What is torque in a car?

• Discover exactly what torque is and why it's important 
• Why do diesels have more torque? 
• Find out how torque and horsepower are related

You might hear the word torque used in connection with cars and their engines. It’s often said in the same breath as horsepower, and is a useful concept to understand. More than horsepower, torque relates directly to how powerful an engine or electric motor feels when you put your foot down.

What is torque in a car?

An engine’s power is measured using two scales: horsepower and torque. If a car engine was a boxer, torque would be the force behind its punch, and horsepower would be the speed at which it’s punching.

When it’s related to an engine, torque is the rotational force. Horsepower is how quickly that force can be delivered. In a car, the more torque an engine has, the more effortless its acceleration will feel.

What does torque mean?

As we’ve seen, torque is a measure of turning effect. In everyday use, when we open a jam jar or bottle of drink, we are applying torque to the lid. Standing on a bicycle pedal produces torque. And if you hear a garage mechanic claim they’re ‘torquing up’ a wheel nut, it’s just another way of saying they’re tightening it.

With an engine, the torque figure is how much turning force it can exert through its crankshaft - the power which eventually turns the wheels.

What is torque measured in?

Most car manufacturers calculate torque in Newton Metres (Nm), but it can also be shown in pounds per foot (lb/ft).

What is considered good torque in a car?

That depends on the car. The bigger the car, the more important the torque figure becomes, because it needs more torque to accelerate quickly. This is why car manufacturers now mention torque in their advertising; cars have become heavier, making torque more relevant to everyday driving.

The speed at which an engine’s pistons go up and down in its cylinders to turn the crankshaft is measured in revolutions per minute, or RPM. An engine’s maximum torque is shown in relation to RPM. The higher the torque number and the lower the RPM, the more responsive an engine will feel.

Why diesel cars have more torque

Originally, the diesel engine was designed to rival the steam engine. It had to pull heavy loads. And to do that, it needed plenty of torque.

Diesel engines develop their maximum torque at lower revs than petrol engines. This is because the pistons in diesel engines travel a greater distance than those in petrol engines and so don’t have to move as quickly.

In addition, diesel is denser than petrol and has more energy per unit. When diesel burns, more energy is transferred into the pistons, onto the crankshaft and, eventually, the wheels.

How this works in reality

An Audi Q7 SUV with a 3.0-litre turbo diesel engine develops a hefty maximum torque of 760Nm at between 1750 and 3000RPM.

In comparison, a Ford Fiesta with a 1.0-litre petrol engine is significantly smaller and lighter than the Q7. Its engine only develops a maximum 105Nm of torque at 4100RPM.

Accelerating from standstill to the 70mph speed limit in the Audi will be a relatively relaxing affair. The huge torque will enable the automatic gearbox to change gears quite low in the rev range. It will feel swift and smooth.

In the Fiesta on the other hand, not only will it be slower, it will also feel more frantic. As there’s less torque and you have to rev the engine higher to access the maximum torque, you’ll be changing gear at higher rpm. This will make it noisier inside the car.

That’s why sports cars are nearly always petrol and are often engineered to develop their peak torque at higher rpm. The higher the engine revs, the racier it feels.

Electric motors are a bit different

The way electric motors deliver their power is different to the internal combustion engine. An electric motor can access all its power from standstill. That’s why even a relatively humble Renault Zoe can feel quite nippy. It’s also why the fastest Tesla Model S can do 0-62mph in a supercar-battering 2.4 seconds.

Where torque is useful

When people talk about how powerful their car’s engine is, they’re usually referring to its torque. That’s the power you really feel when your car accelerates. And if you want a car that feels effortless, it’s quite simple: the more torque the better.

Anyone who’s looking for a car to tow something, such as a caravan or boat, should consider the torque figure of any car they’re looking at. It really will make life much easier. It’s also why people who tow a lot usually choose diesels.

How are torque and horsepower related?

An internal combustion engine produces both torque and horsepower. The two are wedded to each other. In fact, you can calculate horsepower by multiplying torque by RPM and dividing by 5252. That means the more torque a car produces, the more horsepower it’s likely to produce as well.

But if an engine has lots of torque (as in our Audi example above) it doesn’t need to operate at high revs to be driveable. That’s why many high torque diesel engines don’t appear to have ridiculous amounts of power. The torque means they don’t need it.