What posi rearend for 425?

[smitkd]

What posi rearend works best for behind a 425 in a 79 coupe deville? I am looking for better performance to get this caddy moving.

[cadillac_al]

I think the 90's Impala 9C1 cop cars have the same rear end.

[smitkd]

Thanks I will look into that

[Eric85]

The 79' coupe deville is a C platform car. The C platform is a close cousin to the B platform car, The only real difference in the C body has a slightly longer wheelbase. In those years the full sized cars used a 10 bolt rear end with a 8.5 inch ring gear, more commonly called an 8.5 10 bolt. A Few were produced with the big 12 bolt rear, but they are few and far between. So in short you can use any rear end form a B body produced between 1977 and the end of production in 1996, including the cop car rear ends mentioned in a previous post. I will worn you that most B cars have open rear ends, but there are defiantly some out there that are limited slip. hope this helps!

[charles a booth]

my 79 coupe has a 10 bolt rear-end but the ring gear is 8.875
the spin count on the axle is 27. i also want posi but there is no after market support for cadillac rear-ends. the ratio is 2.25:1. I looked , searched, and gave-up! post something if you have better results.
ps; i just learned this info putting rear disc brakes (off a 77 fleetwood) on my 79 coupe.

[jayoldschool]

Charles has it right: the 77-81 uses a completely different rear end than the later GM 8.5 that was used in 90-96. There is a factory posi for it, but hard to find.

[Angry Matt]

'80 factory limo has a 3.08 and '81 optional v6 has a 3.23. I wouldn't go much lower with the 3 speed tranny. Look for the tab on the dif cover that says use limited slip fluid only. They pop up once in a while at the junk yard. A lot cheaper than custom. I found mine for $25.

[charles a booth]

correction ---- axle spline is 28