A friend of mine and myself will be building an ~800hp 427 (stroked 351) with EFI that will be extensively dynoed on the engine dyno to work out the bugs. It will then be chassis dynoed after it is in the car with the racing C4 attached. I am looking forward to seeing the numbers and the true powerloss through the transmission and drivetrain.
It really varies from car to car. rjcuise car was 15.5% loss thru a 4 speed and 8" rear. Other 4 speed cars with 9" rears have been as much as 20% (537 FWHP vs 428 RWHP). 9" rears use more power than 8" rears. My car was a C4 with a 9" rear and RWHP was 315. Then I converted to a T5z and now it's 345 (some other minor mods may have been responsible for a few of those HP). T5z is more efficient than a toploader so maybe the loss is something like 15% with a 9" rear. C4 and 9" rear loss was probably more like 22%. An AOD is more efficient than a C4 I believe so 20% may be a good estimate for a car with a 9" rear...possibly less for a 8" rear equipped car. 25% sounds like a lot of loss for your car.
All of this is just estimates unfortunately since I never had a engine dyno. Don't you just love direct answers...LOL ::
The thing I find puzzling about parasitic loss is how that percentage stays mostly constant regardless of the engine's power. You would think it takes a given amount of power to turn the accessories, the trans, and the rear gear. Let's say it takes 40HP to move all that, so that a 200 crank HP engine shows 160 to the wheels. Now if you dropped a 500HP mill in that same car, it won't make 460HP (500 crank - 40 drivetrain loss). It will likely make about 400 at the wheels, turning the same accessories, the trans, and the rear gear.
I don't confess to understand why.