mk3cn4

iTrader: (3)

I got 99 T/A with ~500hp turbo, stalled A4 using stock diff with "upgraded 3.73 gears" (per prior owner of the car). No problems or symptoms with diff.

Have been doing occasional track runs mid/low 11's around 120-125ish mph with drag radials and always 1.8 sixty with no desire to do harder launches at this time.

I have nightmares about this rear diff going out at highway speed on the street, I occasionally break 100mph tuning "off track" away from public (i think my 3-4 shift happens near 100mph).

I guess my question is: by using the track to flush out problems, does that somewhat guarantee that my diff won't break on the street/highway, because "if" it's going to break it'll be at the sticky track under a hard launch or during the 120+mph trap?

I think I've asked this before in another thread a long time ago but thought I'd get some fresh input.

I basically use the trap speed at the track to measure performance gains, so will likely never do slicks or harder launches than this.

Seems like almost all diff breaks seem to happen at hard launch NOT highway speeds but want to hear IRL expereinces.

Note: I have miniscule budget for a new diff and prices seem crazy high right now. I'm not worried about annoyance/cost of breaking my rear, I'm worried about having an accident at high speed on the street now that I'm applying 500hp to this diff with 3.73 torque. Again, am hearing/seeing no symptoms at all right now and havent for the 8 years I've been driving this car.

Full Power

You're more likely to grenade a driveshaft at speed than a ring n pinion.
As well, rear axle pinion and diff SIDE bearings usually make a lot of noise and vibration warning of their demise LONG before spectacular disassembly.
If you are SERIOUS, drain the oil, inspect for color, and run high grade REDLINE shockproof Hypoid oil in there.

RPM WS6

iTrader: (3)

I agree. ^

Anything *can* happened, there is no 100% guarantee that it will only break in one way or under one condition, but the vast majority of catastrophic failures would be at launch (either from extreme traction - more likely with an M6 - or from wheel hop), or from a really hard-biting 1-2 shift (much more of a problem for an M6 than a stalled A4).

So, best "safe" answer, the odds are on your side. Having said that, I've never known a diff to just fail at high speeds unless it was run low on fluid and overheated (power level notwithstanding).

JohnnyBs98WS6Rag

iTrader: (1)

IMO, at your trap speeds, I'd be worried about that 10-bolt at the track now.

Having said that, if the rear isn't making funny noises on the road back to get your timeslip, it'll get you home. If your rear is on the verge of blowing up, you'll hear it complaining.

mk3cn4

iTrader: (3)

Not sure why, but had no idea this oil existed. Absolutely will be researching this! I check the existing diff fluid very very often but didn't know there was a more durable version.

It still worries me but I'm a little less concerned at the track because I have a helmet and fire jacket and there are safety measures in place that make accidents less deadly (guard rails/rescue teams).

This was the sort of experience I was looking for. Rest assured, if I get the slightest hint that this diff isn't happy I'll be taking care of it immediately.

if anyone else has experience in this area I'd like to hear.

mk3cn4

iTrader: (3)

Ok, thanks for the input, here's where I landed so far.

I am going to change the diff fluid since it's got a ton of miles on it.

Since I drive this car all winter (as long as forecast doesn't show snow), I don't think I'm going to go with the serious racing fluids like the Shockproof stuff.

Once I get under it, I plan first on making sure it's a Torsen not an Auburn. I understand Auburn will have a silver cover and does not have the vertical ribs. Car is 99 so very likely this is Torsen. Then I'm going to drain fluid and inspect diff and old fluid, and if all looks good will then run Amsoil's Severe Gear 75W-110 or similar synthetic which seems to be a popular less extreme option that's safer than base/stock oil and can be run even in cold temps.

https://www.amsoil.com/p/amsoil-seve...gear-lube-svt/

Any additional input welcomed!

RPM WS6

iTrader: (3)

Unless you have a '99 Firehawk or SS, there is no chance of it having come stock with an Auburn. 1998 was the last year that GM issued the Auburn LSD as standard for the LS1 F-body, but it was optional via SLP on Firehawk/SS in '99+. Of course, it could have been changed by a previous owner.

FWIW, the cover was the same for the '98 Auburn vs. the '99+ Torsen, so you won't be able to tell any difference there. But the optional SLP Auburn (again, something that you could only get stock if it's a Firehawk or SS) did come with a cast aluminum AAM cover.

I don't personally see trap speeds as measure of the likelihood of the diff failing. An '02 WS6 that I put together with a friend many years ago was trapping at ~130mph with a Procharger and internal engine mods. It was an A4, launching soft with 60-foots no quicker than 1.7. That car grenaded several "built" 4L60Es and eventually put two holes in the engine block, but the rear never broke.

mk3cn4

iTrader: (3)

That's good to know, sounds like way more abuse than I'm giving my diff LOL

From what I understand, the 4l60e's really hate high input RPMs when using high power. To help protect my transmission I have the shifts down around 6000-6200 and engine cutoff at 6400.

My stall is wayyyyy too high anyway (Yank PT4400 I believe it was from the prior build) and my RPMs only dip a few hundred on a shift. Car kinda sounds like a golf cart going down the track, I'm sure I'm losing some power there but "in my mind" it's like a cushion on the driveline LOL. If I'm able to hit mid 11's in the 120's that's plenty good enough for me for a DD.

JohnnyBs98WS6Rag

iTrader: (1)

I was using the trap speed to gauge how much power the engine is producing. I guess I don't have a good appreciation for how much "cushioning" a typical F-body AT drivetrain provides. I swapped in a CK Performance built 200-4R in my Cutlass and with 3.90 rear gears it bangs 1-2 and 2-3 pretty damn hard even w/ the stock 2-bbl SBO. I'm just glad my rear is a 8.5" Type-O, but nervous about my stock 2-piece driveshaft. I'll be upgrading that after I rebuild my engine (~7L stroker).

Treburkulosis

Agreed I run redline in all of my rear ends.

RPM WS6

iTrader: (3)

The 8.5" is a tough rear. I have one in both my '71 (not original to that car) and my '74. The '74 doesn't make any power nor does it "launch hard" (haha...it's comical to even use those words in this context - 4,000lb car, 2.56 gears and 200hp... ), but the '71 is set up to bite pretty hard from launch when it's on a sticky tire. I have ZERO worries about the 8.5", and I wouldn't go with anything else unless I planned to make it a race car and start dipping into the sub 1.5 60-foot range.