Power vs Torque: What’s the difference?

If you’ve read anything about cars, the chances are that you’ll have come across the words ‘power’ and ‘torque’. But if you’ve seen journalists ranting about the torque of a 4×4’s diesel engine and wondered what on earth they’re on about, then read on…

So what is torque?

Bear with us, because this is quite complicated.

Forgetting cars for a second, a physicist would describe torque as a twist. If you use a wrench to undo a nut (as in a nut and bolt, not a walnut), you’re applying torque. You’re twisting the nut to undo it.

But if you break it down even further, what you’re effectively doing is applying force to a point some distance from the nut. That’s why torque is measured in terms of ‘force x distance’ – Newton-metres (Nm), for example, or pounds-feet (lb-ft). So if you apply 10 newtons of force to a point one metre from the nut, the torque on the nut will be 10 newton-metres (10Nm).

In a car’s engine, though, the twisting force is applied to the crankshaft. The piston is connected to a hinge that’s a certain distance from the centre of the crankshaft, and as the piston moves up and down, it pushes the crankshaft around. Multiply the force of the piston by the distance of the hinge from the crankshaft, and you’ve got torque.

Jaguar Diesel Engine Diagram
A diagram of a four-cylinder Jaguar diesel engine, showing how the piston (centre) turns the crankshaft (bottom)

What is power, then?

According to physicists, power is the rate at which work is done. In automotive terms, that’s how quickly an engine can apply its torque.

A simple one-cylinder engine with 10Nm of torque would apply 10Nm per engine revolution. If it can manage 1,000rpm, it’ll apply those 10Nm of torque 1,000 times per minute (10,000Nm per minute). If, on the other hand, it can do 2,000rpm, it’ll apply 10Nm of torque 2,000 times a minute (20,000Nm per minute), making it more powerful.

Because of the industrial revolution, we measure power in ‘horsepower’, which makes its meaning less obvious. One metric horsepower is the torque needed to lift 75kg one metre in one second using a pulley. Force, speed and distance are all in there, but they needed a unit that resonated in the 1800s. Complaints should be addressed to the Victorians.

Right… I get the science, but why does this matter to a driver?

Although torque and power are closely related, they feel slightly different on the road. In simple terms, and assuming all other things are equal, torque is what gets you moving at lower engine speeds, whereas power gets you up to speed when you put your foot down.

McLaren 570S Spider 2017 Blue Rear Dynamic

Which is more important?

That depends what you want, really. In a 4×4 that needs to get out of sticky, cloying mud, but doesn’t need to go very quickly, then torque is ideal. Alternatively, power would be more important to a sports car that needs a high top speed.

Which engines have the best performance?

Generally speaking, diesel engines will have more torque than petrol engines of the same size, while petrol engines will have more power than their diesel rivals. Because of the properties of the two fuels, petrol engines are capable of more engine revolutions than diesels, while diesels can apply the piston’s force further from the centre of the crankshaft.

That’s why big, heavy 4x4s tend to have diesel engines, while supercars use petrol. It’s also why, during everyday use, people often think diesels are faster than petrols. It isn’t necessarily true, but a petrol will have to work harder (i.e. spin faster) to provide the same performance as an otherwise identical diesel.

For the best acceleration, though, you’ll probably need an electric motor. Whereas maximum torque in a diesel engine is usually around 1,500-2,000rpm, an electric motor provides maximum torque at all times. That’s why the Jaguar I-Pace and Tesla Model S accelerate so quickly.

Jaguar I-Pace 2019 Grey Front Dynamic

 

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