I have a 91 GTA that only spins one wheel when I spin the tires. The RPO sheet says I have a posi. When I do the -up-on-the-lift-wheel-spin-test-, it says I have a posi. My guess is that the posi is just worn out. Do I need to replace the whole unit? If so, I hear the best is the Zexel Torsen T2R is the best for handling applications. (Autocross, etc.) I've also heard that they only stand up to one season of real racing. Supposing I use the car as a daily driver and occasionally race it (AutoX and Drag) can I expect this unit to hold up for the life of the car? Do I need to worry about the number of splines on my rear axle? I dont know how many bolts the differential is. It's in storage, so it isn't convienient to pull it out and throw it on a lift. Did all 91s come with the same differential? Thanks for the help! Bill P.S. Where is the best place to order one?

P.S. Can I re-gear when I'm in there? I think I have 3:23s now. I want the car to be capable at auto X / circle track / street and still drag race it. Should I go to a taller gear ratio?

How much does it cost & what parts do I need to upgrade to 28 spline axles.

(Do I have 26 spline stock?)

Is the upgrade worth it?

I made the switch from an open diff to a complete 4th gen rear about two weeks ago. So far I love it and think the upgrade was well worth it. If you have an auburn posi unit, you cannot rebuild it and will need to replace the whole unit. The local ring and pinion shop around here would charge $150 for just changing the unit, and they wanted $300 to do gears at the same time. You car should already have 28 spline axles, I think they started making those standard in 1988 or 89. I wouldn't know about how long the Zexel Torsen unit lasts, but when I put mine it it had about 80k on it and its still going strong. Since the Zexel uses worm gears and relies on light friction to bias the torque rather than a clutch cone like in the Auburn, it realistically shouldn't wear out nearly as fast.

If I were you, I would just get an entire 4th gen rear from a 1999 or newer. You'd get the Zexel Torsen, 28 spline axles, and disc brakes. You might even get 3.42 gears if you find one that came out of a six speed. You could probably find one at a Junkyard for $400-$500 or look on ebay. I got lucky and found one locally for $250. The biggest PITA I had was installing a prop valve to adjust line pressure to the brakes. There's a good article in the tech article section here on how to do it. I never did get the flares quite right on my brake lines...

Getting a higher gear ratio will help your acceleration, since it will multiply your torque more. I'm not sure because I've never been on a circle track, or road course, but I would think that it would help you get to top speed quicker coming out of turns, although that top speed would be slightly lower and your gas mileage would suffer.

Destructimus, thanks for your reply. I want to keep my GTA rims. From my understanding, the 4th gen rear sticks out much further on both sides, so I'd have to change rims.

What gear ratio would you all reccomend?

I got a 3.42 rear axle from a 2000 T56 SS Camaro with 80,000 miles. These things had Zexel Torsen differentials stock. You don't HAVE to use different rims. Look how mine stick out and decide for yourself.

Personally, I got 12" disk brakes, 3.42 gears, and a Zexel Torsen Posi for $260. I'll live with the extra 1-3/4" on each side!

Actually it only sticks out about two inches extra on each side, which results in it being about as far out as the top of the wheel-well- hardly noticable. As long as your GTA rims are 16", you should be able to keep them. Any smaller and they might not fit over the caliper. What gear ratio you use just depends on what ratio of speed to gas milage you want. 3.73 will get you the most speed but least mileage and so on. If its just a weekend car and not a daily driver, I'd go with that. Otherwise I'd reccomend 3.42 or 3.23.