fazm83 said:
actually i believe the standard is 18% (which is a mix between the manual and automatic). Its just how much they underrate the engine. 210hp with an 18% loss is 172 at the wheels. so if its 191 at the wheels, that equates to around 225 at the flywheel. Underated about 15hp.
the gt's ive seen put down 276 at the wheels stock, thats bout 325, so the gts are about 25hp underrated.
My matco tools guy use to work at a dyno plant for ford. He says the way they use to come up with the hp #'s was dyno numurous engines, take the one with the lowest hp and take about 10% off of that. He says each engine can vary 10% in power (10% of 300 is 30hp at the flywheel)
There is no standard for this as it is FAR FAR FAR to inaccurate.
Loss through drivetrain will vary from make to model even down to individual cars. The same car infact can produce different rwhp numbers without any tuning or mods due entirly to inaccurate measurements.
Drivetrain loss is not a static figure, but nor is it a fixed %. The theory would follow the more power produced the more loss but the % would probably drop, e.g.
15% of 200bhp = 30bhp loss
10% of 500bhp = 50bhp loss
Higher bhp engines loss is a lower % but more bhp.
If follows the other way too, the drive line must require X amount of power to rotate it, but if you hooked it upto a 5hp lawn mower engine the 15% rule would be riduculas as it might not have enough power to actually turn the drivetrain in the first place.
Many people use the 15% for ease of use (automatics lose more so say 20% and 4wd with an auto maybe 25%+). So to give an estimate:
This calculation is an easy way of doing it.
210bhp x .85 = 178.5rwhp
However rolling road dyno's will never give accurate results, if you dyno the same car 5 times in a row you may get upto 15rwhp variance, and not in any particular order, some say heat soak causes a loss of power but other people then get better numbers after each pull even though the car has got hotter.
Use rwhp only ever as a GUIDE which could easiily be 10-15% wrong in either direction. The tyres and tyre pressures will affect the reults and they have nothing to do with the power the engien is producing.