My car has a 2.73 open rear diff. I want to swap in one from a z28 with the 3.42 ( I believe this is the lowest they came with) and a positraction with rear disc brakes. Did all z28 come with 3.42 and posi?

 

No (my IROC, for example, has 3.23's). Perhaps the following link will provide some useful info about gear ratios and limited slip:

https://www.thirdgen.org/forums/tran...-10-bolts.html

JamesC

 

My 87 IROC originally came with a 9 bolt diff and 3.27 gears but that option was for the L98 350 TPI engines.

 

Most thirdgens have 2.73 or 2.77 gears. It's usually a safe bet that Z28's and performance oriented Firebirds will have posi. But it's also a pretty safe bet that the OEM differential will be nearly worn out.

If you're looking for a more performance oriented gear ratio, look for a 6cyl car as most came with 3.23 or 3.42 gears. They most likely won't be limited slip, but they're much easier to find then a performance geared posi rear with a working posi. In other words they're cheaper.

If you really want better gears, and a strong posi, rebuild the rear you'd got with what you want. Rear discs are over rated. The front brakes do most of the braking. A functioning drum setup works better than the 82-88 iron caliper rear discs anyway. The 89-92 PBR brakes are hard to find, expensive, and marginally better. The only real advantage to them beyond cosmetics is they're super easy to service.