What is power (kW) and torque (Nm)?
How many times have you read the word horsepower and had almost no idea what it actually means? We explain the difference between power and torque.
They’re terms we hear all the time, but what really is horsepower? And while we’re at it, what’s torque? And how are they different?
Okay, let’s start at the start. The purpose of a car’s engine is to propel it. To do that it has to produce a twisting force which can, ultimately, act on the wheels and tyres and physically move the car. That force is, essentially, torque, and it relates to how much force the engine can muster. Torque is measured in Newton-metres (after 17th century English physicist Sir Isaac Newton). This is a Newton Metre (or force) applied to a lever that’s one-metre long.
A Newton Metre (Nm) is sometimes mistakenly referenced as being a 1kg force applied to a one-metre-long lever but this is wrong. Rather that 1kg force applied to a one-metre long lever actually works out to be equal to 9.8 Nm. So, the stick and the weight analogy is totally useless when trying to explain torque. All you really need to know is that ‘torque’ is measured in Newton Metres, meaning a force applied to a one-metre-long lever.
Horsepower, meantime, is merely a mathematical expression of how fast the engine can produce that torque; a multiplication factor, in other words. In practical terms in a car engine, that more or less means that horsepower equals torque times rpm. Torque is real, horsepower is abstract.
In the real world, horsepower is what makes a car go fast, torque is what makes it feel fast. That’s because a high-torque engine may only rev slowly, limiting its ability to move things along quickly. But that same, slow, torquey engine will still give you a shove in the back when you hit the throttle, and that’s one of the things that makes driving fun.
Generalising again, race cars work best with high-horsepower engines (for acceleration) while off-road four-wheel-drive thrive on torque (for hauling them through bog-holes and over logs).