Rickard 993 Turbo

Yes but the problem lays in the dics matrail that are brass insteed af steel from the cups

DanH

I spoke to the guys who rebuilt my diff a couple of hours ago. He recommends you don't put too much preload in the LSD if you will use the car on the road at all. He said for road preload should be 10nm or so vs 40nm or so for a race/track only car.

Even with the cup diff plates at 60/40 and road going preload, the car is a different animal now. It requires much more finesses in transition from braking to throttle but if you get it right it just fires the car out the corner.

enthusiast

Additional discussion from Rennlisters on 11-02-2005.


After reading THIS THREAD and especially post #8 and the following ones, I had a couple of long conversations with Brian Copans about the what is going on with the GT3's LSD. When I had my car on the lift last Saturday the differential seemed completely open: when a friend held one wheel I could turn the other in the direction of travel and there was no apparent force on the other wheel.

Brian is very knowledgable about these transmissions and has thought a lot about what is happening to the LSDs in our street GT3s. As NJ-GT notes, the brass pieces in the street car wear rapidly and soon provide virtually no pretension on the LSD clutch plates. The steel pieces in the cup car clutch pack are much more durable, but start out with much more pretension. On a street car that will see duty on wet roads with non-grippy tires the greater amount of pretension of the cup car piece can be expected to lead to severe understeer. NJ-GT has not had that experience, happily.

In addition to pretension in the LSD, though, there are ramps and pins that operate to lock up even the weakened street LSD on hard acceleration. Brian speculates that this is Porsche's purposeful design. The street car LSD will lock up only when the car is driven hard, and then only with the forces are greatest--probably when the car is well through a turn. The cup car LSD will lock up much sooner, putting the power down more effectively through the turn but with some understeer. I'm not sure what difference slicks vs. street tires has on all of this, but I would expect the understeer effect with the cup car LSD to be more severe with street tires. Brian would not recommend the cup car LSD for a car that does not see mostly track use with an experience driver--no rain-soaked drives by the spouse on highway off ramps.

When I talk about the street and cup car LSDs I'm really talking about different clutch packs and internals in the same differential housing. Brian thinks the cup car (Motorsport) internals are superior to aftermarket LSDs.

Installation of the cup car clutch pack can be done without having the transmission. The differential can be removed, the internals replaced, and the differntial reinstalled. This assumes that the differential housing with re-mate to the transmission without changing the pinion depth and lash. That has been Brian's experience, but it would still be best to have the transmission there at the time of the installation to measure.

I'm not sure what I'm going to do this winter. I plan to remain on street tires next year, so maybe I'll keep the LSD as it is for now. I would like to install the single mass fly wheel and RSR clutch, but that might wait, too. I have asked my service manager about the LSD and would like to know Porsche's view about it. I expect that they will think it is just fine as it is (and it might be for a street car), but I do want a ruling from them that if I replace the LSD with the cup car part that it will not adversely affect my transmission warranty.

As a probably unrelated matter, I sometimes find it hard to shift from neutral to first gear when at a dead stop. It feels like the shifter hits a gear with no teeth and just will not engage. Shifting into second (never a problem) and then into first or letting the clutch out and in again usually solves the problem, but sometimes it takes a couple of tries. Have you experienced this?
__________________
Chris


The manual test procedure calls for the LSD to be removed, placed in a jig to imobilize one side and then spin the other side with a beam-type torque wrench. The torque value should be about 15 NM (that's from memory--my manual is at home), which is pretty darn low.
__________________
Chris


Chris, I also spoke to Brian and the dealer tech who checked my car, and they do agree that the freewheeling test with a wheel off the ground isn't a proper test. The other tests I mentioned in the thread aren't valid either. My conclusion is that when the amount of wheelspin you get in tight corners increases dramatically, then it is time to change the plates.

I have not had the neutral to 1st problem, but I do have trouble with the 2nd to 3rd shift occasionally when cornering. Edit: Actually, it may have been the 3rd to 2nd downshift where I had trouble. It hasn't happened for a while, I can't remember.
________________________________________
Last edited by mds : 11-02-2005 at 06:19 PM.

enthusiast

We have the Porsche AG manual procedure for checking the "breakaway torque" on the limited-slip differential when it is removed from the transaxle. We don't have concensus if there is a procedure to check the differential while its in the car, normal drive train configuration.

christallon,

This is for comparison with your GT3 and others if they check. I believe my GT3 operates normally both on the track and street. I checked my differential yesterday as follows.

1. car and drivetrain at ambient temperature
2. raised right rear, turned right rear wheel by hand, light resistance
3. same procedure for left rear, same result
4. both rear wheels clear of the ground. Turned right rear wheel and the other wheel rotated the same direction. Turned both clockwise and counter clockwise, same result. Same results when I turned left rear wheel
5. removed both rear wheels
6. used small torque wrench with 32 mm socket on axle nut, to try and turn the axle & LSD by hand
7. incremented the torque wrench setting in steps until the trial was reached where the rear axle turned, "breakaway torque"
8. my "breakaway torque" value was 9.4 lb-ft (12.8 N m)

Rickard 993 Turbo

I have 150nm..

DanH

preload? can you even turn corners?

997gt3north

I just had a guard transmission 50/80 LSD installed on my 997gt3 (my shop checked the tq number) have to find the email, but they said it was extremely high (a 3 foot breaker tq wrench was used and it was not easy to make it slip) versus the stock unit (almost totally open they said)

I do not have difficulty turning corners or parrallel parking the gt3

paul

Rickard 993 Turbo

I hope so, it's the 05 cup with 8 plates, i think it will be super duper perfect

DanH

I mean on the road. I'm hazy on the conversation I had with the chap who did my diff but I thought he said that much preload would be a problem on the road which is why he didn't do it. Just can't remember what he said the issue was.

Thing is, if you have a full cup diff setup then I guess its intended you run slicks?

Rickard 993 Turbo

Here in sweden the 05 up plates has been used before without any problem, street doensmatter osm much for me, more importent it does good at the track

Weather here has not premited me to test yet and im not finiched yet, im intsllning my Rs cage and rs rear window first

NJ-GT

I have the '04 Cup LSD at 95 ft/lb of lock.

Understeer? that is what the adjustable sway bars are for.

Currently, I'm trying to correct oversteer problems, after the installation of the RS bar with reinforced floor plates.

Rickard 993 Turbo

Nice to hear, i have almost every spring there is so if it under steer i just change rear springs or adjust my sway bars

christallon

Hi enthusiast,

Thanks for that very well thought through response. The dealer got the same number (10 lbs) while running your test on my car last week. That is well within spec. I am satisfied that we have what Porsche intended for the street car. As for track stuff, many opinions and many recipes for success out there. I'm leaving mine alone and will adapt driving style to suit. Thanks again for your very valuable input.

Chris

mjb

Is this the same spec? But I'm guessing form the way it's measured that breakaway torque is not at full lock??

Rickard 993 Turbo

Yes it is, same spec, my 05 cup with 8 plates has 140lbs