: Question about rear diff.....

Night Wolf
03-20-05, 01:59 AM
I am looking to replace the 2.28 open rear end in my '79.... something geared lower and with Posi...

at the local junkyard, there are a couple late 80's/early 90's Brougham *limos* do these use a similar rear end to my '79 DeVille?

There is also a '76 DeVille.... what is the gear in that? and would it be Posi? there is a '78 and '79 as well, but they don't appear to have the towing package, and therefor have the same gears as mine....

I would like to get some info.. this could be something I am interested in....

03-20-05, 02:14 AM
The 76 uses the older Pontiac designed full size rear end. It may bolt in, but they are not related to the 10 bolt we have today. It is a 8 7/8in ring gear. Ratios from 2.41 to 3.23 only. No aftermarket support. The cover has 10 bolts, and it is NOT related to the older 12 Bolt rear end, it is basically a HD 8.5 with a odd housing. Look for the scallops in the sides of the diff cover.

If it was me, I would look for a HT4100 car and snag the rear. Yes, flame me if you will, but the 7.5's under sane driving will survive 100's of thousands of miles. Go out to the drag strip or tow, they won't. But normal driving, even hot rodding some, they will be fine. Then it gives you cheap 3.42's.

Then you can take your existing rear end and fit it with some 3.73's and a Eaton or Auburn Posi, or even a Power Trax unit. They make them to replace even posi rear ends.


Night Wolf
03-20-05, 03:25 AM
there is an HT4100 car at the (cheap) junk yard....

but I think that is too low of a gear... and Posi is a must....

I can't see going from the 8.5" 10-bolt to the 7 5/8".... I was even looking at going to the commercial chassis 12-bolt...

I want something between 2.73 and 3.08... and a Posi.. that is all... I have 15" spoked wheels with normal tires... the engine will be built up some, but not much....

03-21-05, 12:47 AM
Trans AM's from 1970 to 1981 at least were ALL 8.5" 10 Bolts, and all had posi std. I think the Z28 did as well. Look for those cars for parts, but the axle assy does not bolt in (leaf springs).

Ratios from 76 were around 2.56 to 3.08, 77-79 were 3.23 with Pontiac 400/4sp, 2.56 to 2.73 with Olds 403/THM350. 80-81 ALL Turbos were 3.08's but had a HD THM350 (not THM350C!). Again, this is for Trans AM's, not Firebirds. Only the guts will work, but these are high bias posi's. Expect some difficulty in tight corners when they are fresh. I used to spin the inside tire all the time with mine and the 301 Turbo. One tire had to break loose, or the whole back end of the car would break loose.

You can snag a 8.5in 10 bolt G80 Locker (G80 is the option code, it is a locking diff in the trucks) from 1973 to 1999. I haven't see a K1500 Suburban yet from 95 to 99 without a G80 Locker. Check boneyards. Again, axle won't bolt in, but the guts will.
This locker is a std posi with a locking add on that locks below 20 mph and unlocks above. I don't know how well it does in racing or hard use.

12 bolts died off in the early 70's. The commercial chassis got 10 bolts. They are easy to ID. 8.2in 10 bolts and 8 7/8 in 12 bolts look similar, they have the cover with the oil grove that takes ring gear splash from the rear cover to the axle bearings. 8.5in 10 bolts like we have don't, they have a rear cover more like a Ford 8.8.

Don't worry, if you can break a 10 bolt rear end with power, you can break a 12 bolt. They aren't that much stronger.


Night Wolf
03-21-05, 02:28 PM
What is in the late 80's and early 90's (307/350) Brougham limos?

03-23-05, 08:44 PM
Hey Rick, whats so great about positraction for the '79 anyways, i know its not a daily driver for you, and i know its not going to go in the snow, so whats the big deal about spinning just one wheel out of a corner??? I had a Positraction rear end in my Roadmaster and i thought it was ok, it caused you to fishtail in the rain, of course this car was driven by a teenager, but from what i've heard, its not as easy to get posi stuck as it is an open differential but when you get stuck, its very hard to get unstuck. Give me your responses and be careful on that posi if you get it, i messed up my differential in my roadmaster driving it too hard. (it would thunk when shifting from 1st to 2nd or when coming out of a corner w/ a heavy foot.