How make power can a 84-85 GSL Diff take on street tires?

 

I read somewhere that they can hold around 400 to 500 HP......don't quote me on that....not sure if it's true ......

 

Yeah. I was running 440 rwhp through mine for a while with a Guru torsen.
No problems but the axles were quite twisted.
i believe the stock diff centre tends to be the weak point though.
definitely wouldnt try hard launches on hot tyres.

 

The diff is pretty strong. It is the same one used in NA8/NB Miatas which can handle all the power people can throw under the hood of those.

The axle housing is weak. It can flex and cause the axles to break.
Allegedly the '81-83 rearend as an assembly is stronger despite the smaller axles because the housing is stiffer.

I have broken many '84-85 axles and rearend housings but never had a problem with the diff or ring and pinion. This with 180-270hp...

 

The housing flex issue is mostly on tri-point modded housings though, yeah?

 

I started doing 3 links in part because I had twisted a lot of rearends with the 4 link and it was the easiest way to get the pinion angle correct

The diff nose would climb up. Have had them where the pinion angle was actually higher than the driveshaft angle. I think the mechanism was that, we all know the rear suspension binds because of the 4 link's geometry. That binding tries to twist the housing end-to-end. Acceleration forces will amplify the end that is trying to twist the one way and cancel out the other side. So, the pinion climbs over time.

Not just when cornering, mind you, but over every one wheel bump...

Makes one wonder if putting the rear stabilizer bar back on would be a good idea for rearend longevity

 

Oh ok. Im no engineer but the only instances and photos I've seen (online) of the housing deforming that much were with the tri-link, which in my head (probably wrongly) puts the twisting forces into the pumpkin and the whole housing rather than keeping it at the brackets either end.
i had poly bushes in mine, and later introduced adjustable poly arms, the pinion angles were never a problem, luckily binding wasnt a problem with stiffer springs, on mostly smooth roads but i did tear a mount off one time, so there is that.
the axles did have some crazy twist on them but were still straight somehow lol.
this was mostly just street driving and casual track days, so apart from the slightly silly power mine had im pretty sure your cars were getting a propper caining in comparison

 

So, the ring and pinion are not something to be concerned with?

Running 400 or so on mine with moser axles, new/stock bushings, watts link bracket and 245s in the rear.

 

I have never heard of someone damaging the ring and pinion in a Mazda 7".

You're probably more likely to have problems with the Moser axles vs. good condition OE. Good condition OE being hard to find in 2022 but Mosers are pretty substandard.

 

cool. Thx for the heads up on the R&P. The Mosers in mine, according to them are basically cut down Chevy 10 bolt rear end axles, as mine is 5 lug.

I have a good pair of 1980 axles I should probably sell, just not sure they’re worth the shipping

 

I'll buy them if they still have the backing plates on them. Looking for drums and everything else in the brakes too. My '81 was converted to disk brakes a while back and I want to get that junk off the car and return it to stock, and I also prefer the SA drums to the FB self-adjuster type. Pedal feel is much better.

 

Sending pm

 

Have you found a source for SA drums?

 

How to rebuild a third member. He is working on a 1.8L Miata diff but they are the same as an SA/FB so figure this applies

 

I turned my 4.10 gears into confetti with 300whp. Whole ring and pinion and everything. Lots of 2nd gear clutch kicks and 1/4 mile clutch dumps before it finally broke, when it broke I believe it was due to running the 615 azenies and the ambient temperature being about 40f and not warming up the drivetrain before hammering on it.