[MrWesson22]

Due to larger and heavier internal components, the rule of thumb I've heard is that a similarly built C6 requires an additional 20-25% horsepower to spin over a C4, which equates to around a 10-15% hp difference at the rear wheels. I haven't done any testing personally, but if you go to mustangsandmore.com and ask Alex (aka Moneymaker -- SS NHRA nat'l record holder), he says a C4 is generally worth around .3 in the 1/4 over a C6. I'm a 4 speed guy myself, but everyone I've talked to says a C6 is only beneficial if a C4 will not handle the power produced, which will not be the case on a 289.