I'm looking to drop a 383 and a Muncie 4 speed manual into my 85 Trans Am and was wondering what rear end I should use. I currently have a Borg Warner 9 bolt Limited Slip with I believe 3.73 gears and I'm looking to install a Posi unit. My friend tells me that the housing for the gears in a Limited Slip is different from that of the Posi's and that I'd have to install a different rear end, so I figured might as well get what would be best with my engine and transmission combination. While I'm at it, I live in a city with stop and go traffic, what gear ratio should I run in the rear end, I was thinking of 4.11's. I don't like using the highway often and I figured the higher ratio would allow more torque to the wheels making it easier for the engine to move the car, thus better "in city" gas mileage. Ideas? Thank you for your time.

 

Your 9 bolt won't have 3.73 unless someone has special ordered those gears from Australia. Was the 9 bolt offered in a 85 TA? I thought 86 was the first year it was offered in the Pontiacs. 3.70 was the GM over the counter option but was never offered in a production car.

There's nothing wrong with the diff you currently have except that parts are difficult to find and performance parts are not available.

You only have a couple of options on replacement diffs.

Install a smaller and weaker 10 bolt. More parts are available but the tiny ring and pinion will always be the weak link.

Find a Dana 44 that was offered in the SLP Firehawk and was once offered directly from GM but never offered in a factory production car. Chances of finding one in good condition for a good price is slim.

Buy an aftermarket bolt in differential. Your choices are a Ford 9", GM 12 bolt or Dana 60. All will cost about the same price for a complete diff. They're all equally strong. It just comes down to exactly what kind of diff you want under the car.

The last option is to fabricate a junkyard diff to fit. This can end up costing more than just buying an aftermarket diff but really depends on your fabrication skills. A third gen uses a torque arm and there's no other junkyard diff except from a third or fourth gen that will be a direct bolt in.

Fourth gen diffs are no stronger than a 90-92 third gen diff or any other third gen diff with 28 spline axles installed into it. They'll all have the tiny ring and pinion gears.

A good aftermarket differential isn't cheap. Run what you currently have. If it breaks, then decide if an expensive, stronger diff is worth it.

 

I have never before seen "4.11 gears" and "better gas mileage" used in the same sentance.

 

First, limited slip is posi. Posi is Eatons name for their limited slip differential (LSD). The name Posi has been used a a generic name for all LSDs for so long that people think that they are two different things.

A 9 bolt or 10 bolt will not hold up behind what you are building if you drive it hard. You will need a 12 bolt or something like that to live behind your engine and transmission. The Moser 12 bolt is about $2200 plus shipping and is a very strong rear end. I would use a 3.73 in your case. You probably don't have a 3.73 now.

 

Well..... whatever it's called, the one wheel spins if the other can't get traction but they won't spin together. Some people tell me to keep the rear axle I have but I didn't think I should. My one friend thinks that instead I should look for a completely totalled Corvette and put an LS2 with the rear mounted six speed tranny in it and put in a fuel cell. Ideas?

 

As for the 4.11 and better fuel mileage, it sort of makes sense, if the engine isn't going to be on the highway where the better top speed is than the higher gear ratio will make more torque to the wheels making it easier for the engine to get the car up to 40 mph which is the average speed. The average differential and transmission's best fuel mileage speed is 55 and I'm guessing that using the 4.11 would decrease the best economis speed to around 45, just a guess though, haven't tried or did the math.