I have a fresh built vacuum modulated trans with 3800 converter in my car.

The fresh combo has been together with a bad mail order tune for about three months. My fuel has been way lean so I have tried to stay out of it as much as possible but when I did get on it from a roll in 1st or 2nd it would pull great and shift around 6700 where its susposed to.

Saturday I finally scraped up the last of the funds and got the tune ironed out. I took the car to the track that night and it was horrible. No matter if I flashed or foot braked the converter it was shifting about the time the back tires were out of the beams. The car was in the 8.60 @ 70ish mph range
I finally attempted to manually shift (lots of fun with the stock shifter) and managed to get it down the track with a best of 7.54 @92 buts that not how I want to be doing it . What is causing this thing to shift so early from and dead stop but be fine from a role?

 

bump...

 

I can't be the only one that's had this problem.

 

Having the same problem myself and looking for the answer. I feelyour pain!

 

My tranny builder thinks the line the comes with the vac modulation kit might be sucking itself closed. I'm going to switch to hard line and see if it changes anything.

 

if it is the clear little dick cheese line that came with the kit throw it away!!!!!! I had one callapse on me and sucked trans fluid up into my brake booster, it callapsed at the very top of the metal hose inside and made a hole...it was horrible...I believed I used some very stong fuel hose

 

I think you will find that if the tires are spinning off the line, the engine will "see" the shift rpm, and go to the next gear, as this is controled by the computer. This I have seen happen before, so I doubt it is the vacuum modulator causing this. If you think the vacuum modulator is causing this, disconnect the vacuum line to the modulator and see what happens.

 

yeah its like doing a burnout in drive, you will end up shifting through all the gears sitting still because the engine is revving up to the shift point...

 

Its not a spinning issue. Its on drag radials. In the video it pulls the tire about 3" on launch.

It does not matter anyway. I blew it up agian last night. I'm tired of trying to make a OD live and not going to spend the $$ for a 4l80.

Time to put a th400 in it.