I am just starting an 88' Firebird Project and the rear end, suspension, and brakes are first.

I have the stock 7.5 rear end which blows and i need a new one. I'm thinking the 10 bolt 8.5 but don't know what other choices I have. If the 10 bolt 8.5 will be good where should I look into finding one?

After I get the rear end figured out i want to put in a new suspension while the old rear end is already out. Plus my stock parts are basically blown and figured i might as well replace and upgrade instead of fix. I did a little bit of research but don't really know which way to go. I'm going to be getting the adjustable front end suspension for sure just because the suspension will be put in before I get my new engine. I'm going to be finding a 350 to toss into it (after I build it up a bit first) and know that the weight is going to be dramatically heavier because I only have the 2.8L V6 right now. So, I either need to plan ahead and figure that all out or get the adjustable so I can re-adjust the suspension when I put in the 350.

After all thats said and done brakes need to be installed. It has the horrible 88' 10" drum brakes in the rear.... which suck so horribly. So, I'm going to need to replace the rear and might as well do the front as well. There is the 12" disc off of the 89 and up but idk if thats going to be enough. I found this information off of the technical articles [ https://www.thirdgen.org/brakes2 ]
I don't know if its going to be worth it to spend the approximate $2,000 and brakes all the way around or if I have a better option more wallet friendly. lol. I mean, I'm not afraid to spend the money because I know that to do this thing right im going to end up putting about $8,000 into the car before I'm done. Obviously I'm looking to find cheaper ways to do things so I can put more money into the car in other areas.

Well, I have already put a lot of my questions and info down on here so I'll leave it at that. You will be seeing me post throughout the forum with the rest of my project.

 

bump

 

With a lot of fabrication work, you can put any diff you want under the car but because of the third gens torque arm suspension, the easiest and cheapest way is with a direct bolt in diff. You have a choice of a 9", 12 bolt or Dana 60. None are cheap but it makes the swap including brakes, a lot easier. Finding a 8.5 10 bolt is easy. Adapting it to fit under a third gen isn't easy.

If you're really trying to budget the build, leave the diff that's currently under the car alone. Many have survived with no problems. If you do manage to break it, then decide if a better upgrade is required.

A cheap alternative is to buy a 9" housing package. You get a bolt in 9" housing with axles. You provide the center section with gears, posi, etc and you can use your brakes or the larger late model GM brakes. This is probably the cheapest and easiest diff swap to something better than the 7.5"

 

More about brakes:

https://www.thirdgen.org/forums/faq-...iscs-89-a.html

More here as well toward the end:

https://www.thirdgen.org/forums/tran...-10-bolts.html

JamesC

 

Thanks for the help... just tryin to build up info before I start the project

 

Some don't like the option, but I did it, and so I'll throw it out there.

4th gen rear swap. I got my 4th gen rear for under $200 - it's a direct bolt in (except some rigging for the eBrake). I came across a 2000 LS1 rear, with 3.42 posi disc. So, for $200, I now have 3.42 gears, posi traction, and the nice big LS1 rear disc brakes. You do have to have the 16in wheels for the LS1 brakes. The swap will take less than a day, assuming you are by yourself.

This swap will make your rear wheels stick out 1.5 inches further on each side, as the 4th gen rear is almost 3 inches longer total.

 

Swapping in a 4th gen rear is still a 7.5" 10 bolt. Just as weak, if newer, 10 bolt.
What are you planning for the use of the car? Weekend driver, spirited driving fun car, drag car, auto-x etc?

 

Just to nitpick, the 7.625 (7 5/8) was introduced around 86 or so. The fourthgen with its 28-spline axles would be marginally stronger than a thirdgen with 26-spline. For a DD or weekend cruiser, the 10-bolt is, either third- or fourthgen, an easy option. The more wicked you get, as 91, I believe, is suggesting, the more strength you'll need.

JamesC

 

Yeah, the 28 spline was introduced in mid 89 or 90 (I think), so they are a little stronger than the early 3rd gen stuff. I can't remember why I know, but I think it had something to do with the rear from a 90 IROC I was thinking about buying.So, for the money, and maybe one of the reinforcing diff covers, it would be a little stronger than what you have. But, it's not really a great deal stronger.

 

Well, my plans overall is to build a 350, put on a whole new suspension, you name it really. I haven't decided if I want to drag, auto-x, or just mess around with my buddies. I will be putting a lot of power to the wheels tho so I need to know that my rear-end doesn't blow ot on me when I'm tryin to blast some import punk.

 

Well, what you plan to use the car for will determine alot of what suspension and braking components you install on the car.

 

IIRC, from what I read on here, the 4th gen rear will handle up to 400hp (someone chime in here). But as everything else - it all depends on use. 400hp from redlight to redlight with loose street tires is a whole different ball game than 400hp on the track with the traction of sticky drag slicks and really slamming the gas full bore to beat the green light.

But - that's also the beauty of it all - until you get that $6000 built motor that screams in at under 10 seconds, any functional rear will do.

 

4th gen diff is no stronger than a 90-92 diff. Upgrade a 82-89 diff to 28 spline axles and it will be just as strong. They all have the same weak link of a tiny ring and pinion as mentioned above. You'll break the teeth off the gears before you do any other damage to the diff.

 

ok... I never thought about the axles and splines.... makes sense. Looks like I'll just do that and if by some act of god I end up with a 400hp drag car then I'll think about a new rear end.

 

For what it's worth, LS1 cars with a T56 and sticky cars have been know to bust their rears when pretty much stock (other than maybe bolt ons).

 

As others have said, your intended use will GREATLY affect the parts you choose. If you're thinking of a more budget friendly brake upgrade, I have some LS1 calipers for sale that would probably need a rebuild. I'm in Milwaukee too so we may not have to deal with shipping.

Mike