What's a good rule-of-thumb for estimating Rear Wheel HP, given an advertised HP rating for an engine? I've read articles suggesting drivetrain losses range from 15-35%. Others figure the loss as a fixed amount of horsepower, i.e. 50-100 HP. This method would imply that I'm giving up a "fixed" amount of power regardless of the output of the engine. Additionaly... is it fair to assume an automatic with a loose converter is going to have greater losses than a manual car?

I'm trying to figure what to tell my engine builder in order to develop 400 HP to rear wheels.

For an auto divide your RWHP by .85 and that will get you close. thats what i use. a stick is higher. if your trying to find your flywheel hp, and you know your rwhp say 400 divide the 400 by .85, and your flywheel hp would be 470.588. *THIS IS ONLY AN ESTIMATE*

Well if this helps,

This motor made 591 hp on a DTS engine dyno when fresh.
3 years later in the car with 4-speed on a dyno jet chassis dyno It made 477 hp. at the rear wheels. only difference was full exhaust system, mechanical fuel pump, and belt driven water pump with clutch fan.
After reading that Car craft article 21% seems about right.