What's the difference between PS, bhp and hp?
It's very easy to get power measurements mixed up
There's not a more popular way to compare cars these days than a straight-up power figure. With any new performance car release, it's the first statistic that most petrolheads beeline for, seeing as it is intrinsically linked to how fast (or slow) the respective car will be.
Sadly, it can often become a convoluted mess of numbers that are often mistranslated thanks to unitary conventions used in different areas of the world. The Americans, British, Continental Europeans and Australians all use different units when it comes to the power produced by an engine, so it's time to debunk each of the options and decide which one we should all stick to.
Horsepower and brake horsepower
The standard unit used in the USA, horsepower actually originates from the birth of the steam engine. James Watt (the inventor of the steam engine) decided to come up with a unit to directly compare his latest machines to the horses that were dominating the haulage industry at the time. Through experimentation, Watt decided that one horsepower was equivalent to a horse transporting 33,000 pounds of mass one foot in one minute.
Of course, no one has a clue how strong this horse was, what it had for breakfast that morning or if it was a tad asthmatic, and yet the majority of people still use this somewhat convoluted unit to this day! To show just how much of a faff it is to find the unit for horsepower, here is the equation:
Yup...good luck with that one.
To further complicate things, us Brits use brake horsepower (bhp), which often clashes with the straight up American horsepower (hp). Brake horsepower is derived from the technique used to measure power - a large drum with an internal water brake measures the resisting braking force produced by the rotation of an engine's crankshaft.
In America, this form of testing is undertaken with few ancillary components attached to the engine while it's running, allowing the powertrain to breath more freely and concentrate all its efforts on energy transfer through to the measuring apparatus.
In the UK however, we have all the ancillaries attached (stuff like the power steering pump), making for some annoying (but realistic) parasitic losses. This means that bhp values are less than hp values across the board, often leading to confusion with engine power outputs that are translated across the Atlantic.
The final unit we have to worry about is technically the most relevant to performance, and that's wheel horsepower (whp). This is the perfect indicator of exactly how much of the engine's energy is transferred through the drivetrain to the tarmac and can be measured via a rolling road.
The unit of choice for Continental Europe, PS is derived from the German phrase 'pferdestärke', which simply translates to 'horsepower'. PS is calculated in the exact same format as horsepower, but converts the entire equation into metric units.
Known also as 'metric horsepower', 1PS is equivalent to the power needed to lift a 75kg mass one metre vertically in one second. Once the mathematical trickery is over, you end up with a figure 1.4 percent higher than the old school horsepower unit.
What we should all be using - the Kilowatt (kW)
If you come from an engineering background, the kilowatt is the go-to power unit. It is calculated by using other members of the metric system - the second, the joule, the kilogram and the metre. Used worldwide across all areas of engineering, the kilowatt is a measure of energy transfer over time and is the most cohesive and simple way of measuring the power from an internal combustion engine.
To calculate a power value in kilowatts at the peak torque value, you need the torque value and the engine speed that the peak torque occurs at. Bung them into the equation below and you'll have your power figure:
For us bhp and hp loyals, kilowatts only really appear in our lives through watching videos from Australia and New Zealand, most notably Mighty Car Mods. This is because the Southern Hemisphere has fully adopted the now trendy metric system, pioneering the measurement that the rest of us really should convert to. But considering how stubborn us 'Northern' petrolheads are, it'll be some time before we talk about our project car's power outputs in the new world unit.
1kW = 1.341hp
1PS = 0.986hp
To tie this whole issue up nicely, here are a few comparisons of each of the units for some current machinery:
Mercedes-AMG Project One: 1020bhp = 1034PS = 761kW
McLaren 720S: 710bhp = 720PS = 537kW
Jaguar XE SV Project 8: 592bhp = 600PS = 448kW
What power unit do you think should be king in the automotive world? Should we stick to horsepower or convert fully to kilowatts? Tell us what you think in the comments below!