TR6

Apparently, the clutch pack in my LSD is worn out. I tested it by jacking up the rear of the car and turning one wheel with my torque wrench while another guy held the other wheel. There was very little resistance and he was able to easily hold his side while I turned my side. I assume this means the clutch pack is worn out. My car has option 220 (limited slip).

On to my question. What parts are needed to rebuild it and is it something that is with the reach of a DIY for an average home mechanic. Is there an upgraded clutch pack that I can install that will perform better than the factory spec?

One more question: Is there any harm in driving the car on the street or track with the clutch pack worn out? Will it simply behave as a none LSD car or am I going to damage anything? I can feel a difference in the handling on the street. When I put the throttle down in a straight line, the car slightly torque-steers to the right. This is how I first noticed that something was not right and a buddy suggested checking the LSD.

Geoffrey

It will depend on what is worn out. You'll basically need new friction disks and floater plates. The street car came with 2 plates each, the G50/10 came with 4 plates each and are made from more durable material. You may need to replace the pressure rings if they are warped, cracked, or heat checked. You have 40/40, the motorsport version is 20/100. I'd stay with the 40/40.

No, you won't hurt anything unless a friction disk cracks, or otherwise becomes inoperable.

TR6

Geoffrey, thanks for the great information. Can you elaborate on why you would stay with the 40/40 rather than the motorsports 20/100 version? Does the motorsports version have more limiting capability? Not to sound ignorant, but I'm not familiar with what 40/40 vs. 20/100 really means.

JW in Texas

Greg,
It's the lock up percentages on accel/decel. I think The motorsport is 20% on accel/100% on decel. Yours is 40% on both. Geoffrey certainly knows A LOT more then I do so I'm sure he'll chime in with corrections/more details

PS: I would let Todd handle this one since he has set-up & rebuilt a BUNCH of our Cup trannies.

Bill Verburg

As noted above the #s are the accel/decel lockup #s.

You existing lsd can be set up as a 40/40, 60/60, 80/80 etc by juggling plate # thickness and assembly sequence. 40/40 is sort of the standard for street use w/ the the higher #s increasingly biased to track only use.

Starting w/ 964 RS and turbo an additional feature was added to the lsd, variable ramp angles allowing different accel/ decel lock ups.

964RS & 964t used 20/100

993 used 25/65, 993RS 40/65

The variable split diffs have been generally found to be advantageous. But you can't rebuild yours to variable spec, you can swap it for a GT or later variable type

Additionally the 2 plate unit used in your 964 can be rebuilt w/ more, and more robust plates for longer life and greater limiting effect. GT(Guard Transmission) is a great source for this operation

Geoffrey

The 20/100 and 40/40 describe the locking characteristics of the unit. It does not represent a 20% lockup or 40% lockup as commonly refered to. Lockup amount and rate is determined by the preload of the clutch pack and the angle of the ramp on the pressure ring. Altering either will alter the overall locking characteristics. As best as I can tell, the number represents the angle of the ramp milled into the pressure ring, and that is all it means. For simplicity, a higher number represents more pressure on the clutch pack, and therefore more resistance of one wheel spinning at a different rate than the other. You can change the variable rate by purchasing different rate pressure rings and there are two available for a stock diff 40/40 and 20/100.

In my experience with the Cup cars, 20 on the acceleration ramp does not provide enough pressure on the clutch pack to prevent the inside wheel from spinning and 100 on the deceleration ramp causes too much understeer. The 40/40 is a good compromize and about your only option for the smaller diameter diff in the 964 transmission. You can of course replace the entire diff with the larger 993 style or an aftermarket Guard unit provided the mainshaft has the notch for the larger housing.

I hope this helps.

TR6

Thanks, this is great information. Now I just need to see what its going to cost. I asked about whether it was ok to keep driving it this way because I have a couple of DE's coming up in the next couple of weeks. I know I won't have it fixed by then.

PorscheII

You probably have a 40/40 (40%) LSD, which has two friction discs and symmetrical 30-degree ramps. The only alternative when rebuilding your LSD is to squeeze in 4 friction discs, which would yield an 80% LSD. But even just replacing your two friction discs would probably make you happy with improved performance.

Geoffrey

How do you figure that?

Bill Verburg

Here are the internals of a ZF clutch type lsd

4 internally splined friction plates(2 per side), separated by 3 externally splined transfer plates, capped by a thrust washer, the pressure rings have the V shaped ramps in which the spider arms ride. Through '89 the ZF ramps were cut symmetrically. Starting w/ 964RS & Turbo & continuing w/ 993 the ramps are cut asymmetricly(this can be seen in pic 2 of a 993lsd)




On the early 4 plate cars a lsd could be set up as either a 40% or 80% solely by arranging the assembly order of the friction and transfer plates

The lsd was called a 40% when the disks were arranged as follows on a side FFTT, only 2 interactive surfaces(counting the end plate surface)

The lsd was called a 80% when the disks were arranged as follows on a side FTFT, because there were more interactive friction surfaces, 4 interactive surfaces(counting the end plate surface)

As Geoffrey said the % is only a notional #, the actual break away torque is affected by the # and type of friction surfaces and the preload on the assembled unit.

The later asymmetric ramps added the accel/deccel variance to the notional #s as well.

911 4 plate ZFs were used thru ~'83, they were changed to 2 a plate design ~'84 which were continued thru at least the 964 non RS, non Turbo units

PorscheII

Here is some more input:

30 degree ramps yield 80% with 4 discs, 40% with 2 discs
37.5 degree ramps yield 60% with 4 discs, 30% with 2 discs
50 degree ramps yield 40% with 4 discs, 20% with 2 discs

... where disc means "friction disc with two active sides", and % means "percentage anti-slip effectiveness".

All terms/definitions are as per Porsche factory literature.

TR6

Great information! Here's one more question: Does anyone know approximately how many hours a shop should charge to do this for me? Can it be done without removing the transmission completely?

Bill Verburg

the early(>'~83) ZFs used symmetric 30*ramps & 4 friction disks(2 per side) the difference between 40% & 80% was only due to the assembly sequence

The ZFs from ~84 > '89 used 2 disks, 1 per side w/ symmetric 30* ramps, ony 40% is possible w/ these

don't know thw 964RS/964t ramp angles but Porsche Motorsports 40/65 LSD as used in 993RS has a 50-degree ramp on the acceleration side and 37.5-degree ramp on the decel side, utilizing a total of 4 friction discs.

Many pro race teams opted for somewhat more aggressive 50/80 ramps. Some teams even used dual Belleville washers per side, for a tremendous amount of pre-load.

In '03 Porsche came out with a version of their 40/60 LSD that had thinner discs, allowing a total of 8 friction discs and 8 plain plates. Most teams used
40/60 rather than 50/80 ramps. These LSDs have no place on the street .... strictly track, due to their aggressive nature.

To sum up whats available
1.0, 1.5, 2.0, 2.2 Belleville washers... which can of course can be eliminated altogether

2, 4, 6, 8 friction discs

40/40, 40/60, 50/80, 80/80 ramps (which can be reversed.... or figures halved by using only two friction discs or placing discs back-to-back).

All of the above info comes directly from Paul Guard

Blu RS

"Apparently, the clutch pack in my LSD is worn out." Whoa, brother. I went down this road last year and have I got some pertinent information to share with you. I say pertinent because you and I have the same cars (1993 RSA) with the same transmission (G50/05) and the same 40/40 LSD.

First of all, I can tell you unequivocally that the LSD in your car is not the LSD pictured in Bill Verburg's post. You may or may not wish you had that LSD, but in any case you don't. I've personally worked with two LSDs from two separate RS America transmissions in the last year or so, one of which was from my personal RSA with 45K and another RSA (1994 model) transmission with 100K+. They were both identical two (2) friction disk LSDs 40/40 and neither had their friction disks worn out when taken apart. These LSDs are the "old style" 40/40 LSDs and are particularly identifiably because they have squared shoulder tabs for the ramps and plates versus the clearly rounded shoulder tabs you can see in Bill Verburg's photo.

My car with 45K miles had a substantial number of track days under it's belt, while the 100K LSD was mostly a daily driver/commuter. Friction disks from both were similar regardless. Both local Porsche service agents and Guard Transmission took a look at the friction disks from my LSD and concluded the friction disks were not worn out, nor were the preload springs. Yet I thought my LSD was worn out (knowledgeable people in the PCA community led me to believe that LSDs wore out before clutches on track cars) when in fact my clutch was starting to slip. That's what got me going down the same path as you to try to find out what I needed to do to get a "better" LSD. Also I had decided to do a new clutch job at the same time anyway, so what not do the LSD at the same time!

Early on, I called the good folks at Guard Transmission and I absolutely encourage you to do likewise because they're recognized experts; they have tons of practical knowledge. There's also an excellent article on LSDs on their website you should take a look at as well, but that's a separate item for the moment.

So, back to the story, last year I shipped my LSD to Guard Transmission for inspection because I was under the impression that perhaps it could be rebuilt into a 4 friction disc version or perhaps have different ramps installed. Long story short, this model of the LSD cannot/will not be able to be rebuilt into anything other that what it already is, i.e., a two disc 40/40 model. You will find yourself in the same situation with your LSD. However, what they could do was to supply and install their proprietary version of upgraded friction discs. The price is reasonable for the two new discs and I had a reasonable expectation that their discs would be more effective (let's say offer more "friction") than the Porsche parts, which I was going to replace anyway just because I already had the darn unit apart so why not take the opportunity to put new stuff back in, right?

OK, so I had my newly refurbished LSD with GT friction disks put back into the transmission and promptly went out (with new clutch as well) and did a track day. Man, oh man. Things were suddenly different. What had been a nicely balanced chassis now pushed/oversteered pretty badly or bad enough that handling had deteriorated on the track. On the street, the LSD was a bit "crankier" as well, making funny noises and being a little more "grabbie" in first gear turns in parking lots, etc. But nothing really bad, just noticeably different. At first, I didn't really think that the LSD would be making that much difference at the track so I chose to start playing around with camber settings in the front and even added a 3mm additional front spacer. The car still pushed. Now I've gone further and have readjusted my antisway bars to provide more front bite and the car is better but still not quite as balanced as it was before I started with the LSD "upgrade" project. I've got more suspension tuning to do yet still because I'm just not satisfied with the car's handling but at the moment I do not attribute all that blame to the LSD.

So what's the point here? First of all, your LSD may not be actually worn out. Second all, if you decide to upgrade your LSD then consider yourself forewarned. A "better" LSD can make your car take on substantially more understeer and exhibit more "push" particularly in third gear turns. In which case, you'll have to be prepared to retune your suspension to accommodate the additional traction you're getting from your LSD if you have Guard Transmission put in their discs and revitalize your LSD. This has been my experience, and if you read up on other Porsche owners using some of the fancier ramped LSDs in their cars you'll see that my experience is not unique.

Please understand that I'm really, really glad I went with the 2 disc GT upgrade to my LSD. However, I'm also very glad that I did not spring for their big bucks models with the variable ramps etc. They are race gear, and work great in race cars that have been tuned to accommodate them. As I said, I'm already having handling problems/issues with my improved LSD and I'm looking forward to getting it all sorted out because I'm enjoying the clearly improved rear traction. Now if I could just get the front to behave as well! Future options include wider front wheels/tires, fine tuning front camber/toe/trail settings and either RS front control arms or rebushed front control arms. While I welcome the new projects nonetheless I sometimes miss the wonderfully balanced car I had a year or so back. Good luck!

TR6

Interesting. If I can rotate one rear wheel by hand while someone is holding that other rear wheel in place and there is very little resistance, what else could explain it? Sounds like it may be best to go with the stock configuration (40/40) given that this is a dual purpose car. That's also what Geoffrey recommended.