[Bill Wells]

so many variables can cause different losses. side pipes on a roadster alone can eat 50 hp in many cases. no idea what similar losses might be experienced on what appears to be a more complicated under car exhaust system on the spf coupe. more or less ? dunno. an engine on a dyno stand under ideal conditions with a different carb than when tested on a chassis dyno can alter results too...as would ambient temps, humidity differences as well as carb jetting/ timing etc on one dyno vs another. mustang brand chassis dynos usually give lower numbers than a dynojet brand chassis dyno based on experience of some of my friends. typical parasitic loss numbers i have been quoted from engine dyno to chassis dyno would be 17-21% loss differential. some higher, some less. use the chassis dyno as a tuning device and forget the numbers, rather look for directional changes from one xyz adjustment to another, ie, did the change directionally improve or hurt vs the baseline pulls. doesnt roush provide dyno sheets for each specific engine ? if so, you can calculate your own parasitic loss numbers comparing chassis dyno results. i use chassis dyno for tuning purposes, they are usually humbling and perhaps meaningless except for tuning efficiencies. if you have a roush engine dyno sheet , tell us your experience after a chassis dyno tuning session and see if it varies considerably from a 17-21 % range. bill.