It's common for everybody to say C6, but the transmission guys tell me the C4 can be built to handle HP that the C6 used to be chosen for.
Correct. The C6 is a bullet-proof tranny but also a power-robbing SOB. I'd only use a C6 again in a 70ish driver or tow rig. ANYTHING performance would get a built C4 or better if needed. I've never actually wrecked a C6 (says a lot) but I can testify that they'll turn a strong engine into a turd in stock form.
