164runner
Well-known member
new engine 408c, 13.5 to 14 of vacuum at the manifold and at the vacuum modulator. transmission is C-6 from performance automatic. when you drive it easy, it upshifts early and seems to slip before it go in drive. if you stop and take off and give it more gas [ drive it harder ] it shifts fine. stop it, pull it back in low, manually shift it, its ok. 60mph floor it, drops to 2nd and don't let off it upshifts fine. called performance automatic said it needs 15 of vacuum at the modulator, to shift properly, if not need to go with a manual valve body and shift it manually. they said adjust vacuum modulator first , go in 3/4 to 1 turn and play around adjusting it and see if it fixes it. ideas ? am I getting told a line of crap? I did go in 3/4 a turn was no help. letting car cool off and turn it in some more and see if it helps. when engine was stock it shifted fine.