Hello,

This site has been a tremendous help sorting out my '89 IROC-Z 5.7's options. I now know it had an incorrect rear axle when I bought it--drum brakes. The differential (which was not even a posi) went out shortly after I started driving the car, and I replaced the whole axle assembly with a used one that I thought was a match. This was before I knew that I was matching an incorrect setup. It was supposed to be out of an '87 Firebird, with 3.42 gears.

This rear is definitely a POSI! but still has drum brakes. The set-up performs well, although the gears howl a bit and may need attention. The car stops very well.

Chevrolet last offered a 3.42 posi as part of the IROC G92 option for the 5.0 in 1985, according to the 3rdgen.org site. If I have this one, it was at least designed to stand up to the190 HP LB9 (if not my 5.7).

I don't know if it's worth putting money into the present setup, or if should I go back to the original, which was a 2.77 Borg Warner. The car now gets good acceleration and mileage, so I'm reluctant to go to taller gears. However, doing so would make the "numbers match."

If you can answer these 5 questions, you'll make a newcomer very happy:

1. Does it affect the value of the car greatly to have non-original gears and rear brakes?
2. How hard is it to find a decent correct rear? (GH3 Borg Warner 2.77) I've heard the BW's are much more expensive to rebuild than the Chevys, so I suppose they are higher priced, too. Are they worth it?
3. Can the brakes be converted to the correct disks separately from the differential, or should I look for the whole package.
4. Are there problems with rear disks that might be the reason for the original swap?
5. Was an '87 Pontiac 3.42 posi a decent differential that can handle an '89 5.7 that is driven with some attitude?

Thanks for your help.

 

I don't think most people would care about that. Most people who would be looking to get the car would be much happier with a 3.42 posi instead of a 2.77 posi. At least I would.

 

I hope these answer help:

1. Changing anything from "factory" will reduce the value of a numbers matching car.

2. If you do a search on the internet somewhere or maybe even in a junkyard, you may find a BW rearend that matches what is supposed to be in there. They are much more expensive to repair or rebuild because it is an autrailian built rearend and not as common as the 10 bolts that come in these cars. But again, if you want a numbers matching car, it is worth it in the end.

3. You cannot convert a drum brake rearend to disc brakes (not with factory parts anyways).

4. To my knowledge there are no problems with the rear discs on 3rd gens. I have never heard anyone say anything but good things about them.

5. The 7.5 rearend in general is a pretty decent rearend up to about 350-400 horsepower, after that you should start to think about upgrades.

I hope these help.

 

88Camaro350 and MCas7,

Thanks to you both. I'll probably start hunting around for the "correct" setup when I'm done having fun and ready to sell (or if this one breaks). In my humble opinion, CAFE had a lot to do with putting numerically low gear ratios in many of these cars. Anyone else have input on this controversial subject?

 

As far as converting the rear drum to disc's, it can be done. Slight modifications will be necessary but nothing that is super difficult if you have any skills with power tools. Check out the tech article on this site:https://www.thirdgen.org/newdesign/t...eardisc.shtml. I just picked up a complete rear end setup from HP Salvage in Houston from a 2000 Trans am for $200 bucks. Everything needed was included. They don't use the torch when they dissassemble their vehicles. I'll be doing the conversion in the spring on my 91 Formula since winter is starting to settle in here in Mass.. I recently installed a 357 TPI and converted over to a WC T5. The rear got a Zexel Torsen posi with 3.42's it was previously an open diff. with 2.77's. Get the 3.42s you won't be disappointed. It's also a drum rear hence the disc package I bought. I also have read on this site that prior to either 88 or 89 the rear disc set-ups on the thirdgens were pretty much garbage. I don't personally have experience with any but my friend has an 85 IROC with rear discs and he will be doing the LS-1 disc swap also. He doesn't care for the stock setup!

 

Thank you, Luxor,

I like the idea of a Torsen--no clutches! I did not know it was available in 3.42 ratio. The car seems to like this gearing, and I wouldn't have to change speedo gear. As I understand it, the '89 and later rear disks have larger rotors(11.65") and do work better. I wouldn't consider anything smaller. Thanks also for the price and availability info. This is great!