I was at a local junkyard today and found a 1989 GTA Trans Am.It was a 350 tpi car and had a 9 bolt disc brake rear.What years were these used and were all 9 bolt rears posi traction?When I spun the 1 side i noticed that the other side moved exactly the same so i am assuming it is a posi rear.Also what gears were installed in these 9 bolt rears?

 

I believe all 9 bolts were indeed posi as it was supposed to be a performance option i dont see them using an open carrier.

The gears could have been anything, pull the cover and count teeth.

Now comes the fun part, do you really want it? They may be somewhat stronger and have a posi carrier but are pretty rare and expensive to fix/repair.

 

I can't say if all the 9 bolts were posi or not but they were considered the performance diff and should all be posi. The GTA diff will probably have 3.27 gears in it.

9 bolt diffs were offered from 1987 to 1989 with the 89 disk brakes getting the PBR calipers and large rotors. The most common gear ratio was 2.77 offered to the 5.0L. 3.27 was the common performance option for the 5.7L engines. A rare option was a 3.4x ratio and GM offered a 3.70 gear set over the counter.

You can assume that diff has 3.27 gears but you never know what a previous owner had done. It's always possible it was upgraded to 3.70.

It's a good find if it's complete and the junkyard doesn't want an arm and a leg for it. As mentioned above, it can get pricey if it needs a major rebuild since parts are hard to find and expensive.

 

2:77 gears were the standard ratio for the 350 TPI for both the Camaro/Firebirds and the 3:27 were the option at extra cost. 3:27 ratio was never standard on the 9-bolts.