Power And Torque Explained
by David Finlay (20 April 2002)
Browsing through a handful of motoring websites the other day, we were interested to see that a reader had asked one of them to explain the relationship between power and torque but had not received any kind of answer. So we thought we'd supply that service instead.
It is, after all, a fair question. Everyone knows that cars produce power, and bhp figures are banded about regularly. People with more enthusiasm about the subject are aware of torque, and that it has something to do with performance. On the whole, though, it's only engine builders and tuners who know how the two relate to each other.
Some basic stuff first. What an internal combustion engine actually does is take in a mixture of fuel and air and set fire to it, by using one or more spark plugs to introduce an electric charge if the fuel being used is petrol, or by compressing it in the case of diesel. In this process of combustion (which is forceful but more controlled than a mere explosion) the resulting expansion of air pushes a piston down a cylinder and turns a crank. The turning motion of the crank is transferred through the transmission to the wheels, and the car moves. The extent of the combustion depends on many things, including the dimensions of the cylinder, the type of fuel used and the accuracy of the ignition process itself.
The bigger the "bang", the more the crank is pushed round. The force involved is called torque. To some extent it could be said that the engine is working at its best when it is producing the most torque, and in the case of a relatively simple engine this will happen at one particular speed.
In recent years this has become less of an issue, because thanks to the incredible advances in electronic control of the fuel and ignition systems - plus, in some cases, variable camshaft technology - the characteristics of the engine are continually being monitored and altered. This can happen for reasons of fuel economy or emission control, but in performance terms the idea is to allow the engine to produce nearly its maximum torque over as wide a range of speeds as possible.
So far we've been looking at an engine only as a torque-producing device, but torque itself is not what causes the car to accelerate. For that you need power. The basic relationship between the two is very simple - power equals revs times torque.
Physicists define power as "work done", and a good analogy here would be to imagine hammering a tent peg into the ground. If you gave the peg one almighty wallop and then walked away, the peg might not be fully secure, so although you had done the equivalent of applying a lot of torque, you wouldn't have done it for long enough and the total amount of work done would not be very impressive.
If, on the other hand, you tapped the peg gently for half an hour, you would eventually get the job finished, but it still wouldn't be a work rate to be proud of. Hammering the peg strongly a few times would make it secure far more quickly than the other two methods. In engine terms, this would equate to producing decent amounts of torque at a suitable number of revs.
One other point to note about the power/torque equation before we look more closely at engines: we need a constant to make sure we're able to calculate the correct number of units. The figures most commonly used in the UK are brake horsepower, revolutions per minute and pounds/feet of torque, for which the constant is 5252. In other words, to find the bhp, multiply the rpm and the lb/ft and divide the result by 5252.
So, if an engine produces 150lb/ft of torque at 3000rpm, the power figure at that speed is 150 times 3000 divided by 5252, or just under 86bhp.
(Note that the torque figure, and therefore the power figure, is a maximum which can only be achieved if the throttle is wide open. If you're in a situation where producing 86bhp would make the car accelerate, backing off the throttle will reduce the amount of power being produced. When you press the throttle pedal only to the point where the car's speed remains constant, what you are doing is matching the power to the resistance against the car caused by tyre friction and aerodynamic drag.)
The power-equals-revs-times-torque equation looks deceptively simple. But the engine speed has a double effect - not only is it part of the power calculation, it also has a bearing on the amount of torque it's being multiplied by (because the engine produces different torque figures at different rpm). That's why, if you plot power and torque figures on the same graph, you end up with curves of quite different shapes.
This is the sort of graph you would get if you compared the power and torque of a moderately powerful road car - a warm hatch, for example. You'll see that the torque curve reaches a peak quite early on and then falls away gradually. Long after that peak has been reached, though, the power curve is still climbing sharply (because the revs are increasing more than the torque is dropping), and it only starts pointing downwards again when the torque curve has begun its final descent high in the rev range, when the engine is spinning too fast for the combustion process to work efficiently.
If you've ever compared maximum power and maximum torque figures you'll have noticed that the former always occur at far higher revs than the latter. The graph shows why - there would have to be a catastrophic collapse in torque production immediately after the peak in order to halt the increase in power.
In most car applications, this is very unlikely to happen. If an engine produces maximum torque at 3000rpm, it will probably still be producing quite a lot at 4000rpm - it would be very surprising if the torque had dropped by as much as a third, which is the proportional increase in engine speed. The only way maximum power and torque figures would coincide would be if a device such as a revlimiter cut in at the appropriate number of rpm.
It's clear from all this that the best way of getting power from an engine is to persuade it to produce lots of torque at very high revs. Unfortunately - and this will come as no surprise to anyone who has spent more than five minutes talking to an engineer - there are practical limitations to this.
The fastest-accelerating cars on the planet are Top Fuel dragsters, the best of which erupt to 320mph from a standing start in four and a half seconds. To do this they produce more power than it is possible to measure directly with any existing device, but estimates of around 5000bhp are common. They rev far higher than most road cars, but not nearly as high as a Formula 1 car.
Their awesome power outputs are due to the fact that they produce simply unbelievable amounts of torque. Their engines are enormous, and are force-fed gallons of nitromethane (a fuel which is less volatile than petrol but contains its own oxygen, so you can use much more of it) by means of huge superchargers. The resulting explosions are colossal, which is why even at tickover a dragster makes a noise like eight machine guns.
They are also staggeringly unreliable. The amount of running time you can expect from a Top Fuel engine between rebuilds is measured in seconds.
Formula 1 engines have to last several hundred miles between rebuilds, and they are also restricted in terms of engine size and the fuel that they use. Power figures are known but jealously guarded, but 800bhp seems to be a good average these days. That's an awful lot from a three-litre engine, but surprisingly little of it is due to the amount of torque they produce - the ability to achieve very high revs is far more important. If 800bhp is produced at, say, 16,000rpm (and some Grand Prix engines can go far higher), that equates to a torque figure of only 262.6lb/ft, which is easily beaten by several high-performance road cars.
In some ways a racing engine is a relatively simple thing to design. Within the regulations, you are looking for the best possible performance characteristics without necessarily paying much attention to other considerations. The engine of a car built for normal road use is a different matter entirely. Its performance potential is limited by several factors: it must use normally available fuel, preferably in small quantities; its maximum torque figure has to be achieved at relatively low speeds (otherwise the engine will be likely to stall if the revs drop too low); and it must be reliable enough to last for many thousands of miles without shaking itself to bits.
These and other factors make it difficult to achieve high power outputs, which in any case are not always desirable for a car used daily in heavy traffic. Perhaps the most remarkable road car engines commonly available today are the latest-generation turbo diesels. 130bhp, which ten years ago was a good figure for a hot hatch, is now by no means exceptional for a diesel, even at an engine speed of as little as 4000rpm. As anyone who has ever driven one of these cars can confirm, their performance is remarkable. What not everyone realises - or, if so, only vaguely - is that it's the torque that makes this performance possible.
