What is torque? Plain english guide to lb ft and Nm

Choosing what engine sits under your bonnet is one of the most important decisions you make when picking your car.

You may be familiar with power (usually called horsepower), but you may not know what that other rating means – the torque. Power is simple – the more you have of it, the faster you’ll go. But that only tells you one side of the story. Torque is a better indicator of how an engine will feel in use.

Take the BMW 320d and the Caterham Supersport R – both cars have roughly 180hp, but feel significantly different from one another. The Caterham is a light car so only requires 143lb ft of torque to go quickly, but the big diesel BMW has nearly twice that.

What is it?

The easiest way to think about it is – if horsepower tells you how fast the car can ultimately go, then torque tells you how hard it can push you there. Torque matters as much, if not more so, than horsepower because it’s the force you feel when you accelerate. If you regularly tow a trailer or tourer, or frequently carry a fully-loaded car, then having plenty of torque is important because it helps you pull your load onwards.

The best analogy to describe torque is to compare it to turning a nut with a spanner. Say your spanner is a meter long and you apply one newton of force to the end of it – you’re turning the nut with one newton-meter of force. Double the length of the wrench or the weight applied to its end and you double the torque being applied to the nut. In the UK, torque is typically measured in imperial foot-pounds rather than metric standard newton-meters – one Nm is equivalent to 0.738lb ft.

Why does it matter?

As mentioned, torque is a good indicator of how an engine will feel when you accelerate. Critics often describe diesels as having ‘lots of low-down torque’ – this means they develop most of their torque at lower engine speeds, which makes them easy to drive because there’s a big ‘safety net’ of torque to help you from stalling or pulling away too slowly.

Petrols can typically run at higher engine speeds than diesels and develop less torque in the lower ranges – most is made at the higher end of the rev-range which is why petrols are often said to feel more exciting because the driver must hold on to gears longer to access maximum acceleration. That’s why petrol-driven racing cars usually rev really highly.

What about electric cars?

Electric cars have motors which don’t have optimum engine speeds as combustion engines do. Reviewers frequently mention the ‘instant torque’ sensation of electric cars – this refers to the fact that electric motors can deploy 100% of their torque from rest. This means, around town, electric cars such as the BMW i3 or Renault Zoe can feel very sprightly indeed.

Got the power?

So now you can talk the torque talk but have you got the power? Read our guide to horsepower to help you make the most informed choice when picking your next car.