Adam wrote:kevm14 wrote:I could always have the axle shortened, but that sounds like a good way to spend lots of money between the housing and getting new (shorter) axles then a new POSI unit with possibly different gearing. It would probably be cheaper to upgrade the carrier in the stock axle then run it until I start making more than 260hp. Or just run the stock axle and not do a lot of one-wheel burnouts.
Anecdotal experience: a guy I used to work with at Ford had an '88 Monte SS. He swapped in an early '90s 454 with truck oval port heads and a mild cam upgrade. He didn't touch the trans or axle (non-posi). He drove the car for at least two years like that w/o doing burnouts and didn't have a problem. That was probably at the 300hp/400lb-ft level with the cam/intake/headers combo. Doesn't sound like amazing output, but considering that is 50% over stock, it really woke the car up.
So axle/diff strength requirements depend on power/torque, transient spikes, and weight/traction. If you don't increase traction, chances are pretty good that you could put 500 hp into the axle and it would be fine. It's when you start adding traction that weaknesses will be revealed.
Think about it this way: what is the maximum input torque that the axle will ever see? The answer is however much torque is required to break the tires loose. And that number doesn't change if you double, triple or quadruple engine output. And in gears where you DO have traction, you are, by definition, putting less torque into the axle than you were in 1st gear when breaking the tires loose. Rolling burnouts at 80mph don't put any more stress on the axle/diff than a burnout at 10mph. And burnouts aren't really that stressful. Now doing one against the brakes does put more stress on everything, but I can't say off hand how much that really adds.
With no reverse peelouts, no one wheel peels and no neutral drops, I'd imagine that rear would be just fine. Until you start adding traction. And on the street, I doubt you can really add enough to destroy the axle if you follow the rules I just mentioned. Oh, and the formula changes if you have a manual trans and you're banging gears. The shift shock transients can cause issues.
The benefit of the 8.5" is really more just because you won't have to think about the axle as much when hooning around. Also hotrod parts for the 7.5" pale in comparison to the 8.5", I think. I'd be surprised if you could even get a Truetrac for example.