LSD repair

[TheMentat]

From my limited understanding of how a Limited-Slip equipped 915 works, when the friction disks wear, it will effectively perform like a non-LSD equipped transaxle.

Is that right? Would there be any reason to refresh the LSD components other than to improve the transmission from the otherwise "standard" performance characteristics?

[175K911]

Yes, the friction discs wear out and will slowly render the LSD functionality less and less active. But it can be rebuilt pretty easily. The diff with ring gear can be removed while the trans is still in the car by pulling the side cover. You can rebuild with either the OE friction discs or the Guard Transmission parts. At that point you can build back to stock or enhance the performance depending on your needs.

[Elombard]

WOW I did not realize you could do that in the car! I thought when you reset the lash it had to be on the bench?

Very cool.

[Bill Verburg]

Originally Posted by TheMentat

From my limited understanding of how a Limited-Slip equipped 915 works, when the friction disks wear, it will effectively perform like a non-LSD equipped transaxle.

Is that right? Would there be any reason to refresh the LSD components other than to improve the transmission from the otherwise "standard" performance characteristics?

There have been roughly 3 generation of lsd used in 911/964/993
gen 1 ZF 4 plate used thru early '80s
gen 2 ZF 2 plate used thu '94
gen 3 ZF & GKN asymmetric used on some models from '90 and across the board from '94

the effectiveness of a lsd depends on
ramp angle
preload
plate #, material and assembly order

all of the gen 1 & 2 were symmetric w/ fixed 50* ramps, gen 3 used various ramp angles to achieve nominal % locks from 20/100 to 40/65 to 50/100. 993 40/65 used 50* ramps fpr acceleration and 37.5* ramps on the deceleration side.

preload is varied by using different thickness plates which are preloaded by by spring washers, the more preload the more breakaway torque is delivered.

gen 1 used 4 plate diffs, 2/side. Stock they are nominally called 40% but that is when the 2 friction discs on a side are assembled next to each other for 2 friction faces /side, when the same discs are assembled alternating driven and drive discs there are 4 friction faces/side. these are nominally 80%

gen2 were simplified to have only 1 friction disc/side for 2 friction faces /side. These are also called 40% even though their breakaway torque is much less tham from the gen 1 40%.

gen 3 street also only have 1 friction disc/side but there are also Motorsports versions w/ 2 disc/side. Street version are 22/40 or 25/65 or motorsports 40/65

any of the diffs can be rebuilt to 4 plate spec w/ more or less preload to suit the usage.

[KTL]

It should also be noted that the original friction driven discs are rather prone to wear, especially on the later diffs. Aftermarket or Motorsport friction discs can be used which have better friction characteristics. But be aware that the upgraded friction discs should be used in unison with upgraded drive discs. If you use the upgraded friction discs with the standard drive discs, the friction discs can eat up the drive discs.

When you rebuild a diff, you have to be sure to select the proper friction disc thickness to get the correct preload which determines your breakaway torque/setup as described by Bill. Herein lies the problem, the disc thickness is not a standard thickness. It depends, and you need someone with experience or the appropriate reference information to instruct you what your thicknesses and/or total diff. stack height should be.

Here's a couple of good topics for reference:

LSD Rebuild Advice and Instructions

LSD Side Bearing Preload Specs

[TheMentat]

I appreciate all of the detailed responses above. They will be useful references going forward.

My question however is a bit more basic: If you has a burned out LSD in a 915 Carrera, would you fix it right away, or wait until you rebuilt the transmission to address it? Would it effectively be performing the same as a "non-LSD" transmission?

If the work can be done with the transmission in the car, as 175K911 mentioned, then I suppose the decision would be pretty simple.

[TheMentat]

I appreciate all of the detailed responses above. They will be useful references going forward.

My question however is a bit more basic: If you has a burned out LSD in a 915 Carrera, would you fix it right away, or wait until you rebuilt the transmission to address it? Would it effectively be performing the same as a "non-LSD" transmission?

If the work can be done with the transmission in the car, as 175K911 mentioned, then I suppose the decision would be pretty simple.

[Bill Verburg]

Originally Posted by TheMentat

I appreciate all of the detailed responses above. They will be useful references going forward.

My question however is a bit more basic: If you has a burned out LSD in a 915 Carrera, would you fix it right away, or wait until you rebuilt the transmission to address it? Would it effectively be performing the same as a "non-LSD" transmission?

If the work can be done with the transmission in the car, as 175K911 mentioned, then I suppose the decision would be pretty simple.

As the plates wear the performance asymptotically approachs that of an open diff. If you don't need the limited slip properties then there is no need to do an immediate rebuild. Hopefully the fluid has been regularly changed to remove the wear particulates.

r&r'ing an existing lsd is far easier(and cheaper) than the installing one in place of an open diff.

[Matt Monson]

Originally Posted by KTL

It should also be noted that the original friction driven discs are rather prone to wear, especially on the later diffs. Aftermarket or Motorsport friction discs can be used which have better friction characteristics. But be aware that the upgraded friction discs should be used in unison with upgraded drive discs. If you use the upgraded friction discs with the standard drive discs, the friction discs can eat up the drive discs.

When you rebuild a diff, you have to be sure to select the proper friction disc thickness to get the correct preload which determines your breakaway torque/setup as described by Bill. Herein lies the problem, the disc thickness is not a standard thickness. It depends, and you need someone with experience or the appropriate reference information to instruct you what your thicknesses and/or total diff. stack height should be.

Two very good points. Even on the newer GT3 factory diffs. when guys replace just the bronze friction discs with ours and leave the stock plain plates, they are wearing out far more quickly than the ones that also go with Guard plain plates at the same time.

Furthermore, it's important to have a proper way to test any discs you intend to reuse for flatness. Discs and plates that visually appear to be "flat" and not worn out may in fact be done and in need of replacement. Ultimately the best approach is to replace all of the friction discs and plain plates on any differential rebuild.

Ultimately if one is at all in doubt, take it to a professional and have it gone through. It's not a terribly expensive service if you are able to pull the differential out yourself and take or mail it to whomever is going to rebuild it. If there is any way I can be of service in this respect please let me know.

MM

[Determined]

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[kodioneill]

That thread is 5 years old.