Audi-Dudi

I'm rebuilding an OEM LSD from an '83 944 to install in the back of my AWD A4 -- save for some minor machine work, the two diffs are interchangeable! -- and have found the ramp angles are 30/30, which is quite an aggressive setting for a street car, even one made by Porsche.



Since I also found evidence this diff has been apart at least once before (two sets of witness marks on the pinion shafts, which means they've been swapped top to bottom at some point in time), I suspect the ramp angles have been increased from the original setting to increase the amount of lockup under acceleration and braking.

If so, then I'll probably go shopping for another diff, since I don't want or need any lockup on braking, let alone the maximum amount possible. On the other hand, if this is how they all came from the factory, then I guess I'll just go ahead and install it regardless since I desperately need a rear LSD (because of the Torsen center diff, AWD becomes ZWD [zero-wheel drive] when it lifts the inside rear wheel in corners) and something is better than nothing.

Oh, and as an interesting aside, the plates and friction discs in this LSD appear to be the same as those in the ZF LSD I have out of a '73 BMW 3.0SI ... I have not checked yet, but it's possible the BMW parts (if still available!) could be less expensive the equivalent Porsche parts. Something to look into...

Thanks!

Premier Motorsp

Interesting research.

I think the two marks you are seeing are normal. One for the coast direction and one for the power direction at each end. Seems like the power direction mark should be bigger, but your picture looks normal.

Those ramp angles appear to be stock. That diff is in no way going to be aggressive at all. I can't remember the number of plates in that one, but no factory LSD's in 944's are at all noticable (in a negative way). You could remove the coast lockup by machining the pressure plates. I'll bet that a four banger Audi engine produces so little decel drag that after you split it two ways the force left to engage the LSD will be next to nothing.

In the distant past I needed plates for a BMW LSD and found that BMW does not sell anything but the complete diff assembly. I tracked some plates down from ZF USA after a great deal of effort. My application was a 91 M5 if I remember right.

Interesting comment about the ZWD system. 993s are the same way. Even with AWD you still need the LSD to go fast.

Chris Cervelli
Premier Motorsports

Sanjeevan

Chris, I am surprised to hear that the 993's will do ZWD if the wheels lift, has it been corrected with the 996's. I think even the lexus awd detect wheel lifts.
Thanks
Jeeva

Premier Motorsp

The 993s and 996s have essentially the same AWD system. It is a simple mechanical/hydraulic system so there isn't any way to direct torque away from the spinning wheel.

What they do have is ABD (automatic brake differential) which clamps the brake on whichever rear wheel is spinning. There are two problems with this: First, when you are accelerating out of a turn on your way to a heroic lap time, you don't want the brakes activating. Secondly, the system is inactive over 50kph (not sure of the exact speed, but it is pretty low) so the brakes don't get overheated.

Putting the brake on the spinning wheel does sort of send the torque away from the spinning wheel. This seems like a backwards way of doing this for a sports car to me.

The other thing about 993s and 996s is that since they are 911s they have 60% of the weight over the rear wheels. It takes a pretty extreme situation to get wheelspin.

Chris Cervelli
Premier Motorsports

Audi-Dudi

True, but I was referring to the shafts that left those marks on the ramps, not the marks on the ramps.

There's only one friction disc per side, although I'm probably going to add a second disc and plate, if only to increase longevity. The dogs on the side gears are just deep enough to support a second friction disc and by machining the reverse side of the pressure plate shown in the photo, I can make enough room for the second plate as well.

As for negative effects resulting from 30/30 ramp angles, don't forget that my Audi's basic nature is to be an understeering pig ... anything that causes even more understeer or reduces its turn-in ability (like locking on braking) is something I'd prefer to avoid if at all possible!

Hmm, you may have a point there. That said, machining the pressure plates will also have the effect of reducing the amount of preload, and since preload is the only thing that limits slip when a wheel's off the ground -- the ramps don't come into play then -- and this is my primary reason for installing an LSD in the first place, I'd rather address the lockup factor through the ramp angles.

Well, I wasn't thinking about BMW itself as much as the BMW aftermarket suppliers. We'll see...

Unfortunately, this is a real problem with the way I have the suspension setup and will become even more so after the new 2.0L engine is installed (335hp at the wheels!) If I had my druthers, I'd lock the center diff -- I've never been a big fan of Torsens used fore-and-aft -- and I'm actually considering adapting a 1st generation Type 016 tranny with a locking center diff to replace the (I'm told) somewhat fragile Type 01A that came with the car. That said, I also have a spare Torsen center diff on hand and if only for experimentation purposes, I plan to modify that to increase the bias ratio from the OEM 2:1 to (again, I'm told) as much as 4:1 ... it also appears the 3:1 Torsens Audi used in their 2nd generation Type 016s will swap into a Type 01A tranny, and as I have one of those on hand as well, I can experiment with this for sometime until I figure out what the optimal setup is...

Anyway, thanks for the input!

JoelG

As far as I can tell there is no aftermarket for BMW diff parts and the last model that had parts available from the factory was the 2002 (production ended in '76).

There are a handful of places that rebuild BMW diffs as a business and most of them dont offer parts, however I think diffsonline.com does. I assume they buy the bearings and other generic parts like that direct from the mfgrs.

I have a couple of diff clutch discs that I bought from Bimmerworld. I assume they had them made locally. You could call James and ask him.

If you learn anymore it would be great if you shared it here.

Audi-Dudi

The 2002 diff is smaller and lighter duty than the "senior six" diff I mentioned, and while 2002 production did end in '76, the "senior six" diff BMW was using in the 70s was still being used in the 6 and 7 series cars through the late-'80s (or at least a variation of it). I know for a fact that parts were still available from BMW as late as 1985, because I rebuilt the 3.0Si LSD I installed in my '78 530i that year...

BTW, the ramp angles on this diff are 45/45, so if the internal parts are indeed interchangeable (as I believe to be the case, but I'll have to put the two side-by-side to confirm), I could remachine the drive ramp angle to 30 degrees and leave the coast angle at 45 degrees ... not a big change, perhaps, but every little bit helps and it's a better solution than machining new ramps in between the existing ramps of the OEM part.

Of course! BTW, thanks for the parts source tips ... I'll check them out ASAP.

BTW, Metric Mechanic offers a rebuild/upgrade service for BMW diffs and they do some interesting things to them. For one, they remachine the ramp angles to 30/30 (from 45/45) to increase the lockup under acceleration and braking, but then decrease the preload; they also modify them to use additional friction discs and plates, but in some instances, add them to just one side, not both (probably because there isn't enough room to do both), which is odd because this will cause one wheel to behave differently than the other.

Of course, they always have gone their own way with modifications and maybe they know something about those cars that I don't...

Audi-Dudi

I received a private email from someone I believe to be a reputable and knowledgeable source who confirmed that the OEM ramp angles for the 944 LSD are indeed 30/30, and that these are also the ramp angles for the OEM 930 and 915 LSDs as well.

In addition, I learned that with one set of discs and plates, this diff is 40% locking, and with two sets of discs and plates, it's 80% locking. The discs and plates are also available (at least at the OEM level) in different thicknesses so as to make it possible to adjust preload. Lastly, he confirmed that ZF did/does indeed use the same internal parts to make diffs for several different manufacturers and as such, it's highly likely the parts from my old BMW LSD will interchange with those in the 944 LSD.

Isn't the internet wonderful?

JoelG

I didn't mean to imply you couldn't get parts for the older diffs. I think there's a very good chance you still can thru Mobile Tradition (Maximillian Importers).

I just meant that as far as I know (and I dont know a lot) you never could get parts for the medium case diff used in the E30s and E28s from the dealer. Now, if the senior 6 diff is one and the same, I am clearly wrong. I do know for sure there was never a field manual issued for the E30 that covered diff rebuilds like the was for the '02. If its the same diff as the senior 6, that manual should be out there somewhere.

BTW, I left my diff with York Automotive for a rebuild yesterday. I dont have the know-how or tools to do it myself. It was a 40% lockup built by Koala Motorsports. Pretty much down to 0% lockup now.

Audi-Dudi

My knowledge of BMWs pretty much stops at E12s as the last one I owned was a '78 530i (albeit with triple webers, headers, 9.5:1 pistons, etc.) While the guts of the LSDs used in 6- and 7-series cars through 84 or so were the same as those used in the earlier "senior six" diffs, I believe the cases themselves were slightly different so they weren't a direct bolt-in swap.

As for the factory service manuals covering the LSD rebuild procedures, this was clearly spelled out in the two-volume 530i/528i manual I had. Oddly, the one-volume manual that covered just the 530i didn't have it.