I thought I saw a thread on this before but can't seem to find it. The th400 that I put behind my 455 just doesn't shift nearly as crisply as I had hoped. It's a rebuilt unit out of a 73 grand prix. I had a local trans shop rebuild it with a B&M overhaul / shift improver kit. Still slides a little on the 1-2 shift but worse on 2-3. At wot I don't think it would ever make it into 3rd without lifting. Any ideas?
1st thing that needs to be done is verify pressures that are being seen in each gear. sounds to me like pressures could be off. There could some pressure leakage inside, there could be too much clearances in the clutch pack, or there could not be enough fluid flow. some of these can be addressed with tearing the trans apart, but clutch pack clearances to adjust its has to come back apart. there is a pressure test port on the drivers side of the case about 1/2 way between the shifter shaft and kick down plug. pressure specs from tci website
Have you tried an adjustable vacuum modulator? Or, if there is an adjustable one in there, have you tried adjusting it? Reason I ask is because the adjustment can be abused and that will result in lower line pressures. Try turning the adjustment in clockwise for firmer shifts.
