What is the difference between brake horsepower and horsepower? It’s not as simple as you think.

It’s time to stop getting our BHPs and our HPs mixed up because, if you didn’t know already, they’re actually completely different but actually, the same?

Horsepower is the measurement of choice in the US whereas brake horsepower is often preferred by British manufacturers.

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Horsepower originally was invented by the man who improved upon the steam engine invention, James Watt. The measurement initially was calculated to compare the power of a steam train to the power of horses. Somehow, Watt came to define 1 horsepower equating to it taking a single horse to haul 33,000 lbs of mass up a pulley system 1 foot in 60 seconds. It’s overly complex and makes no sense as the average horse actually has around 15 horsepower but it probably made more sense at the time.

Anyway, brake horsepower is a little different and is the term us Brits use instead of horsepower. It was once measured differently and therefore, the numbers were not the same either.

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A common misconception, one that you will see across the internet, is that bhp is slightly lower than hp figures because the engines are tested with auxiliary functions still active, such as power steering, exhaust mufflers, catalytic converters, lights, non-essential electrical functions and so on. This drains power from the engine and results in a slightly lower but more realistic horsepower figure known as bhp. This is no longer true and is, in fact, a confusion of the old net horsepower and gross horsepower figures. As soon as net horsepower was introduced in the early 70s in the US, the term brake horsepower kind of lost its meaning because it was replaced by net horsepower and became the standard measurement.

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Before 1972, US automakers often used gross horsepower which is a measurement taken with these auxiliary functions removed from the engine, resulting in inflated figures. After 1972, this was no longer allowed in the US so technically after that (up to today) US manufacturers were all essentially using bhp anyway, right?

So, what’s the actual difference between bhp and hp? Well, it’s unclear.

In summary, engineers and laymen alike could argue about the differences between hp and bhp all day but at the end of it all, the difference is moot. The original horsepower was calculated using a literal horse whereas brake horsepower, a later invention, was measured at the crankshaft of an actual engine and instead of using a horse, the method used a brake dyno. As the engine powered up an equal braking force is applied to resist it, therefore measuring the brake horsepower.

Engine on a dyno testing machine

As time has gone on, the differences have become increasingly unclear as US testers adopted the same methods meaning that the only differences left are the standards set by the government.

US manufacturers follow SAE regulations and UK manufacturers don’t, which could result in some differences. The US also uses metric horsepower whereas UK manufacturers may use imperial. And of course, British car makers will favour bhp whereas those in the US will use hp. To confuse things further, other measurements such as PS and kW are also popular and can sometimes be skewed depending on which kind of horsepower they are converted to or from.

Volkswagen Racing - dyno machine

So, it’s all a bit of a mess really which is why you’ll often see auto journalists, manufacturers and ordinary motor enthusiasts switch between the two measurements like it’s no big deal because at the end of the day, no one really understands the difference.

Think you know the difference? Let’s talk about it in the comments!

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