I have researched this myself. As you pointed out there is no vaccum to make the tranny shift. So, to make a SC work with an C-4 you have to bypass the vaccum modulator and put in a manual valvebody in the transmission. This means you'll be "shifting" the car as if it were a manual. Might be wise to just do a T-5 swap to save all the trouble.
you don't have to do anything to the modulator, nor do you need to switch to a manual valvebody. You won't be in boost at normal acceleration, cruise speed, or just going down the highway, so the modulator won't know the difference. When quickly accelerating, you'll be in boost (at least somewhat) which the modulator will interpret as 0 vacuum, just like it would if you were naturally aspirated. The modulator only controls shift rpm when the tranny is in Drive. More throttle equals less vacuum (in your case this leads up to boost) which equals higher shift points (in rpm) when in Drive. The governor still controls max shift points. So, in other words, casual driving, shifter in D, tranny shifts like normal with the modulator hooked up in normal fashion. When driving a little harder, getting into boost, the modulator might raise the shift points a little, but modulators have enough adjustment to tune it out, if you like. When wide open throttle, just shift it manually like most people do anyway. Absolutely no need for a manual valvebody.
A C4 shifts more quickly, precisely, and consistently than any T5 could ever be driven. Never sell the C4 short, especially in racing. Overdrive is the only redeeming quality of the T5, been there done that, back to a C4 now and happier than ever.
A C4 shifts more quickly, precisely, and consistently than any T5 could ever be driven. Never sell the C4 short, especially in racing. Overdrive is the only redeeming quality of the T5, been there done that, back to a C4 now and happier than ever.
Well, I guess I would then turn to the C-4 experts, because on TCI's website (for example) they clearly state that their C-4 is NOT for use with a supercharger. When purchasing my street fighter I questioned why, their answer...it won't shift correctly because of the boost effects of the supercharger on the valvebody. That's just what they told me. I personally haven't tried to run a supercharger with a C-4. The guys I know that have run superchargers with a C-4 have had the best results with a manual valve body. The guys that do not have the manual valve bodies get VERY unconsistant shifts and erratic behaviour from their C-4's. This may be a Ford / Chevy debate as to what is better, I'm only offering what I know as I have seriously looked into a supercharger and what I've posted is the knowledge I gained from researching the idea.