Mr. Foreman: All about horsepower

February 25, 2018 @ 12:30PM

HORSEPOWER. The word is a staple of car brochures and on the lips of car fanatics everywhere.

It gets used so often that we get stickers on generator sets, lawnmowers and outboard motors that shout out the horsepower it produces.

We all know that the higher the figure, the better it must be (or is it?) But what is horsepower exactly and why does it seem so important?

The term was invented by James Watt (1736-1819), who is most famous for his work on improving the performance of steam engines.

The legend says he was working with ponies lifting coal at a coal mine and wanted a way to measure the power available from these animals. He measured that on average, a mine pony could do 22,000 foot-pounds of work in a minute.

He then increased that number by 50 per cent and pegged the measurement of horsepower at 33,000 foot-pounds of work in one minute.

Quite arbitrary really, but then that measurement has become the standard for power output.

The question is; does one horse produce one horsepower?

But we are more interested in cars so we will leave that question for later. A car’s engine is always rated in horsepower. To measure it, we hook the engine up to a dynamometer. A dynamometer places a load on the engine and measures the amount of power that the engine can produce against the load.

A dynamometer measures the load the engine can handle at different engine speeds. What dynamometers actually measure is torque (in pound-feet) and to convert torque to horsepower, you simply multiply torque by rpm/5,252.

The result is expressed in BHP (brake-horsepower). Where does the term brake come from? Well, in the early days of dynamometers, they were called engine brakes and used some sort of mechanical brake to load the engines instead of the dynamometers’ electrical drum or dynamo (essentially also a brake).

If you see the term BHP, it means the engine has actually been tested on a dynamometer. A HP unit is essentially just an estimation of the actual output of an engine and not actually measured.

The dynamometer will produce a chart or graphical representation of the output of an engine. It will show the peak horsepower and peak torque at a specific rpm. This is what you will read in a brochure or a review in a magazine as 145BHP @ 9500 rpm, 100 lb-ft torque @ 8000 rpm. An engine that has lots of low-end torque means that the peak torque occurs at a fairly low rpm value, but a similar torque output at a higher rpm will feel like a revvy engine with low torque.

Since an engine makes power in a certain rpm range, gearboxes are needed to multiply the power in order to go faster. We all know the feeling of acceleration and also subsequently the lack of it once we get to a certain higher rpm. This is the torque climbing and subsequently, peaking.

A car which has a steep graphical torque curve will feel very strong in acceleration relative to one that has a flat torque curve. Typically, you will find that a small 2.0-litre four-cylinder engine has a steeper curve than a large 5.0-litre V8 engine. The torque value is, of course, important but a good manufacturer will have compensated a lower torque value with a suitable gearbox to suit.

If you time your upshifts (or downshifts) correctly at peak torque you will maximise acceleration and also top speed. This is also why you often downshift to accelerate. Downshifting moves you closer to the peak power point on the curve.

Of utmost importance, and also much neglected by car fanatics, is kerb weight. A high torque output will effectively be negated by a high kerb weight. A Lotus 7 might produce a relatively feeble output compared to a Rolls Royce Phantom but its light weight makes the car feel and go faster.

So a car is only considered to be a high performance vehicle if it has a lot of power relative to the weight of the car. A vehicle’s power to weight ratio is actually more important than the actual power output of its engine.

You will see a very definite correlation between the power-to-weight ratio and the 0-to-60 time. So in the end, horsepower means little if hampered by weight.

There are other factors in play, of course, but if you have played Top Trumps before, it seems we got it wrong every time we quoted a heavy weight card as a winning card. Try and dig up those cards again and play it properly this time.

Light weight and high horsepower wins every time!

A Lotus 7 produces a feeble amount of power but its light weight makes it go faster.
A Top Trumps card.
More horsepower is generally a good thing.