LSD options and Hard Shifting
11-02-2005, 06:18 PM
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LSD options and Hard Shifting
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?
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?
11-02-2005, 06:39 PM
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So, is the freewheel normal when jacked? No need to panic? What is the proper test? Mine has no resistance from one wheel to the other when jacked. A technician at the dealership explained that the LSD worked under heavy load only. Previous posts have concerned me, so I asked the dealership to provide the proper way to test the LSD and I've had no reply. I'll appreciate any further comments.
Hans
Hans
11-02-2005, 06:47 PM
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Thread Starter
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.
11-02-2005, 08:39 PM
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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.
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 09:19 PM.
