[Bill Wells]

most commonly used 'parasitic loss' numbers in the states are 15% or 17% for a manual trans cobra replica . some like to use 20-25% so they can increase the estimated calculated crank hp. so, pick a number ! real world though it is probably 15-17%. auto trans maybe 22-25%. pulls need to be done in 4th gear for a 1:1 ratio, not fifth gear if a 5 spd.

the conversion to est crank hp is done by taking the rear wheel hp and dividing it by the reciprocal of the loss % used. ie, if you choose 15% as the loss number, then 315 divided by .85 equals 371 est crank hp. a 17% loss would equate to 380 engine hp and a 20% loss would = 393 est engine hp. same math applies to rear wheel torque numbers. some dyno guys will tell you what % loss figure they think their dyno experiences. ... not sure they know in all cases but the dyno mfr probably has more exacting data for that purpose.

most engine stand dynos are under ideal conditions. rear wheel chassis dyno numbers are real world in a real car under more normal conditions. i tend to believe the rear wheel hp from a chassis dyno as more important in setting up a car , dialing it in so to speak.

some will argue that chassis dyno's have variability in ratings between brands. i hear engine dynos do too. however, to really dial in the carb/timing etc one needs to forget what the hp pull is and concentrate on the 'directional' movement, ie, did the change to the carb or timing improve my hp/trq ratings or not ? and importantly , if you get a proper reading on your exhaust, preferrably from a bunge welded into the collectors vs a staff probe put up the end of the pipe, you can get your carb dialed in real well relative to air/fuel ratios.

many a person is humbled on a chassis dyno expecting much bigger numbers vs what hp their engine builder told them or claims of a crate engine mfr. case in point was the chassis dyno numbers used last yr at dvsf2 in ohio...not all were smiles after their chassis dyno pulls . real world to me : chassis dyno numbers.

just my opinion and experience. bill.