Hey all, new here. I'm looking for any and all helpful suggestions. Over the weekend, I broke my pinion on my borg warner in my 89 firebird LT1 running CMC2. I had been running a 3.45 gear but was considering going to a 10 bolt and a 3.90.

Any suggestions regarding the rear end and differential is appreciated.

I have been running that combo for two years, with an Auburn Racers Diff.
I use a late 4th gen rear, with LS1 4th gen rear brakes, which are adequate.
I have only found 3.90s from Motive Gear (made in Italy). All is fine so far!

Rob,

You can't run the 4th gen rear in a 3rd gen in CMC. These are about 3 inches wider so be careful when picking one up used if it's already out of a car.

Sidney

It looks like it's allowed by the rules. Sidney is right about the width though, it requires 4th gen wheels.

Does changing the track width (if you don't swap wheels to 4th gen wheels or make any other changes) fit under keeping the same rear end geometry? I'm not picking a fight, I'm really asking for clarification. That rule could be read to allow the rear, or not. Typically I didn't think update/backdate crossed generations, so I didn't think that would allow it.

Any thoughts?

7.29.1 should allow the update/backdate across the generation. Otherwise, 3rd gens have to run adapters to accomplish it. A 4th gen housing is cheaper then a set of adapters, it lessens the weigh gain vs. adapters, and it allows the use of cheaper OEM wheels. Based on "spirit of the rules" arguments, this should fly.

I know it is allowed to have 4th gen rears now in 3rd gen cars as they made me take one out when it wasn't! I helped get that rule changed - economical racing and parts was a good part of the argument, also that using 4th gen wheels makes it all good with the track width. This rule has been in place for many years without dispute to my knowledge.

It is a legal swap.
The question was asked about the increase in track width. No matter what rear you use, the wheels/tires must remain inside the fenders per the CMC rules. So you would likely have to use 4th gen offset wheels.

I would think that a 4th gen 10 bolt will be heavier than the 3rd gen 10 bolt since the 4th gen would be wider. Longer axles, and axle tubes equals more weight. The wheels don't factor at all since the change in offset of the wheel does not add or subtract weight.
If it was me, I would use a 10 bolt since its plenty strong for CMC use.

I'd do a 4th gen rear and avoid the wheel spacers. Wheel spacers = 2x the number of wheel studs to worry about keeping tight, and if you ever need to change the brakes its another part that needs unbolted. The wheel bearing is farther away from the wheel center which adds stress, and these shafts are know to spall. There is quality and some other design improvements in the later axles too.

Though they are aluminum the spacers will be heavier than the wider 4th gen rear. I'm calc'ing 1.67 lbs for the added tube and shaft length per side, versus 4-5 lbs per wheel spacer.