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Explaining engine torque

By raccars Published

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You might feel you have a solid grasp on BHP and what it means in a car, but what about torque?

Torque is often compared to power output and it is indeed about power, but it's not the same as brake horse power. As modern cars reach ever higher levels of performance, torque is becoming more important when comparing vital statistics. However that wasn't always the case; just a couple of decades ago torque was rarely mentioned when discussing a car's capabilities. Today it's routinely mentioned as a selling point and a comparison tool. When there's not a huge difference in horsepower, torque discrepancies can still make a car feel very different to drive and pointing out a torque advantage is one way for manufacturers to regain the upper hand.

Pulling power or twisting force

Torque is essentially the force which moves a car, and as cars became heavier this characteristic grew in importance. Torque is often described as 'pulling power', but in physics it means the amount of power required to twist something. Power drills often have a torque setting, for example. In a car engine, torque effectively equals brute force. By comparison, horsepower refers to how quickly that power can be utilised.

When written about, torque is usually described in Newton metres (Nm) or in pound feet (lb-ft). It measures how much twisting force is generated at the crankshaft at different rev levels. In the context of cars it's important not only to have access to large amounts of torque but to be able to use that power quickly. Torque plus horsepower equals acceleration.

Horsepower

The more commonly discussed unit of power for a car is horsepower, or brake horsepower, HP and BHP respectively. Horsepower was the term coined by James Watt, an engineer who determined it as a horse lifting a weight of 33,000lb over a foot in height in one minute. BHP figures are usually slightly lower because they are recalculated to assume a certain amount of power lost to the internal friction in an engine, so they are probably more accurate and appropriate. Horsepower can also be expressed in metric terms as PS or 4,500kg metres per minute, equal to 0.97HP.

Calculating torque

In basic terms you can measure torque by making a calculation of force and distance. If you need to use 50 Newtons of force to tighten a wheel nut with a 50cm long spanner, the calculation is 50 (Newtons) x 0.5 (metres)=25Nm or Newton metres of torque. In old money, you could use a spanner of 18 inches and a force of 20lbs to make the calculation 20 (lb) x1.5 (feet) = 30 lb feet of torque.

An engine might produce 500Nm of torque. To visualise this, if you are using a one metre long spanner to prevent its crankshaft from turning, you would need to exert 500 Newtons of force on the spanner. In physics there are about 9.8 Newtons to every kilogram of weight, so you'd need a reasonably slim woman or a large dog to stand on the end of the spanner to stop that crankshaft. This doesn't mean that a small human being can stop a car dead however, as torque is multiplied at the wheels according to engine revs.

Torque and horsepower are mutually dependent in an engine. Horsepower equals torque multiplied by revs per minute, divided by 5252, so it is always the case that higher torque will produce more horsepower at any RPM. However as the final figure relies on engine revolutions (diesels tend to rev lower than petrol car engines), and as a petrol car can reach higher revs it will have access to a higher top speed thanks to torque.

A high torque engine can therefore make your car's engine power more easily available, and is essential if you intend to carry or tow heavy loads such as caravans or trailers.

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