Just a note, I never claimed that pic1 it was a 964 diff, it obviously is not as it has 4 frction plates, 964 has 2 and pic 2 is clearly identified as an asymmetric 993 unit.

Yes, a fully functional lsd will increase udersteer, the amount will depend on how the lsd is set up. The beauty of the 40/65 is that it doesn't induce too much understeer yet adds a lot to braking performance

 

The 40/40 LSD in our 964's is known to use low preload spring rates on the spring washers. This isn't a bad thing as it permits the car to execute low speed TIGHT corners and turns (like in a parking lot) without the LSD becoming "grabby" or acting up like a locked differential. Low preload lets the separate drive wheels operate independently until far more substantial torque load gets imparted. Higher preload spring rates would have the LSD operate more like a locked differential at lower torque values, which can create some peculiar driving characteristics on the street and particularly in bad weather or slipper pavement conditions. With low preload spring rates, the LSD becomes more forgiving under moderate torque application (i.e., light throttle) and therefore is more "streetable" versus a locked differential which causes more tire scruffing, more noise, and generally acts more "grabby".

Anyway, it may well be that your LSD's friction discs are shot, but it's certainly plausible that they are still good. Even though you can rotate the wheels by hand that still doesn't mean the friction discs have worn out, but rather you're getting a demonstration of the characteristic low preload spring rate on this LSD. I suspect that the only way you'll know for sure is to disassemble the LSD to see what's worn and in need of replacement.

In the final analysis, the 40/40 LSD is still a good to excellent choice for a C2 964 that's driven mostly street, occasional track. It may well not be the best however. The 40/65 993 unit mentioned by Bill Verburg is probably better, and certainly is so on a theoretical basis, because it offers far more lockup capability on the deceleration side of the equation. However, having said that, it may be too good on the acceleration side for your 964. Because it's a 4 friction plate LSD, it will work REAL DAMN GOOD and you'll get tons of torque transfer from one driving wheel to the other. I'm already trying to cope with more torque transfer than I can handle with the 2 disc 40/40 unit, so I don't know how I'd cope with even more potential front end "push". Perhaps there's someone out there that's installed the 993 40/65 LSD in a non-racecar 964 that will share their experience and perspective?

 

The 993 street LSD is larger than the 964, but still has only 2 plates in it. The racing versions and RS versions have 4 plates in them. The racing ones have more preload as mentioned with the latest 8 plate 997 Cup differential having a breakaway torque of 160-180ft/lbs compared to the 964 street specification of around 7-15 ft/lbs.

 

How difficult is it to remove the LSD if I wanted to ship it out to someone like Guard Transmission to rebuild? I'm told it can be removed with the transmission in situ. I don't suppose a DIY exists for this. I'm actually more worried about reinstalling it without screwing something up after I get it back rebuilt.

 

I've the 996 Cup LSD (40/65) in my C2 and I'm so totally pleased with it. Very stable under braking and I can hit the throttle very early with the grip it delivers. I still have the stock sway bar in the front, but 18 mmin the rear. In some corners there' some understeer but it's not a problem. To fix this I'll buy the RS sway bar before next season. This is one of the best things I've done with my car.

 

Thanks for the write up Blu (and everyone else..)

 

Taking the diff out is pretty easy if its similar to doing a late G50 3.2 one - just undo both inner driveshaft flanges from the gearbox, undo the stub axle bolts(not very tight), remove the diff cover and the diff should just slide out. I would drain the oil before removing the diff cover and also make sure the diff doenst fall on your head

I'm not sure if the ring and pinion will need to be re-set post diff rebuild - I didnt re-set mine when I pulled the diff to get at a seal that had been pushed in too far and all has been fine since.

 

Sorry to keep asking all these questions, but:
1) While it is ok to drive it on the street with a worn out clutch pack in the LSD, is there any risk driving it on the track this way? I have a track event coming up in less than 2 weeks. Problems with handling?
2) Would a worn out clutch pack in the LSD explain the symptom I described where if I'm driving at 70 on the highway in a straight line and mash the gas pedal to the floor, the car seems to pull slightly to the right? Kind of like torque steer.

 

I have see differential problems cause a car to pull one way or the other on acceleration and deceleration.

No, not likely to have an issue on the track either. Unless the disk cracks which I've not seen

 

Bumping an old thread...

... what is the ballpark cost of rebuilding a factory 964 LSD to stock factory spec?

Who does this work? Guard has been mentioned - anyone else?

 

Rebuilding to factory spec should be a piece of cake for your local Porsche dealer. Really, it's no big deal mechanically but there are several hours of labor involved getting the lsd unit out of the transmission and back in again. Give your local dealer a call to get a base rate quote established. After that ask some of your local track junkies who they use to do their engine/tranny work then call them for their quote. At this point you'll have a better idea of who you should use and why. Good luck!

 

Paul Guard sold the business to Matt Monson, contact them at Guard Transmission

they can rebuild w/ better than oem friction disks

 

Beware! The Guard Transmission (GT) friction discs are basically not compatible with the 40/40 two friction disc LSD in the normal 964. After about a thousand miles with the GT "tweaked" LSD in my car, the whole transmission had to be torn apart and rebuilt a second time by Jerry Woods, the same folks who had just rebuilt the tranny . GT had rebuilt my LSD and installed their proprietary discs, by the way. The cost of this mistake ran in the thousands of $$. The LSD had to be rebuilt as well and this time with Porsche OEM friction discs. The problem was directly attributable to the GT friction discs aggressively grinding up the floater discs into a massive slurry of metal that wreaked havoc with the tranny's bearings, many of which had to be changed. Fortunately the error was caught early enough that no gear sets had to be replaced, but of course some inordinate and premature wear was caused regardless. When confronted with the issue, GT responded that they had not anticipated such a problem but nonetheless they refused to offer any financial consolation or even a refund. Perhaps I should have taken them to small claims court (and I'm still mad as hell about it) but nonetheless I didn't. To their immense credit, the guys at Jerry Woods did give me a financial break on their cost of doing the rebuild. The tranny now works just fine and the LSD works just fine with new OEM friction discs. If you contact GT about having your 40/40 LSD "tweaked", I would hope that the new owner would be honest enough and well enough advised to council you against such an installation. The old owners said they had stopped offering their discs for a 40/40 installation (after my disaster) but I don't know what the new owners will do, so again, be aware.

 

so - for those of us with no LSD, is it a case of buying one, and it can be fitted without engine \ tranny removal?

 

While I am sorry to hear that your transmission failed on you, I have to say that I am pretty skeptical that where you are laying blame is really where it lies. The reason for this is that you present this like it's something unique and specific to the 964 2 plate LSD. But the fact of the matter is that every single GT LSD uses the same friction disc material, so this would be a problem with every single LSD that Guard has ever sold or rebuilt. We're talking literally thousands of units in service out there. I have now owned this business for more than half a year and I haven't gotten a single call from a car owner reporting this complaint. But please understand that there are exceptions to every situation. Who am I to say that something unique to your gearbox didn't happen that was somehow related to how Paul Guard rebuilt your LSD. But I just can't get on board with the idea that the ZF G50 LSD is so unique that this problem is something that's going to happen everytime one is rebuilt with our parts. Hell, the internal parts are the exact same parts as a ZF 930, 915, and 914 LSDs that came before it. The only thing that's unique about the ZF G50 is that it was only used for a couple of years before Porsche went to GKN for all of their LSD manufacturing, which has it's own list of both assets and liabilities with respect to design, preformance and durabilty.

There has long been speculation that the sloughed off friction material from our LSD, as well as pretty much any LSD, factory included, could accelerate the wear on other components inside of the gearbox. This is part of why we tell people to do the following:
1. After initial break in of 500-1000 miles or so change your gearbox oil after installing our LSD or getting yours rebuilt. When you buy a new car, you do an initial engine oil change at an abbreviated service interval. You should do the same with our, or any new LSD you put into your gearbox.

2. Change your gear oil at a maximum of once a year. I don't know what Paul used to tell people as a mileage for service interval, but I suggest 10-15k miles. These are high performance cars that get beat on. Guys change their engine oil at 2000-2500 miles against published factory service intervals of 3000-5000 miles, but they never seem to think that they should change their gearbox oil more frequently when they drive their cars sportingly. I change my own gear oil every 6 months on all my cars because in all honest I rarely make it to a year because of #3.

3. Change your gear oil after every single race. If you "just" go to track days and run hot laps you can probably push it out to 2-3 track days/test-n-tunes. I personally mostly do casual track days and don't race much outside of auto-x and rally-x. But if you drive the car in a racing environment at it's mechanical limits for an hour or more, it's cheap insurance to spend $50-100 on gear oil.

These are things I have been telling people for years and not just something I say now that I have a vested interest in the performance and longevity of your gearboxes. I am sincerely sorry to hear when anyone suffers a premature failure on a gearbox. And if anyone ever has a problem with a gearbox that they believe is related to one of our products, please let me know. Just be warned that I don't just hand out checks or replacement parts. If a product that I sold fails you, I'm going to want the whole gearbox shipped to me and to be able to assess it with my own technician. I don't mess around when it comes to product quality. I'm not going to pretend that what we make and sell is perfect and even though like most motorsports parts manufacturers we've got that little note on our invoices that says these parts are for racing and carry no warranty expressed or implied, the fact of the matter is if something is defective I'm going to stand behind it. There is that old adage of "that's racing" but if there's a flaw in something I am putting out there it's my intention to correct it and make it better. This is my livelihood we're talking about here. We're not some major corporation with deep pockets and multimillion dollar budgets. If there's a problem, it's in my best interest to fix it and fix it quickly. When you call us, you get me or you get my wife. That's it. That's the business, and if we don't do right by the brand that Paul Guard spent the last 20 years building and refining, then we don't deserve to be the caretakers of the name.

Regards,

Matt Monson
Guard Transmission LLC
303-530-1094