I have an 89 rallysport with a tbi 305. Its got the stock rear end, is this a 10 bolt rear end? I know it came stock with a 2.73 gear ratio, but thats about it. Im looking at upgrading it, but am not sure what to look for, is it classified as 7.5 or 7.675"? Also, how does it compare with an iroc reaer end? are they the same size/ toughness, just without posi and discs? Is the rear end in my rallly even worth building? Any input is appreciated

 

It's a 7.5 or 7.625", whatever you want to call those, they're the same thing. Yes it is the same as the ones in some IROCs, but not all; some years of IROCs, Z28s, Trans Ams, etc. (the performance line cars, not the base models like the RS and the plain Firebirds) got a different rear, the 9-bolt.

You have probably the worst possible "core" to rebuild. You probably have 26-spline axles, a 2 series carrier, and drum brakes. I would not recommend spending your money on that rear.

The ones to get are the 90-92 disc brake ones. Those have 28--spline axles (the same as the LS1 cars) and the better PBR rear disc brakes. They're an altogether better platform for putting money into. One like that would simply bolt into your car with no other changes. They're not easy to find though; don't count on going to your local U-pick and finding one, although it's not impossible; there weren't very many of them to begin with, and they're in high demand for some reason.

 

because mine is an 89, it is 28 spline, does that change anything? and im not too worried bout disc brakes, would rather not have em for the headache of the conversion

 

Odds are about 80% that it has 26-spline axles. But 89 being the changeover year, you never know until you take off the cover and look, it could have either. DON'T assume anything about any rear end until you take off the cover and look; least of all, the spline count of a 89 10-bolt.

That's why I recommended the later model ones, with the PBR calipers..... there is no headache. It's a bolt-in.

You could spend lots of money putting a paint job on a 72 Pinto.... and when you got through, you might maybe have a great paint job. But you'd also still have a 72 Pinto. Same principle with your rear end. Some things are worth putting money into, some aren't. If you're going to sink money into something, start out with the best "core" you can.

 

ok, whats the thing about this 2-series carrier? what is that? and i was under the impression id still have to at least change the combo vlave and some line hardware for discs, ur saying i dont?

 

Think about gear ratio for a minute....

There's more that has to change than just the teeth count. The gear ratio is also the ratio of the diameters of teh pinion and ring gears. But, the ring gear is the same size for all ratios of a given model of rear end, namely it has to fit on the carrier. So, what the factories do, is as the ratio increases, they use a progressively thicker ring ear, to move it over more and more toward the ever- smaller pinion. Eventually they get so thick that they can't be heat-treated without warping. At that point they make a step, they move the flange on the carrier over toward the pinion, and start over again with the thinnest possible ring gear. In the 7½" 10-bolt, that happens between 3.08 and 3.23; 3.08 and down are 2-series, and 3.23 and up are 3-series. The carriers have the flange in a different place. You can get extra-thick special gears, for a price; but then you're stuck with a 2-series carrier for as long as you want to use those gears. A smarter choice, short of buying the right varrier, is to use a spacer. It is totally impossible however to put 2-series gears on a 3-series carrier.

The later-model PBR calipers have about the same hydraulic requirements as drum brakes, so the hydraulic system is the same between the 2. The older crappy cast-iron caliper system was not like that, you do have to change a bunch of stuff to make that swap.

I have a PBR brake rear in my 83.... went right in, all I had to do was deal with the 83-back lines, as opposed to the 84-up metric ones, which was pretty easy actually. The parking brake hooks up perfectly and works great, and the brakes themselves are far better than the drums.