come. Pressure x Area equals a force right? So, if you have a fixed frontal area, then the faster you go, so the greater the drag force gets.

Friction is a bit less easy to quantify, and depends on a lot of things, like where its coming from, but essentially it follows the same rule. The faster you go, proportionally the more you have to over come.

Which means that there is an 'exponential' at work here. Quickly, the relationship is something like:-

We can use this later. Its probably not that accurate, but it serves for illustration. If you look the scale along the bottom tells you vehicle speed. The axis up the side tells you the drag you get at that speed. What you need to notice is that the relationship is a curve, and that if you double road speed, you more than double the drag force.

If you multiply speed by force, you get power, so we can actually use this to work out how much power we would need to achieve a certain speed.

But, if we were to then work that back through the torque on the wheel and the transmission to find out what power we needed the engine to make, and what revs, what we would find is that we don't have a suitable gear ratio to get the maximum speed for the power the engine could make, at the exact revs it makes it.

What we would probably find is something like that in third gear, we could get the thing up to maximum power, but the gearing would be too low. So the engine would be making more force than was needed to over come the drag, but not able to turn any faster.

If we then changed up to forth gear, then the gearing would be too high. Now the gear ratio would mean that at the maximum power engine speed, the road wheels would be trying to drive the vehicle along at a speed where the force it could make wouldn't over come the drag. So, the vehicle would accelerate up to some speed beneath its maximum power rpm, where the force it could generate at the wheels equalled the drag at that speed.

Which brings us neatly to the idea of the power curve. Its starting to get a bit technical now, and I think that I'm raising more questions than I'm answering. But, I said that I was going to start by trying to explain how power was used. And that is what this is all about. Basically the power the engine makes is used to over come drag, and that brings about this interplay between the power need to push the vehicle along verses the power you can deliver to the wheels.

So then, Power curves. I'm sure that the bit about gearing and matching engine speed to road speed has started raising some questions in your mind. So now its time to look at a bit about what goes on in the engine to make power, and compare the theory to practice.

OK, the power trace on the left is the sort of thing you'd expect from a typical kind of car engine.

You know, your sixteen hundred Sierra or whatever.

So, lets have a look around it.

Blue line is power, pink one is torque.

Scale on the left hand axis is power. Scale on the right hand axis is torque. In between on the bottom axis is the engine speed.

So, if you look up an engine speed, and go straight up until you reach the line, you can go left or right and read off the power or torque the engine makes at those revs.

Now, this example shows an engine that revs to 5000rpm.

If you try and find the peak of the blue line, you find that it makes about 90BHP, around about 4250rpm.

If you find the peak of the pink line, you'll find that it makes about 250Nm of torque at about 1750rpm.

If you were looking at a spec sheet, these would be the figures quoted. But more important than the peak figures, and the speeds they are made at, if they are provided, is the shape of the curves.

The shape of the power curve gives us some idea about the nature or character of the engine in the way it makes power, and how it will feel or behave in the car.

Any way, we said earlier that Power = Torque x Revs. And if you look at the curve you'll find that that relationship follows.

But, what is curious is the way that the power builds up in a kind of ramp, that plateau's off and then drops off.

If you think about it, what's happening is that the torque ramps up pretty quickly, but then trails off. So as both torque and revs increase, so power ramps quite steeply, but then as the torque trails off, and starts to decay, so the multiplication of the engine speed sort of compensates and keeps the power growing or constant, until the torque is so low that it cant keep up and it starts to drop off too.

Now, to understand what is happening, we have to have a look inside the engine. Basically its all to do with air flow, and cylinder pressure.

Another bit of physics for you, working back from torque. We have an engine. That engine is a piston, in a cylinder, connected to a crank-shaft, pushed down by the pressure of the burning gasses.

Right. The torque is a force times a lever. The lever we have is the crank shaft, or the engine's 'stroke'. The force, has to come from the cylinder pressure. Pressure times area = force. Right, speeding things along, Pressure times piston area gives us force, times stroke gives us the torque.

Which is Neat:-

Torque = Cylinder Pressure x Engine Capacity

And by if we multiply by speed, to work out the power, we get:-

Power = Cylinder Pressure x Engine Capacity x Engine Speed

So, the controlling thing here is cylinder pressure. Engine capacity doesn't change, and we are looking at the torque and power at any particular speed, so the thing we need to look at is pressure.

Right. To get cylinder pressure, you burn fuel in air. You need about a pin head sized drop of fuel in a pint pot sized volume of air to make a bang in the cylinder.

Now, we could get as much fuel into the cylinder as we wanted, but the biggest limitation is the air to burn it. Realistically, best we can hope to get is a volume equal to the volume of the cylinder, right?

Now, in truth, we'd be lucky to get that. There's actually not a lot of pressure difference between the suck in the engine and atmospheric pressure to drive the air through the small passageways into the cylinder, so in a 'normally aspirated' engine, we'd be doing well to get a complete fill on every induction stroke.

And, as we increase the engine speed we have to get that air in to the cylinder in a shorter time. Thinking about how the drag on a car moving through air increases exponentially with speed, if you kind of flip the logic you can see that the drag on moving air through a metal tube will also increase exponentially with speed.

Now, there are a few other factors at play here, but, look at the torque curve and find the peak. That is the engine speed where you are getting most cylinder pressure, hence you must be getting the most complete cylinder fill, after that, the speeds are increasing and the drag stopping enough air getting in fast enough to fill the cylinder properly, so you get less cylinder pressure, so less torque.

But what about the bit before hand? Well here there must be some other factor stopping the cylinder filling properly, as the engine speed is obviously low enough that the air should have sufficient time to fill the cylinder. And in fact, there are lots of contributing factors.

The first one to mention is a thing called 'scavenging'. Basically, when the power stroke ends, the exhaust valve opens and the residual pressure in the cylinder pushes the burned gasses into the exhaust pipe. Now, when the engine isn't running that fast, the gas has a lot of time to escape, so will kind of mosey out through the port in an unrushed manner, possibly lingering in the cylinder longer than needs be. This means that there will be more exhaust gas left in the cylinder when the inlet valve opens, and that will effectively limit the amount of space that can be occupied by fresh air.

But, in a similar manner, the fresh air, if it's not sucked in in a hurry, will also tend to dally, and that may have a similar limiting effect to the drag at higher speeds.

Now, please excuse me if this is a bit of an over simplification, but I dont want to get bogged down in talking about cam timing and port velocities and gas inertia right now. There is a lot more going on than gasses just lingering about like drunks not wanting to leave the pub at closing time, but for now the analogy will suffice, and you get the idea that the engine isn't working as well as it could at low speeds, gets better as it gets faster, then starts to drop off after that.

That explains the shape of the torque curve, and that in turn is explained by the amount of air that can get into the cylinder at a given speed.

So what if we could have an engine that made the same cylinder pressure all the time? Well basically, you could, but practically the only way to do it would be to restrict air flow into the engine down to the lowest common denominator, and accept a constant torque lower than the engine could make at peak, and what you would have is a horizontal line for the torque curve and a straight diagonal one for the power curve.

And it wouldn't actually be all that helpful. The force delivered to the rear wheels would be the same at every speed, but remember drag increases with speed, so we generally want more force the faster we go, so an imperfect torque curve isn't that big a disadvantage, and in fact could be an advantage.

Hmmm. Interpreting the power traces then. Now, we know that the engine torque is the thing that ultimately gets translated as the force at the wheels. The more force we have the more acceleration we can get, or the more work we can do.

But its a bit of a strange concept, because when we are driving, we feel speed and we feel acceleration, so when we describe the way things feel we tend to say things like 'you can feel the power coming in hard' or 'it just pulls'

And making it more complicated to figure out what is going on, we have a gear-box, that is giving us torque multiplication. I mean, in the lower gears it transfers the same power as it does in the higher ones, but by reducing the speed, it increases the torque.

So, from the drivers seat, it can feel like you have a really powerful engine when actually you don't, or vica-versa, depending on how the gears are spaced.

As an example, I had an old Montego 1600 estate. From the drivers seat, it felt really lively and quite sporting, and gave the impression of having a lot more power than it really did. The main reason being that the gears were quite closely spaced, and all but the fifth gear, actually giving quite a bit of reduction.

Overall, what this meant was that you could get quite a bit of force at the wheels, simply because the engine was spinning faster for a given road speed. It meant that you would change gears more often, to keep it the engine revs where the power was, but that just accentuated the effect, because it felt like you were really having to work the car hard, just like a sports car.

In comparison, I had a 2.0l Granada. Bigger engine, with a lot more power, but taller more widely spaced gears. Consequently, the force you felt at the wheels was a lot lower, and you couldn't work the gearbox to make it move, so it felt a lot more sluggish and lethargic.

OK, so the Montego is a lighter car, and there's a big expanse of body work about you in a Granada, but it illustrates how the installation and gearing can effect the way an engine feels.

What we are getting to is the difference between a 'slogger' and a 'screamer'. Now the best way to illustrate this is in relation to motorbikes. I know that this is a bit off topic, but motorbikes tend to exaggerate the subtleties of a lot of automotive dynamics.

Right. The power we need to go a certain speed in a certain vehicle is the drag force at that speed multiplied by the speed. To provide it we need an engine with a sufficient power rating, geared so that it is putting enough motive force to the road at that speed.

Power = Torque x Revs, and Torque = Cylinder Pressure x Cylinder Capacity. So, for the same size engine, we can get the desired motive force to the driving wheels either by having an engine not turning many rpm, making lots of cylinder pressure, and geared relatively high, OR we could have an engine that turns a lot of RPM, but doesn't make so much cylinder pressure, and just gear it lower. Yeah?

Right, in the first instance what you have is a 'Slogger'. The engine doesn't wind many RPM, but it's tuned to get good cylinder filling at the sort of rpm it does turn so it can make a big bang and give plenty of motive force, or 'torque'. It would be described as a 'Torquey' engine, or a 'Thimper'.

Back in the 1950's, Norton built a bike called 'The Manx'. I could critique this machine for hours, but essentially it was a pretty rudimentary machine, even by the standards of the day. Norton would have described it as 'Simple and Effective', though, and I guess that's reasonably apt.

Basically, it won races because it rarely broke. Even for the era it wasn't a fast machine, but it worked well as a package. It's simple single cylinder engine made it light and nimble, and relatively reliable. On the track, it didn't make huge amounts of power, or rev to heady heights. What it did was make a modest amount of power from a modest rpm limit, that gave the rider power he could use easily.

At the Isle of Man, during the TT races (from where 'the Manx' got it's name), spectators commented that coming through Douglas, on the closed streets, the Manx Norton's would be coming through at about seventy miles an hour and the engines revving so slowly, that you could almost hear each individual power stroke, and lore would have you believe that they fired every lamp post!

Quaint as that bit of lore may be, I don't actually think that it's wholly true. I mean, those bikes could get up to speeds of around 100mph or 160 Km/h. That's about 2.6Km/min. Given a 1:1 gearing to the rear wheel, that would be about 1500rpm. Realistically, a very tall overall gear ratio would give about a 3:1 reduction from crankshaft to rear wheel, so the engine speed that would drive them at 100mph would more likely be about 4500rpm.

Even so, that is still pretty low, and remember that these things only had one cylinder, so there would only be one bang every other revolution. That's, 2200 bangs a minute, or 32 bangs a second. Remember audible frequence is 22Hz to 22KHz. 1Hz being one 'cycle' a second. So a frequency of 32Hz is only just into the lower bass spectrum of the audible hearing range!

OK, bit of a diversion, but yes, we have a low revving engine, a 'Slogger', not making many revs, but geared incredibly high to get most effect from a very big low down torque peak.

So, at the other end of the spectrum, the 'screamer'. About ten years after the Manx Nortons, Honda went to the Isle of Man, and they took with them a motorcycle that was just simply outrageous for it's era. It was a 250cc machine, with six cylinders, and it revved to a reported 20,000rpm.

Now, the Honda 'Six' (or the 'Hailwood Six' to aficionados, as it was ridden to victory my the late great Mike 'the bike' Hailwood) was the complete opposite of the Manx Norton. It was the pinnacle of technology for the day. It was EXTREMELY complex and won races through being technically more advanced than the competition, and piloted by a great rider. Where the Nortons might have been treated to a new spark plug before each race, the Honda was completely stripped down and rebuilt, and where even average club riders could get results from a Norton, only the superstars could get the Honda to work well.

But, for the minute, we can forget the complexities and subtleties of the engine design, and just look at what it was like to ride, and how it behaved on the track.

The Norton, we said revved to about 4500, maybe 5000rpm. The Honda to four times that speed. The Norton's made do with three or four tall, wide spaced gears, but the Honda had six of them, all giving a lower final drive ratio, and all a lot closer together.

Now, here's something to explode a myth for you. The 'Power Band' this is the bit of the rev range where the engine's making serious and useful power. It's the plateau, or crest of the peak around maximum power in the power trace.

People will describe 'screamer' type engines as being 'peaky'. By that they mean that the 'power band' is relatively narrow, or that to keep the engine making good useful power you have to keep it spinning at close to the peak power RPM.

Well, here's a novel thought for you. The Manx Norton's revved from 0 to 5000rpm. Lets assume that they are the complete antithesis of a screamer and make useful power across the entire rev range. That gives them a power band 5000rpm wide.

Now lets look at the Honda 'Six'. That revved to 20,000rpm. The power came in hard at around 12,000rpm and it held most of that power to about 16,000rpm. That means that the 'power band' was only just a bit smaller than the Manx's at 4000rpm wide. BUT, the big difference was that while that may have been the 'sweet spot', there was still useable power from about 9000rpm, and the thing could be over revved to the 20,000rpm red line, so it had a 'useable' power band of 11,000 revs, more than twice as wide as the Norton's.

Trouble is, we don't 'Feel' the power band at the crank shaft. We feel it from where its used, at the driving wheels.

So, get on a Norton, and engage first gear, slowly release the clutch, and instantly, you feel the torque trying to push you along.

Get on the Honda, engage first gear, slowly release the clutch, and .... you stall.

Reason being that at or close to tick over, the Honda was hardly making any torque at all, and the force it could put to the rear wheel, even in first gear was less than the drag.

OK, lets start again. Get on the Honda, engage first gear, OPEN THE THROTTLE, and make the thing rev to 60,000rpm, and slowly engage the clutch.

Sorry, I should have said 'Slip' the clutch, and feed the power in. You haven't broken your neck have you? No good. It's OK, that bruise on your chin where the handle bars reared up will go down in a day or two. Would you like to have another go?

Right. Bit of satire, but you get the idea. You could stick your grand mother on a machine with the easy power delivery of the Norton. The Honda was a handful. If you got too few revs off the line or were in the wrong gear coming out of a corner, then it would bog, as you dropped into a portion of the rev range where it just simply didn't make enough motive force for the speed you wanted to do. Go the other way, and the gearing was such that you could easily be in to low a gear at to low a speed, and find it making such ferocious motive force from that gearing, that rather than pushing you forward, it would lift the front end and smack you on the chin.

So even though the engine technically had a wider power band, and more useable power, what you felt at the rear wheel was something sharp and unforgiving, or 'peaky'.

This, for years have given rise to the bit of law that says that screamer engines have no torque, they make power from revs. Actually, that's a fallacy, and they do make torque, often more than similarly sized engines reputed to have loads of the stuff, trouble is, that peak torque and peak power are a long way up the rev range, and usually close together, giving this very sharp feel to the way the power is delivered.

mean, here's a 'Screamer' type power curve. If you look, you'll see that in this particular instance, it's on the same scale as the 'typical' engine's power trace.

It still shows an engine that revs to 5000rpm. Peak torque is still 250Nm, but has been lifted up the rev range to a heady 4000rpm, rather than 1750.

And this has lifted peak power to nearly 140BHP, still at about 4250rpm.

But look at the shape in comparison to the 'Typical' trace beneath.

The power trace is 'peaky', the 'land' around peak power is only about what, 3000rpm to maybe 4500rpm.

In the car, it would feel like you had to keep it spinning to get any where, and you would want to work the gears hard to keep the engine spinning in this region.

But take note, the real difference is in the range from 0-2000rpm.

For the 'Typical' engine, the power rises quite steeply from 0-2000rpm, at which point its making about two thirds of its useful power, that it delivers in a nice broad plateau.

For the 'Screamer', up to about 3000rpm, the engine is only making about half the power that the typical engine can, but from there on, it takes off steeply, going on to make about half as much power again, as the 'typical' engine.

But what does this mean?

Well, it depends on the installation, but, obviously, the screamer engine needs to be revved above 3000 rpm just to make the same sort of power that the typical one does.

On the road, that would mean that to maintain the same kind of acceleration, you would have to have lower gearing to put the same motive force to the road to keep up.

Which is important, because cars tend to get driven in city traffic where they are constantly accelerating up and down through the gears between traffic lights. So, the 'typical' engine would be less tiring to drive. But as speeds don't tend to get much over 40-50mph in town, the 'screamer' engine might get revved harder and so feel more 'nippy'.

On the motorway, though, cars tend to hold a constant speed for long periods (If you've tried to get round the M25, or between J2 & J14 of the M6 recently, you might disagree, but you know what I mean!) In this environment, the power curve is actually ''clipped' because you will use a wide open throttle to give you maximum motive force to accelerate up to your road speed, but then, having achieved it, you will back off the throttle, choking the engine and deliberately preventing complete cylinder filling, so that the motive force you get at that speed matched that needed to balance the drag at that speed.

OK, so, lets say we have a vehicle and that to propel it along at 70mph, needs a torque of, say 150Nm. Both the screamer engine, and the typical engine can make that torque quite a long way down the rev range. In fact, for the 'Typical' engine, it can make that torque as low down as 1000rpm. For the screamer, though, it needs to be turning at nearer 3500rpm.

Remember though, that the engine is making that force with the throttle wide open. So, lets say, that the engines were both geared the same, and in top gear, gave 70mph at 4000rpm.

Both would be cruising at the same road speed and engine speed, and the only difference would be how far open the throttles would be.

For the 'screamer' 4000rpm is pretty close to peak torque, and 150Nm is just over 1/2 the torque the engine could make at that speed, so the throttle would probably be just over half way open to restrict cylinder pressure down to give the desired torque.

For the 'typical' engine however, 4000rpm is quite a way past peak torque, and in fact, the max torque it can make at that speed is only about 180Nm. So to sustain a cylinder pressure capable of producing that torque, the throttle would need to be perhaps a bit more than 3/4 open.

Now, here's the rub. If we were to raise the gearing, we could drop the revs at that road speed back to maybe 2000rpm. That is pretty close to the peak torque for the 'typical' engine, so it could sustain 70mph with that gearing, with the throttle just over half open.

The screamer engine on the other hand, would barely be able to make the torque to sustain that speed at those rpm, so would have to have the throttle almost wide open.

OK, lets drop the speed down to 60, and keep the tall gearing that had the screamer engine suffering. Now, the typical engine is still pulling strongly, and can hold that gear, because it is still capable of making far more torque than is demanded. But the screamer, well, that has had to drop down a gear. Simply, the speed reduction has taken the engine down to an RPM range where even with the throttle wide open, it cant make enough motive force. So, the driver would have felt the engine start to bog, had no more accelerator pedal travel, so changed down a gear, and got the revs back up to some where it can make the motive force.

So, the conclusion is that up to 70mph, the 'typical' engine is going to be a bit less nippy. It can probably accelerate better than the screamer motor, but for the most part, some-one with the screamer engine is just going to hold the lower gears more and use more revs to get the same if not better effect.

On the open road, the 'typical' engine will cruise just as happily at the same speed, but could more easily pull a higher gear set to do the same speed, and could rely on using lower rpm to do the same job, and with less need to change gears for hills or small changes of speed.

So, really, there doesn't seem to be much between the two engines at the moment, except that the screamer needs more attention from the driver to work the gears, but rewards by offering better acceleration and top speed if you do.

What about economy?

Ah, well, there's the rub.

The general consensus is that high revs and wide throttle openings mean more fuel, but while there is some truth in that, to get the real truth we need to go back to the fundamentals.

Power = Work Done x Speed.

Work Done = Force x Distance

We get our force from burning fuel and air, and we get our speed from how fast we turn the crankshaft to burn it. So, the more fuel we burn in our cylinder, or the more often we burn it, the more power we are going to make, and the more fuel we are going to use.

So, simple linear relationship Fuel Consumption is directly proportional to Power usage.

Back to our two cars, both the screamer engined one, and the typical engined one, at the same speed, under the same conditions, were subject to the same drag, so the same amount of power was needed to push both of them along.

Same Power = Same fuel.

Mess around with the gearing so that one is turning slower than the other, or look at how wide open the throttles are; it doesn't matter.

Provided that both cars are the same, and they are both going at the same speed, it doesn't matter whether the engine is turning high rpm with the throttle a long way closed, or low rpm with the throttle a long way open.

Power used will be the same, so the fuel used will be the same.

Or will it?

Practically, the answer is no, it wont. But, the lie that is in there is nothing to do with throttle opening or engine speed, but efficiency losses.

And in general, the higher the speed the greater the drag, so the lower the efficiency. Which would suggest that the higher speed 'screamer' engine would be less efficient. Except that depending on the loading, it might not be. A wide open throttle gives no artificial impediment to cylinder filling. So, if the screamer engine, is made to turn at a speed where it needs to use a wide open throttle, then its possible that it might be slightly more efficient than the 'typical' engine, that is running with a half closed throttle.

So the rules, when it comes to economy, are just as vague as when it comes to power, and the generalities don't always apply and vehicles can defy common convention.

At the end of the day, what it comes down to is the 'topography' or design of the engine, the way that it is tuned, in terms of giving a 'Slogger', 'Typical' or 'Screamer' type power curve, then the overall gearing that the engine is attached to and that the driver has to take the engines torque and apply it as motive force, and then that is all dependent on the mass and drag that the vehicle is working against.

But I think that you are getting the idea.

To recap.

Power is how fast you can do work. Work is how far you can push a load. So power is how fast you can push.

We need power to over come drag, and drag increases 'exponentially with speed'

Motive force is the force you get at the wheels, and depends on the torque you get from the engine and the gearing between the engine and wheels.

Maximum Speed is the speed where the power needed to over come drag = the maximum power from the engine, and is only achievable if the gearing is optimised to match the engine's peak power to the drag at that speed.

From the engine, we get shaft power and shaft torque. Power is torque times revs, and torque is cylinder pressure times cylinder capacity.

We get more power from more cylinder pressure, a bigger cylinder or more engine revs.

Cylinder pressure is dependent on getting air into the cylinder. The more air you get in, the more pressure you get, so the more power.

Getting air into the engine is the difficult bit of engine design, and generally you wont get complete or maximum cylinder filling at very low revs, as things wont be working quite fast enough to all come together. As speeds increase, so the cylinder filling will get better, but then as speeds start to get very high, the air will be subject to the same sort of drag as the car, and you'll start to loose cylinder filling again.

This accounts for the hill shape of a power curve, with the power and torque varying with engine speed.

Different 'tuning' or 'states' of tune can provide different shaped power curves, from a 'Slogger' type curve, with a lot of torque made low down in the rev range, but not a lot of power further up, or 'Screamer' curves, with the torque made a lot higher in the rev range, giving a lot of high end power, but not a lot of low down 'grunt'.

The different 'natures' of the power curve can give very different driving characteristics, but ultimately it is matching the power curve to a set of gear ratio's and to a vehicle that effect how a car behaves on or off road.

Economy, ultimately, is dependent not on how much power you have or can get from an engine, but how much you use. Power = fuel. The more you use of one, the more you use of the other.


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