Power, torque, finally

2y ago


There has never been as much crap talked within a single arena of human interest as has been talked amongst car enthusiasts on the subject of power and torque.

I thought I’d have a crack at explaining them to anyone who doesn’t get it. I realise this might come out as a bit simplistic if you’re an engineer or physicist and you do get it, but in that case, you won’t be reading this anyway. Anyone else, keep going. Also, there won’t be any calculations in this, because we’re concerned only with conceptual understanding. And numbers make people reach for a service revolver with a single round in it.

The first thing to understand is that torque is simply a force; a force like the one you apply pushing a broken-down Alfa Romeo off the road. It’s a slightly confusing force because it works in a circular sort of way instead of a straight line, but we needn’t worry about that too much. If you like, think of trying to undo a nut with a spanner. How hard you push and how long the spanner is are both significant. So torque is (think of the units, like lb-ft) a force at a distance.

Torque is quite difficult to illustrate, so here are some 911s in the mist of misunderstanding.

The second thing to understand is that power is defined (in physics) as ‘the rate of doing work’. So there you go. Torque is the size of the job you can do, power is how quickly you can do it. To be honest, we can leave it at that. I would.

You want more? I got it, brothers and sisters, and here it comes.

A car engine can’t produce anything unless it’s running. Once it’s running – the crankshaft is spinning round and round – it is producing power and torque. The two are inseparable, and anything else you may have heard has come straight from the horse’s arse.*

Because the crankshaft is turning, it is producing that mysterious circular force mentioned above. Because it is turning at a speed, it’s capable of applying that force at a certain rate, which is the definition of power. Power is torque multiplied by engine speed.

So power allows you to do work at a certain rate, and let’s say the job in hand is accelerating out of a tight bend. Here, 300 horsepower is 300 horsepower. It could be reached by a high torque engine turning quite slowly, or an engine with puny torque turning very quickly. That ‘torque times speed’ thing works out the same. So the torque and power curves you see on those graphs can be manipulated, and that’s what leads to ‘engine characteristics’, but they’re both going to be there.

Something else worth remembering. An engine with 500 horsepower has that as its peak power. It doesn’t give that everywhere. In modern engines the power peak generally occurs just before the rev limiter is reached; in old racing engines, just before the whole thing disintegrates.

Power? God knows, but how quickly this bugger goes up and down has summat to do with it.

An engine with 350lb ft of torque generates that at its peak, too. Not everywhere. In modern engines, with electronic ignition and injection control, variable valve timing, and so on, that torque peak might be spread quite lavishly throughout a broad part of the rev range. On older engines, such as a small capacity Japanese racing bike engine from the 60s, it is often a fleeting thing, like the beauty of the cherry blossom.

But wherever there is torque, there is also power, and power cannot exist without torque, because it would have nothing to give. Torque and power, together, are a sanctified union the division of which leaves both parts purposeless, like John Donne and his mistress.

We are now at the edge of a terrifying abyss at the bottom of which is the reason for needing a gearbox. Do you want that too, or are you ready to end it all?

Picture no-credit: the author

* Electric motors and steam engines are slightly different. Leave it…

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