Crank horsepower and wheel horsepower
Engine ratings are usually measured at the crankshaft, on an engine dyno. A chassis dyno measures at the wheels, after the power has gone through the transmission, driveshafts, differential, axles and tires. Some of it is lost on the way as friction and heat, so wheel horsepower is always lower than crank horsepower.
What the inputs mean
- Drivetrain loss: the share of crank power you assume is lost before the wheels. It is an assumption, not a measurement. The default of 15% is a round starting point, not a figure for any particular car.
- Crank power: the engine rating, to estimate what a chassis dyno might show.
- Wheel power: a chassis dyno result, to estimate the crank power behind it.
The formula
wheel hp = crank hp × (1 − loss ÷ 100)
crank hp = wheel hp ÷ (1 − loss ÷ 100)Note that going back from the wheels is a division, not an addition: at 15% loss you divide by 0.85, which adds 17.6% to the wheel figure, not 15%.
Worked example
- An engine rated at 400 hp at the crank, with an assumed 15% loss: 400 × 0.85 = 340 whp. 60 hp is assumed lost.
- A car that makes 300 whp on a chassis dyno, with the same 15% assumption: 300 ÷ 0.85 = 352.9 hp at the crank.
- At 10% the same 300 whp gives 333.3 hp; at 20% it gives 375 hp. The spread shows how much the answer depends on the assumption.
Why the loss is not one fixed number
There is no single correct drivetrain loss. It changes with the type of transmission, the number of driven axles, gear oil temperature, tire pressure and the tire itself, and even the gear the car is tested in. Different chassis dynos also read differently: two dynos can give different results for the same car on the same day, and many dynos apply their own correction factors.
The percentage model is also a simplification. Part of the loss behaves like a fixed drag that does not grow with power, so a single percentage can overstate the loss on a high-power car and understate it on a low-power one. Treat the result as an estimate to compare with, and compare dyno runs made on the same dyno under the same conditions.
Questions
How do I convert crank horsepower to wheel horsepower?
Multiply the crank figure by one minus the loss. With an assumed 15% loss, 400 hp at the crank is 400 × 0.85 = 340 hp at the wheels. The loss percentage is an assumption; real losses vary.
How do I work out crank hp from a dyno wheel figure?
Divide by one minus the loss. 300 whp at an assumed 15% loss is 300 ÷ 0.85 = 352.9 hp. Adding 15% instead (345 hp) is a common mistake.
What drivetrain loss should I use?
There is no single right value. It depends on the drivetrain, the tires and the dyno. The table shows the result from 10% to 20% so you can see the range: 300 whp works out between 333.3 and 375 hp at the crank.
Why do different dynos give different wheel horsepower?
Dynos measure in different ways and apply different correction factors, and the result also changes with temperature, tire pressure and how the car is strapped down. Compare runs from the same dyno.