rear end

im in the process of getting a new rear end for my 94 caprice which has a 7.5 inch. i was looking into the 8.5 inch and found a few at some local junk yards. the best one i came across was for $150 and came out of a caprice wagon and is a posi unit with 2.93 gears and drum breaks. I have dreams of sometime in the next couple of years putting a 383 stroker motor under the hood of my 94. how much power can these 8.5 inch rears withstand? if im going to do this i want to do it right the first time. i know it will be much easier to get a 8.5 out of an impala, caprice wagon or roadmaster but i would be willing to go the extra step now so that i wont have to redo the job again down the road. should i try to find a ford 9 inch or will the 8.5 do the trick?

The wagon rear has an overall track width about 1.5 inches wider than the sedan. I don't think you want the wagon rear.

Are you sure the current one you have is a 7.5? I thought all of the 91+ cars got the 8.5". If not, keep looking for a 9C1 or ImpSS rear.

ya i do have a 7.5 inch. i also found a 8.5 out of a roadmaster but they want $250 for it. it also has posi but only 2.56 gears but i am changing the gears anyways. would this be the correct width?

The Roadmaster Sedan, proper width. The Impala SS also has a 1.5" wider track width than the Caprice, so unless you plan on running rims with the offset that Impala SS wheels have, stick with a Caprice or Roadmaster Sedan (or Cadillac Fleetwood of the same year) rear end.

Again, cheap parts abound here in the Phoenix area, I got my rear axle from a '94 9C1, complete, rotor-to-rotor for $129. It's 3.08:1 and 8.5". I did, however, have to rebuild the carrier and replace the axle bearings.

Keep in mind that if you want to go 3.08 on that axle you're speaking of, you will also have to replace the carrier (the Series I with the 2.56 gears will not work with 3.08s, you need a Series II carrier).

The Roadmaster sedan and 9C1 got a axle with the proper width mounting but the overall width is a little narrow due to clearing the skirts on the Roady and to leave snow chain clearance on the 9C1, will work perfectly though even with SS wheels, they just sit a little further in the wheel well. Roady got drums, 9C1 got discs.

The SS and civi LT1 Caprice sedan had a wider housing, the civi one had drums the SS had discs.

As covered the wagon axle is wider yet as are the mounting points, you do NOT want to try and put it in a sedan.

The rear discs as setup by GM are NOT an upgrade, but rather quite the contrary for daily driving, system can be modified to work right but honestly the drums work quite well so long as you aren't road racing.

94-6 the civilian L99 cars often came with the 7.5" axle, mine did, I put a Roady sedan axle in it and then later stumbled on an SS axle with some mods I wanted and that would allow my 10" wide drag rims to fit better so that is in the car now.

Saw your question about gears and power in another thread, will answer here.

Currently is an SS 8.5", stock axle shafts, Eaton posi and 3.73 gears.
Stock LT1 b-bodies put about 220hp to the rear wheels after drivetrain losses, my car currently puts more like 400hp to the wheels and the axle is about the last thing I am worried about.
The 7.5" though will not stand up to a stock LT1 if you get traction, that is the same internals as they use in f-bodies and those lighter cars break them.

When you start looking to make more power start another thread, LOTS of missunderstanding out there about making power. Guys are way too quick to do crappy 383s and then someone like me walks all over them with heads and cam on a stock shortblock in a car that sees more miles. Don't repeat the same mistakes everyone else makes. Problem is most guys are never able to admit a mistake and instead try and get others to do the same because if lots of people make the same mistake it must be "right".

My car certainly does not sound stock and the behavior is certainly not stock, but it is still mellow enough for me to hand the keys to anyone and easy enough for anyone to jump in it and run the dragstrip numbers I do with it. A lot to be said for keeping things simple and understanding NOBODY has a better R&D budget than GM, even if their ultimate goal is not the same as ours they still make a lot of good stuff that most guys mistakenly write off as "stock and therefore no good" and endup wasting money in the wrong areas.

I still get about 19mpg in mixed driving with it and may be able to improve that with more tuning.

Except the 9C1 cars also got discs, not drums.

I went and edited, was just a matter of typing speed and thinking speed being out of synch.

wow, thanks for all the help guys. 3.73s is exactly what i want to do. right now i have only come across two 8.5 inch rear ends and i called over 20 junk yards in the state of connecticut. one is out of a wagon like i mentioned and the other is out of a roadmaster. maybe ill wait a little while longer to see if anything comes up and im still slightly confused about the width factor but im sure ill figure it out. thanks again to all

The sedan axles all had the same width mounting points, the overall width on the skirted rear wheel well cars is narrower to clear the skirts better and they used that width on the 9C1s in order to make room for snow chains on the tires should departments want them.

Very slight cosmetics that the vast majority of folks wont notice is the only "downside" to the Roady sedan axle and if you plan on getting different rims you could oder offsets to accomodate and hide that easily.

ok so if i went with the roadmaster rear i found; 1. could i keep the posi unit that is in it now or should i get a new one and 2. would i have to replace the carrier or could i just put the 3.73s in the stock carrier? or would both of those questions be something that would have to be looked at first?

what is the difference between a series 1 and series 2 carrier.

what is the difference between a series 1 and series 2 carrier.

THe location of the flange the ring gear mounts on.

2.56 gears have a huge pinion gear so the ring gear flange on the carrier is moved further to the driver's side to make room for it. I find 2.56/posi to be an unlikely setup. 2.56 is certainly possible but if it is posi I would expect to find 2.93s which would have the right arrier for 3.73s.

If it is a 2.56 carrier then I would get a new posi fair number of choices all with plusses and minuses. Lots of guys argue for the Eaton and similar because they are "rebuildable" but I looked into the cost of the clutches and such and you might as well buy a new one. The Torsen do not have clutches to wear as they are gear operated but they can be a little fragile if hit with serious shock loads, usually only a problem in manual tranny cars. The Auburn are not rebuildable.

They do make "ring gear spacers" so you can use 2.93 and numerically higher gears on the 2.56 carrier but that is added cost just to reuse a likely almost worn out stock piece so I would not try that.

ya i got the vin off the car the rear was in and called a GM dealership and they ran the numbers and said it was an 8.5 out of a roadmaster, posi with 2.56s and drum breaks. but i will call again just to make sure

The VIN will not contain any information aside from giving GM a go point if their information goes back that far and the car is original.

There is a decal under the decklid with a listing of RPO and SEO codes - called the Service Parts ID or SPID label. This will give you the codes that are specific to what was installed in the vehicle. These are 3 digit alphanumeric codes (such as Z24, Z28, 9C1 etc) that pertain to certain options installed on the car. The codes you are looking at all start with a "G", but they might not all be present.

2.56 gearing is GU1
8.5" is G87
7.5" is G89
Limited Slip (Positraction) is G80