Read the Engines section of any Parkers car review and you’ll come across the term ‘torque’. It’s distinct from power and arguably more important for real world driving, having a huge effect on what an engine feels like to use.
In essence, torque is about how much ‘work’ an engine can do. At Parkers, we give torque figures in metric units of Newton-metres (Nm), rather than the imperial pounds-per-feet (lb/ft), largely because the car manufacturers provide metric data.
In this guide, we’re going to explain everything you need to know about torque including exactly what it is and how it affects a car. We’ll also consider the relative merits of engines with high torque and low torque.
What is the exact meaning of torque?
Torque is the force required to rotate an object around an axis. Here’s an illustration using the imperial pounds-per-foot (lb/ft) measurement, which we find easier to visualise.
You’re undoing a nut with a foot-long spanner. You hang one-pound weights on the end until the spanner moves. With ten pounds hanging off it, the spanner finally moves – that means torque of 10 lb/ft was applied to the spanner.
Torque increases proportionally to distance. A two-foot-long spanner will move the same distance with the same 10lbs hanging off, equating to torque of 20 lb/ft. Torque has doubled, even though the force applied has halved. It’s exactly the same principle as levers – the longer the lever, the less force is required.
How does an engine produce torque?
Here’s a very simplistic way of thinking about it. An engine’s horsepower is generated by the downward movement of the pistons. The pistons are connected to the crankshaft below and turn it as they move up and down. The engine’s torque is generated by those rotational forces at the crankshaft.
The torque generated by a crankshaft spinning at several thousand revolutions per minute (rpm) is fairly useless as-is, so it’s transferred to the wheels via a gearbox and differential. But that has to done without losing any torque, or at least minimising the loss. It’s a very tricky balancing act but, done correctly, it has a profound effect on what a car feels like to drive.
Why does torque matter?
The power and torque figures for a car’s engine tell you something about what that car feels like to drive. In simplistic terms, a car needs power to accelerate from a standstill, and torque to accelerate once it’s on the move. Note that engines generate maximum power at higher rpm than maximum torque.
Let’s say you’re doing 40mph in top gear on a country road and pull out to overtake a lorry. In a car with a low torque engine, you’ll probably need to shift down a gear or two so you can tap into the engine’s power and accelerate past the lorry swiftly. With a high torque engine, you may be able to just plant your foot on the accelerator and use the torque to whip by.
Torque is sometimes referred to as pulling power; at Parkers we often describe high torque engines as feeling muscular or strong. Other outlets might describe a car as having ‘stump pulling torque’, or ‘enough torque to pull down a building.’
High torque engines generally sit at high speeds more comfortably and quietly, and are usually more fuel efficient. They can get up long hills more easily and tow heavier trailers. However, they’re not necessarily as engaging to use as low torque engines, which may require lots of gearchanges and attention to preserving momentum to travel swiftly.
Why sort of engines produce lots of torque?
Here are some rules of thumb. A big engine produces more torque than a small one; a diesel produces more than a petrol engine; a turbocharged or supercharged engine produces more than a naturally aspirated engine. That’s why most of the best tow cars have big turbo diesel engines that, by their very nature, produce an awful lot of torque.
But It’s not necessarily as simple as that. The amount of torque – and horsepower – a modern engine produces is controlled by computers as much as anything else. By changing the computers’ settings – known as a map – it’s possible to quite radically alter the amount of torque an engine produces.
The results can be truly remarkable. Ford’s tiny 1.0-litre, three-cylinder turbocharged EcoBoost petrol engine has, in certain models, been mapped to produce as much torque as some engines more than twice its size.
Equally, there are some large engines that produce relatively little torque. That’s usually because the engineers wanted lots of high-rpm power, rather than low-rpm torque. Though it is possible to achieve a balance of high torque and high power, as the latest generation of turbocharged supercars vividly demonstrate.
It’s worth noting that high torque engines are generally best suited to an automatic gearbox. The BMW Z4 M40i is a good example of why. With an automatic gearbox, the car’s various computers essentially work together to ensure as much torque as possible is on hand at any given moment. With a manual gearbox, there’s so much torque in fourth gear that it can take the car from walking pace deep into three-figure speeds. So there’s not much point in changing gear, therefore there’s not much point in having a manual gearbox.
Is an electric car’s torque different?
An electric motor generates torque as its crank spins, in much the same way as a combustion engine does. But there’s a crucial difference. An engine generates more torque as speed increases, whereas an electric motor produces maximum torque immediately. That’s why so many EVs seem to surge forward with a relatively light touch on the accelerator.
Looking for more jargon-busting motoring meanings? Head over to our Parkers Car Glossary page and take a look at our other definitions
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