Thread: 1999 Pontiac Engine And Tranny Swap

As long as you're taking the whole engine and transmission, the converter size doesn't matter. If you grab the wire harness and PCM from the donor car, you won't have to worry about the different gear ratio.

Originally Posted by PGT

Alrite then thanks alot for the info but where exactly would the pcm be

In the bottom of the airbox.

HOLD ON A MINUTE!

The diff ratio is different, 3.05 in the Regal vs 3.29 in the GP GT. This means you would need to reprogram your computer, since I'm pretty sure you can't use a Regal computer in a Grand Prix (BCM issues, I believe). If the diff in your current trans is still in good shape, you could swap that in and you should be fine with your stock computer.

Originally Posted by Scimmia

HOLD ON A MINUTE!

The diff ratio is different, 3.05 in the Regal vs 3.29 in the GP GT. This means you would need to reprogram your computer, since I'm pretty sure you can't use a Regal computer in a Grand Prix (BCM issues, I believe). If the diff in your current trans is still in good shape, you could swap that in and you should be fine with your stock computer.

I thought GP BCMs were pretty accepting. It's the Chevy and Buick BCMs that give issues.

If you run a 3.05 diff when the computer thinks you have a 3.29, it'll throw a code and max out the line pressures, it'll be shifting at the wrong times, your speedometer would be off, etc. Not a good idea. Either reuse your current diff, get another 3.29 diff, or reprogram your computer.

Sabrewings, I know for sure you can't use a GP PCM in a Buick, and I've always been told you can't go the other way, either. Even the gauge clusters in the Buicks get their signals differently than the GPs. Mechanically the cars are pretty similar, but electrically the Regals and the GPs have quite a few differences.

Dave did a good write-up about swapping the differential.

http://www.grandprixforums.net/f21/4...pics-1928.html

reprogramming the computer requires a tuner like HPTuners or the DHP Powrtuner.

Originally Posted by Scimmia

Sabrewings, I know for sure you can't use a GP PCM in a Buick, and I've always been told you can't go the other way, either. Even the gauge clusters in the Buicks get their signals differently than the GPs. Mechanically the cars are pretty similar, but electrically the Regals and the GPs have quite a few differences.

I wish I still had my GM SI manual installed to look at the differences.