How to Calculate the Horsepower of a Car

‘Horsepower’ was originally dreamt up as quite a random, abstract set of numbers, plucked out of the fragrant 18^th Century air by one James Watt.

At the time he was observing the workload that pit ponies were capable of and, needing a standard unit of measure for his steam engines, decided to allot a unit of work to a horse, using the pony as a guide- and then adding a bit. So a pony is this big… a horse is that big… Ok, call it 50% more. Yeah. That random.

However random they may indeed have been, these numbers remain the same to this day: One horsepower is 33,000 foot pounds of work in a minute, meaning (theoretically anyhow) a horse, working to one horsepower, can raise 33lbs of coal 1,000 feet in one minute, or any variation of that equation, so long as the end result when multiplied is 33,000ft lbs.

All that being clear, how on earth does one translate that to your Lamborghini Aventador? I mean, it’s a bull after all, not a horse.

As we all know, what a car engine does is harness the explosive power of a combustible liquid, translates it into a rotational force (usually) via pistons and a crankshaft, then transmitting this to the wheels, thus providing forward motion. To most effectively measure the output of an engine therefore, it is necessary to harness the rotational power of the crankshaft and this is done with the use of a dynamometer.

Via the ‘shaft, a dynamometer provides resistance to the engine, in the absence of the gears transmitting the power straight to the road and it is this resistance that can be measured… say, how much resistance is required to reduce the engine rpm from 6,000 to 5,000, for example. This measurement is in fact ‘torque’ and it is through finding out the torque that the horsepower can be ascertained, by use of a simple equation.

Horsepower = torque x rpm/5,252

Personally, I prefer to look them up, either online or in the copious blurb that comes with a new car.

There’s not a vehicle out there, from Alfa to ZiL, that doesn’t proudly exhibit its inside leg measurement for anyone interested enough to read it. The automotive industry has made great strides in successfully selling the notion of horsepower to the general punter, so much so that even though we don’t even really have the faintest notion what it really means, we’re still very much of the opinion – “Hm. Horsepower. Me like Horsepower. Horsepower good.”

This knowledge however is nevertheless regarded by many as a huge leap forward in evolution from the dark days of walking around a nebulous vehicular shape, kicking a tyre and grunting grudging approval of the fact that at least it’s “in metallic”. But only just.

Baldly translated into car terms:

A new Ford Fiesta (in metallic) will offer up 120hp at 6,350rpm.

A new Bugatti Veyron (any colour) will offer up 1,200hp at 6,400rpm.

Hm. Me like horsepower.

Published official horsepower figures, along with top speeds, will decline year on year, as the engine wears and becomes less efficient. And be careful that it’s horsepower you’re after, not torque, before you go splashing out on an earthmover. Now they’re expensive…