Power can be a confusing thing, right? Be it bhp, hp, PS, kW, it seems there are infinite ways of representing how much grunt an engine can offer.
For us in the UK, we’ve at least remained settled on bhp for the most part. But then there’s an extra layer of confusion to throw in there – power measured at the engine, and power measured at the wheel – which can vary wildly. What’s the difference, and which should you trust? We explain.
What is brake horsepower and how is it measured?
Brake horsepower (bhp) is, simply put, the measurement of a car’s power at the flywheel. That means it takes into account frictional losses from within the engine (which horsepower does not, but that’s a whole other explainer) and offers up a figure for the power delivered to the car’s gearbox.
How is wheel horsepower different?
As the name suggests, wheel horsepower (whp) is the figure measured at the wheels. This is always lower than a bhp figure as it also takes into account losses of power through the rest of the drivetrain including the gearbox and drive shaft.
There’s no hard rule for an exact percentage loss of power between bhp and whp. This will vary on a car-by-car basis depending on factors like the drivetrain layout, the weight of components within it and the type of gearbox used. The weight of wheels and tyres also have an effect.
A very loose rule of thumb is 20-25 per cent, but don’t take that as a guarantee.
Which is used more often?
Manufacturers pretty much exclusively quote power measured from the engine as this is always going to be the higher figure, and the more marketable one.
Most dynos, meanwhile, will measure wheel horsepower, simply because it’s much easier to get a car on a rolling road rather than going through the trouble of removing an engine to simply measure power. However, most companies offering dyno services will also have their own calculations for converting that to bhp.
Which should I brag about?
Really, this depends if you want to boast about how much power your car is putting down to the road or how much its engine can produce. Most companies with dynos will have their own calculations for converting wheel horsepower back to brake horsepower, but it’s never truly going to be 100 per cent accurate. Wheel horsepower is going to be your fairest comparison for useable power in truth, but it’s also going to be the lowest figure.
Can I improve wheel horsepower without improving brake horsepower?
Yes, you can, but any gains will be marginal. Using lighter alloy wheels is probably the easiest way to improve your wheel horsepower, or you could go to the lengths of a lighter driveshaft. It’s not the most effective way to make your car faster, though – if you want more speed, it’ll be easier to focus on pure engine upgrades.
