Carlarena1

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!

olds 307 and 403

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.

Carlarena1

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!

75 W-30 H/O

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.

olds 307 and 403

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

joe_padavano

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

Carlarena1

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.

brownbomber77

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.