Pulling the right rein: Horsepower explained

How much power does a horse actually produce and who worked it out? Why use horses as a measurement of car’s power? What does it all mean anyway?

There is logic to the madness, but to explain it all we’ll have to go back. Way back.

A steam genesis

It all starts with the appearance of the first steam engines in the early 18th century and the need to explain what these new-fangled machines could do.

With horses doing the heavy lifting in farming, mining and other industries, and steam engines being pitched as a replacement, people sought to compare the rate at which they could do work, otherwise known as ‘power’.

The concept really grew legs, however, when James Watt – the Scottish engineer honoured with the ‘watt’ measurement – started selling a new and improved steam engine in the late 18th century. He actually developed the vague concept of ‘horsepower’ into a specific metric. After studying a range of horses, he concluded that a theoretical single horse could carry 33,000 foot-pounds per minute (ft-lb/min).

Other steam-engine makers would follow Watt’s lead and also use horsepower to rate them, as would those creating other motors – including the internal-combustion engines powering the first ‘horseless carriages’.

Not all ‘horses’ are the same

The definition of a horsepower has expanded over centuries to suit different regions and purposes, including – among others – the addition of a metric unit to go with Watt’s original imperial measurement (also known as mechanical horsepower). For car buyers in the US, the UK and other countries – and a big chunk of motoring culture – it’s still the favoured measurement of an engine’s power.

Here in Australia, its use is strictly colloquial – we’ve officially been using the metric unit kilowatt (kW) to rate engines since 1977. In case you’re wondering, 1 kW is equivalent to approximately 1.34 mechanical horsepower.

What does it mean?

Power is what makes acceleration happen, so, more power – assuming all other factors are equal – means faster acceleration. Which means the car with the bigger power number is always better, right?

This is where it gets complicated because there’s another key contributor to acceleration – torque. Torque is a measurement of the rotational or twisting force produced by an engine’s crankshaft. It’s a measurement of an engine’s capacity to perform work – in human terms, its sheer strength – where power, contrastingly, conveys the rate at which work can be performed.

A good way to think of the difference between an engine with an emphasis on power and one with a lot of torque is to picture two people – ‘torque’ and ‘power’ - and two same-sized piles of dirt. Both successfully shovel the pile in the same duration but ‘torque’ has a big shovel and does the job with fewer, larger shovel-loads. They’ve both ended up with the same result but ‘power’ has worked more frenetically to get the same result.

How a vehicle accelerates in the real world comes down to a complex interplay between power, torque and revs, along with weight, aerodynamics, gearing, frictional losses and other factors. While an engine with a bigger power number will deliver better acceleration, if it’s still short of the revs needed to deliver peak power when a less powerful engine is already doing its best work or cranking out more torque, it may feel less responsive most of the time (at low to middling engine speeds).

In the end, it’s really horses for courses. If you’re a racing driver, revving an engine harder is a small price to pay for more power and going faster. If you’re just tootling around, however, it’s worth studying the specifications, because more power doesn’t always mean a ‘better’ drive.