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Discussion Starter #1
I'm looking to install a posi rear end from a late model Impala SS or Caprice into my '79 Deville. What's a good price for one with 3.08 gears and disc brakes? I'm just looking for a ballpark figure to start with! Also if anyone has any good sources for one, I'd appreciate that as well! :thumbsup:
 

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$429 will get your a posi unit that you can install in your existing rear in a couple hours from PowerTrax.

Ballpark I would expect is $500. I have seen cop cars used in rough shape for for $600 with the rear. So keep an eye out for that. You should be able to bolt on the front suspension for the stiffer bushings and beefier sway bars and springs (springs are somewhat harsh from what I have heard).

1994-1996
Impala's have 3.08's. Cop Cars have 3.23's. Both have 4 Wheel disc.

Roadmasters have 2.93's, no discs
Fleetwoods get 2.56's no discs
Fleetwood Broughams have 2.93's no discs
Fleetwood Broughams with RPO V4P have 3.42's (rare)

From what I can tell (please correct me if I am wrong on this guys) the cars with traction control could NOT get RPO G80 (posi). But only the Fleetwoods/Fleetwood Broughams could get traction control. Limo's and Commercial Chassis, Impala's, Caprices and Roadmasters all could get G80.

But, be away the Limo rear end may be the 9.5", not the 8.5". It will bolt in, but replacing gears may be more difficult.

Armored Chassis got 3.42's also.
 

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Discussion Starter #3
Thanks for the help! I thought about upgrading my current rear end, but I would like disc brakes as well. Since you mentioned prices over $500, I found a 1996 Impala SS rear with 3.08 posi and disc brakes and 70k miles on it for $349. So I guess that's a pretty good price?
How much torque can these stock SS axles take? I have a 472 in my Deville that is putting out over 500 lb-ft.
 

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If you ever can break it with street tires I wanna see your engine! The 8.5" came behind cars like SD455's and Buick GN/GNX and GSX 455's. No, unless you are at the drag strip with slicks and a 5000 trailer, you will likely never be able to blow that rear up.

Good price. It is possible prices are on the way down with current economic conditions. I may have to go look for one!

I have always heard if you can break a 10 bolt (8.5") you can break a 12 bolt (8 7/8") Chevy. (this is not the rear end on the 74-79(80?) B/C/D cars that had the Pontiac 8 7/8" rear end.
 

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I just found a slew of them! http://www.car-part.com/ that someone posted on another thread. Down as low as $200 w/3.42s, disc and no posi! Probably a Cadillac V4P Rear end, no other car could get it in 1996 as it is listed.
 

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Discussion Starter #6
I think I checked that site out, but none of them were close to me and had the posi.
That's great about the durability you mentioned! I am not too familiar with the strength of the stock 2.28 rear in my '79, but I do want one that can take the 500 ft-lbs or more from my 472.
 

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It is possible GM went with the 2.28's for a couple reasons. These are theories now.

1. GM had excess parts from the 74-76 cars that needed to be used, might have been carriers, or something. So worth the tooling costs of keeping it around. Still saves money even if you have to spend $ to not scrap material.

2. GM may have determined in extereme testing, like towing or ? that the 2.28 ratio needed for fuel economy in 1977-1979 that the 8.5" and 8.75" (rarer?) would not survive with the Cad 425 V8 well enough for reliability.

3. It is exclusive to Cadillac. Cadillac loves exclusivity....
 

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Discussion Starter #8
I'm sure it was for fuel economy, since a 2.73? option was available. I got as high as 17 MPG on the highway with the 425 doing 70 MPH, so that's not bad! I just didn't know how strong this axle is compared to the newer Impala SS axles.
Also, do you have any recommendations for what to look for with trouble spots, etc? I plan on at least changing the fluid in it, but any other things I should do or look for?
 

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The 8.5in got as low as 2.41's, never lower. So maybe they just needed the lower gear and the 8.5" ring was too small to support it safely for reliability.

8.5's are tough, they can take cop car pounding and laugh at most of it. A 350 Chevy in factory trim will never hurt a 8.5". A 2.1 gallon Cad, maybe, with slicks. But not without. If you can hook up, there is an engine problem! If you can't, the axle isn't seening big power.

Not really any trouble spots, the 8.5" came out in 1970 for the F Body, and promptly was stuffed behind 400's and 455's, as well as 396's and LT1 350's. And they didn't have reliability problems. If you are concerned on power, get a rear cover girdle. That can help, but I don't know honestly how much. I can say it likely won't hurt, as it braces up the rear caps to the housing, that should help cap distortion under serious power with slicks. The shock is what gets it.

Don't swap axle shafts from side to side, else all should be solid for you.
 

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Learn something new every day.

Thanks for the site info. on car-part.com I had not heard of them, and I'm in the market for a left side headlight. Mine has a nice hole in it from a rock. It's still functional, but only time before someone sees the hole during an inspection and fails the car. Then I'll be in a bind to get one fast.
 

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Discussion Starter #11
Is there an easy way to identify the axle? Like how to tell if it is a posi, and what gears it has?
 

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Years ago, GM posi differentials came through with a little metal tag that sez to use gear lube additive to keep the thing from chattering around turns. I look for the little metal tag attached to one of the cover bolts. This can be left off when work is done so is not 100%. When cruising junkyards, I'm always on the lookout for those tags! Ratios can be had from the axle codes which are on the axle tubes in a few different spots depending on years. It can be figured also from the option code tags. It's easy to mark a tire and driveshaft on a posi axle and count the turns.
 

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As far as I know, the Impala SS rears came with 3.42's too. Don't depend on factory limited-slip diffs for handling a lot of torque though. The clutches tended to get trashed pretty easily. The 9.5" rears in limos were VERY heavy-duty but early ones had a loud pinion whine since they came directly from the truck division, and also beware they didn't come with ABS. Installation in older applications wouldn't be an issue obviously. If you want one of those, the '93 was available with 3.73 gears! Come to think of it, I guess you probably don't have to worry about that though with the kind of torque you're putting out.
 

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Discussion Starter #14
Yeah, I'm kinda thinking the 3.08 would be about perfect, especially going from the stock 2.28. I don't want to make an all out drag car, but have a nice streetable car. I think with all the torque of my 472, a 3.08 would be about right. Any suggestions?
 

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caddydaddy said:
Yeah, I'm kinda thinking the 3.08 would be about perfect, especially going from the stock 2.28. I don't want to make an all out drag car, but have a nice streetable car. I think with all the torque of my 472, a 3.08 would be about right. Any suggestions?
One question.
You mention having 500lb/ft in your '79 so I'm assumming that you dumped an old engine in your car. You ARE aware that those old torque & HP specs were bogus, right? Your engine is probably a lot more like maybe 350lb/ft at best, maybe even lower depending on the mileage and condition of the engine. The point being is that a little over 300lb/ft of torque isn't all that much so I'd be cautious about going with too tall of a rear.
 

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Discussion Starter #17
The Ape Man said:
Your gas mileage will go way down with those cogs.
It shouldn't be that bad with 3.08's. And this isn't my daily driver, it's my 3rd car, so the fun mileage is what I'm looking for!
 

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Discussion Starter #18
Katshot said:
One question.
You mention having 500lb/ft in your '79 so I'm assumming that you dumped an old engine in your car. You ARE aware that those old torque & HP specs were bogus, right? Your engine is probably a lot more like maybe 350lb/ft at best, maybe even lower depending on the mileage and condition of the engine. The point being is that a little over 300lb/ft of torque isn't all that much so I'd be cautious about going with too tall of a rear.
Yeah, I dumped that weak 425 about 5 years ago! I now have a modified 472 that's putting out some good power now. Conservatively it has at least 380 Net HP and if not 500 lb-ft, then very close to it. I had a machine shop that does mostly race engines do the work, like head porting and balancing. It now has higher compression (9:1), Comp Cam, Edelbrock intake and Q-jet, shaft mounted rockers and better springs for the higher lift cam. I hope to get the headers I have to fit, and it has dual exhaust now with DynoMax mufflers.
That was the reason I don't want like a 3.73 gear, I have the torque to not require it. I think a 3.08 should be about right to make it very streetable, yet have good acceleration. I've done some weight reduction to the car (aluminum hood, fuel cell, etc.), so I'm sure it's under 4,000 lbs now.
 

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If you don't care about mileage then the 3.08 will work. 3.41 or 3.73 will be great out from a standing start if you have good traction. If you are using street tires and are interested in embarrassing Hondas then stick with the 3.08's. Want best ET at track with slicks same rollout maybe 3.41's are a bit better. Your car will fly with 3.41's even with a 425. See you at the gas station.
 
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