Tom Trew

Last year, after 6 years of track time on the Limited Slip in my 993 transmission, I had it disassembled, and replaced the stock LSD with a Porsche Motorsport LSD. I also had steel synchros installed while I was at it (in fact worn synchros were the catalyst to do the work in the first place). I have had the old LSD on the shelf since then, and finally decided to look inside. Like many of you, I kept reading about friction disks and ramps and such, but had no good mental picture of what was going on inside the unit. Hopefully the following will help make it more understandable.

First thing I noticed before even removing the transmission, was the color of the transmission fluid. I replaced it a couple of times, and each time it was as black as tar. I now know that is the friction disk material as it is worn off of the disks themselves. I will watch the color of the transmission fluid carefully to assess the wear on the new LSD.

The first picture is of the differential in it's case as it is removed from the transmission. The large flange on the left is where the ring gear bolts on. This whole unit turns as power is applied by the pinion gear which is the output of the transmission gears. The two drive shafts insert into the two splined holes on either side of this unit.

http://forums.rennlist.com/upload/as..._lsd_copy1.jpg

Here is the LSD with the cover plate removed. You can see the first stack of friction disks/pressure plates still inside the case. What is inside is a standard differential gear set along with two sets of friction disks on either side.

http://forums.rennlist.com/upload/cover_removed.jpg

Here is the first stack of friction rings being removed from the differential housing.

http://forums.rennlist.com/upload/fr...ng_removal.jpg

Here is the actual differential gearset removed from the housing with the two stacks of friction rings (one on either side) still in place. This is essentially all that is inside the differential case.

http://forums.rennlist.com/upload/diff_and_rings.jpg

Here is the stack of rings as they came off of the differential gears. Notice there is one friction ring, one pressure ring, another friction ring, another pressure ring, the rightmost ring is a beveled washer that appears to keep tension on the stack of friction disks. I think this means that this LSD is one of the "4 disk LSDs", since it has two friction disks on either side.

http://forums.rennlist.com/upload/ring_stack.jpg

Close up of a friction ring. The material seems to be some sort of carbon material much like a clutch disk.

http://forums.rennlist.com/upload/friction_ring.jpg

Close up of a pressure disk. These two types of disks are alternated in the stack.

http://forums.rennlist.com/upload/pressure_ring.jpg

Here is the actual differential gear cluster itself. This also shows the infamous "ramps" we keep hearing about. It appears that as one output shaft turns faster than the other, the planet gear shaft inside the differential will begin to ride up this ramp. As that happens, it forces the two halves of the differential casing apart, which applies pressure to the friction disk/pressure plate stack. That serves to lock the differential to the outer casing, which means the differential "locks" and forces power to go to both drive shafts.

http://forums.rennlist.com/upload/diff_with_ramps.jpg

Here is the planet gear set inside the differential shown above.

http://forums.rennlist.com/upload/planet_gears.jpg

All in all, it is a fairly simple device. Mysterious only becuase I couldn't see what was happening. It is clear that the friction surfaces wear just like a clutch does, and that at some point, it will lose the ability to lock up at all. I measured the 4 friction disks and the least worn measured .096 inches while the most worn measured .064 inches. The way the unit is designed, the first friction disk closest to the differential gears takes very little wear, while the one in the middle takes most of it.

Tom

clubsport1

Thank you very much for that informative write up.
My plate lsd is not working at 100% i am due to replace it with the mechanical gears next month, I assume you favour the upgrade, however i wondered if there were any downside to street driving as the car will be used mostly for street and 5+ track days a year,,thanks again..

Tom Trew

Kim - I spent some time at my shop as they were disassembling my transmission. It is not so much that you need special tools, but more that you need to know where the little spring-loaded pawls are that will jump across the room if you are not careful. The shop owner that did my work had learned the lesson the hard way, and knew how to take the gearsets out of the tranny without it happening. Once you know that secret, it looked like a pretty straight forward process. Now I know how to take the LSD apart, and how to replace the frictions disks. That can be done with the transmission still in the car too, since there is an access panel on the side of the transmission.

Clubsport - You should do some research on this forum to understand the difference between a geared LSD and a friction plate LSD. Bottom line seems to be: geared for Autocross, friction plate for Track. The Motorsport LSD certainly has a more aggressive lockup than the stock unit. I can feel it even on the street accelerating out of a turn. Of course, I am also comparing it to my stock LSD that was likely worn out for the last couple of years. Not a big issue on the street, you just know it is there.

chris walrod

Tom, thanks for the info and pics..Really apprecitate it.

Those ramp angles for accel and decel are worlds apart. Ramps are typically symetric in the way they are splined, or keyed into the diff housing. This will allow the ramps to be reversed to change the behavoir of the diff. I wonder what the angles are from that diff? And, if they varried in the production 993's...

Thanks again...

FastMaz

Tom,

Thanks very much for the informative photos and write-up.

Another black box opened up!

FastMaz