How do you calculate brake hourse power from wheel horse power?

Answer 1

"they are basically equal"- Previous Answer

They are not even close to equal. You can't calculate brake horsepower from wheel horsepower unless you know the exact losses through the drivetrain of the vehicle in question. The actual percent lost through any given drivetrain can vary dramatically. In order to know the losses you would need to measure the % difference between the brake horsepower and the wheel horsepower. Then you would already know the brake horsepower, and any calculations would be unnecessary.

FYI-

Brake horsepower is a rating of the engine output before the drivetrain/water pump/alternator/other attachments. It will be a higher number than wheel horsepower. It is measured by a brake dynomometer at the crankshaft of an engine.

Wheel horsepower is generally 15-20% less (sometimes more than 20% less) than brake horsepower depending on whether the vehicle is FWD or RWD, or manual or automatic transmission. Usually the least losses will be in a FWD, manual transmission, and the most losses will be in a RWD, auto transmission.

Auto manufacturers rate their engines in brake horsepower, as it allows them to advertise higher horsepower numbers than what is actually going to the ground.