If you really want to know, put the engine on an engine dyno... What does it really matter what it makes?
That said, it is a percentage, but it is not how you guys are calculating it. The proper way to calculate it is to know what percentage of power your Clutch/Torque Converter loses. Then you have to figure out what your transmission loses, then your driveshaft, then your differential/axles and half-shafts if your car has an IRS. Change any of those variables( ie lightweight driveshaft ) and the formula changes again. It only works if you have SAE corrected RWHP numbers as the engine originally was rated in SAE.
That said, it is a percentage, but it is not how you guys are calculating it. The proper way to calculate it is to know what percentage of power your Clutch/Torque Converter loses. Then you have to figure out what your transmission loses, then your driveshaft, then your differential/axles and half-shafts if your car has an IRS. Change any of those variables( ie lightweight driveshaft ) and the formula changes again. It only works if you have SAE corrected RWHP numbers as the engine originally was rated in SAE.