I have an open rear in my 77 Cutlass S and i want to swap it out for a posi unit. This isnt a race car, and i appreciate my decent gas milage. i just want it to have a little more get up and go, and not be a 1 wheeler. Two questions, what is the highest ratio posi rear i would be able to get that would bolt up to my car? And, what would i find such a rear underneath of? Thanks as always!

 

Do you know what ratio you currently have? It is 2.56 and down and 2.73 and up, common GM 8.5" 10 bolt. I assume you want above a 2.73 gear, get an posi, they list what ratio they fit. Monzaz on here is a good source for parts. For a whole rear end, look for the Impala SS and Caprice rear end from the late 80's to mid 90's. You will see a mileage drop with gearing that will improve acceleration, basically above 3.08. The best of both worlds is the 2004R swap and something like a 3.42 or 3.73 gear. A near stock 2004R is fine with 170 HP. Also do dual exhaust as well. Good luck.

 

Hey, not sure what ratio i have currently. Ive tried (and failed) to locate an axle tube # I've gotta get the car into my buddy's auto shop on a lift to try and find it. So is 3.08 the highest ratio offered in a posi? Ill probably be sticking with the th350 for now, but i may go with a gear vendors unit in the future. Also will the unit from the Impala or Caprice rear end be a direct bolt in? or will i need to do some modifications? thanks again!

 

I replaced my 2.56 open with 3.23's and an Eaton posi. It definitely woke the car up but I think I would have been okay with a 3.08. I'm turning 3000 rpm at 70 mph. 455/400 with a 255/60-15 rear tire.

 

The Caprice and Impala rears are direct swaps except wheel bolt pattern, pretty sure they are 5x5 vs your current 4 3/4. Posi is available with nearly any ratio but is an option even with performance cars. Forget gas mileage turning 3000+ rpm going down the highway. Choose your ratio according to what you are comfortable with. I use this calculator but add 200 to 300 rpm for torque converter slippage. Put 1 to 1 for transmission, it defaults to the .68 2004R ratio.
http://tech.oldsgmail.com/ch_axle.php

 

The 1977-90 B-body cars used 5x4.75 except for cars with HD suspension and brakes (police and taxi, primarily).

 

Looks like its time to go junkyard surfing. I know of a 74ish skylark in a local you pull it. There is also an 80 something caprice. I believe the rears are still there, as both cars are still fairly intact. Hopefully it wont be too hard to get the cover off them both and have a look at which 1 (if either) is a posi unit.

 

89-92 B Body SEDANS with towing package typically came with a 3.23 or 3.42 posi. Swap the axles out with 89-91 Police or Taxi ( SVL-2022648 ) and take a step bit to drill the stud holes a little wider and you got yourself a (nearly) bolt-in rear end. An easy-ish way to determine the 8.5 is they most likely will have a 5x5” bolt pattern.

The B-body bushings are a little wider so *MAY* have to trim them to fit inside the upper control arms. I used a couple fender washers to pick up the gap on the lower control arms.