Engine power ratings - what's the difference?

When Ford Australia announced power figures for the locally delivered Mustang, we were outraged by the V8-engined pony car apparently losing over 20kW in the migration south of the equator. Ford couldn't initially explain the discrepancy, but investigations revealed the engine hadn't been detuned, rather it was simply the victim of a different method used in Australia for calculating power.

In the world of cars, there’s one number which gets more attention than the rest. Power. It’s the headline maker for sports cars and hot hatches and one of the first pieces of information you’ll find on the technical specification page of the brochure.

But, as you’ve probably noticed, we often see or hear power being discussed or claimed in different terms, most commonly horsepower ('hp') and often from European sources as 'PS', 'CV' or 'ch'. So what’s the difference?

Horsepower ('hp') has been used the longest. In fact, well before the first car was ever built. The unit was devised in the late eighteenth century by engineer James Watt as a standard to compare the output of steam engines versus the work of draft horses.

It was calculated that one horsepower was the amount of work done to lift 33,000 pounds of mass, one foot vertically in one minute. In metric terms that's about 15,000kg lifted approximately 300mm in a minute. It turns out, this measurement of mechanical horsepower, also known as imperial horsepower, is the equivalent of 0.7457 kilowatts.

In North America, power outputs are typically published in imperial horsepower to an 'SAE' standard, from the Society of Automotive Engineers.

The often seen 'PS' unit is actually horsepower too. 'PS' is an abbreviation for the German word pferdestärke (horsepower) and is known as metric horsepower. Similarly, the Italian use of 'CV' and the French 'ch' are also metric, and abbreviations of the word ‘horsepower’ in their respective languages.

Used commonly across continental Europe and Asia, metric horsepower was derived from the same theory as Watts' original, but as you might guess, uses metric units. It was determined that one metric horsepower was equivalent to the amount of work done to lift 75 kilograms, one metre vertically in one second. Which ends up equalling 0.7355 Kilowatts.

Although 'PS', 'CV' and 'ch' figures are still widely used in advertising material by many European car manufacturers, the official EU measurement of engine power is actually the kilowatt, an internationally recognised unit of power with a globally agreed definition.

Blurring brake horsepower
Many older standards have been used previously to calculate the horsepower rating of cars. In the past, the SAE has promoted gross and net power ratings, the former calculated from the engine's crankshaft, but without ancillaries such as the exhaust, alternator and cooling system connected.

It's rarely used now, and has been discredited for producing optimistic figures which were not only unrepresentative of how much power the engine could produce in the real world, it could be 'fudged' by manufacturers conducting their own testing. It was a test also at odds with other more realistic standards such as the SAE's own net power standard, the German DIN standard (Deutsche Institut für Normung) and later standards from the ECE/EU (European Union).

All of which ultimately explains the Mustang anomaly – boasting better horsepower figures out of North America versus power figures for Europe and Australia.