Two places to measure
A car maker rates the engine on its own, at the crankshaft. A tuning shop usually measures the whole car on a chassis dyno, at the wheels. Between the two sit the gearbox, the driveshafts, the differential and the tyres, and each one turns a little power into heat. So a wheel figure (whp) is always lower than a crank figure for the same car.
- Nissan GT-R R35. Nissan rates the 2017 US car at 565 hp. Cobb measured a stock 2017 car at 501 whp on its own dyno, about 11% lower (GT-R guide).
- Litchfield's LM1000 GT-R. Litchfield published both numbers from the same car: 1,047 bhp at the engine and 824 bhp at the wheels, about 21% lower. At high power the gap grows in horsepower, not only in percent.
hp, bhp, PS, kW and whp
- hp and bhp are the same unit in practice: horsepower at the engine. UK tuners such as Litchfield write bhp.
- PS is the metric horsepower European and Japanese makers use: 1 PS is 0.986 hp, so 600 PS is about 592 hp.
- kW is the official unit on European papers: 1 kW is 1.341 hp, so 300 kW is about 402 hp.
- whp is measured at the wheels. The guides on this site label every figure hp or whp, and say which one each tuner published.
Why two dynos give two numbers
Even at the wheels, two dynos rarely agree. Dyno types measure differently, shops apply different weather correction, and the same car reads differently on a hot day. On the Audi RS3 8Y, APR measured a stock car at 412 hp on its own dyno, above Audi's own 401 hp rating. That doesn't make either one wrong: it is why a single peak figure tells you little on its own.
How to compare tunes honestly
- Compare the gain, not the peak. Stock and tuned runs on the same dyno, on the same day, show what the tune really adds.
- Compare like with like. Crank with crank, wheels with wheels, and the same fuel: a figure on E85 or race fuel is not a figure for Israeli 95 or 98.
- Ask the shop three things. Is it at the engine or the wheels, on which dyno, and with which correction? A good shop will show you the before-and-after sheet and the logs.
- Watch for heat. One cool pull flatters every car. If the power drops over several runs in a row, the car is heat-soaking (see Do you need a bigger intercooler?).