I just saw this in a different thread on another site I think. I don't remember what exactly they said, sorry. But you could change the stock valvebody to a manual for like 75 bucks. If you must retain it, go to Fordmuscle.com. They have a c-4 forum there and it is FULL of c-4 guru's. Hope that helps.
TOM B said:
Trans-go does. I used it, and love it. It's a shift kit, and if you go through it and change a few more parts that are supplied, it changes it to a complete manual valvebody. Ithink they even sell them on e-bay for 57, but then shipping and what not. You may be able to save a few bucks.
10secgoal said:
When you go into WOT there is no vacuum. It should be at 0. Then when boost kicks in it pushes the one way check valve shut thus not allowing the boost to screw with the vacuum modulator.
Under normal driving there should be no boost thus the transmission will operate no problem.
I would suppose that since the tranny's modulator sets the firmest shifts for 0" vacuum(which when n/a would mean wide open throttle), that when blown and equipped with the check valve it would still shift the hardest at 0" vacuum. In a blown application, 0" vacuum could occur at nearly any rpm or throttle position(except closed). If your blown engine reachs 0" vacuum at half throttle, then your trans will now shift its hardest at half throttle b/c the modulator would normally interpret 0" vacuum as WOT. Because of this, I would say that some means of boosting line pressure would be strongly recommended in a blown application.