We discuss the limited-slip differential (LSD) a lot so I am reposting useful information from various authors. The majority of the information is on the premature loss of the basic locking torque. I will edit and add to this post as needed. Originally posted 12 April 2009 on 996 GT2/GT3 Forum, current version 6 April 2010.
Key note on how the Porsche LSD functions with basic locking torque.
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.
(from Brian Copans information discussion provided in Topic Section #2)
1. General LSD Information
2. Discussion on Porsche LSD in GT3 street and Cup Cars
3. Testing the LSD
4. Transaxle Lubricant
5. LSD Part Numbers & Transmission Models
6. Upgrades (to be compiled)
1. General LSD Information
By Andrew Warren, San Diego Region
Have you ever wondered what a limited slip differential is; what is does; how it works? When your car turns a corner, its outside rear wheel must travel farther than its inside rear wheel. The open (non-limited-slip) differential allows that to happen. It's a clever arrangement of gears that can transmit torque to either the left or right wheel and still allow the other wheel to rotate at a different speed. In fact, you can hold one wheel completely motionless and still drive the other wheel.
Limited-slip differentials (LSDs) work the same way, but they LIMIT the differential (the "slip") between the two rear wheels. The main reason for wanting a limited-slip is to improve traction in a turn. In a hard turn, the inside rear wheel will tend to lift off the ground (either completely or just partially) due to weight-transfer to the outside wheel. Because of the way an open differential works, the engine torque is directed to the wheel with the LEAST amount of traction, so in a situation like this, the inside wheel will tend to spin -- losing traction -- and the outside wheel, which is in the best position (since it's heavily loaded) to drive the car through the turn, will just coast along.
A limited-slip differential will allow SOME slippage of the inside wheel, but it will still direct a significant portion of the engine torque to the outside wheel where it can do some good. This, by the way, is the reason that I don't recommend using a rear sway (or "anti-roll") bar on a car with an open differential. The sway bar tends to pull the inside wheel off the ground in a corner, which is the last thing you need if your engine torque is going to go to the wheel with the least amount of traction.
There are numerous LSD designs. Many, including Porsche's LSD, use friction plates (like clutches) to accomplish the torque split. Some (like the Quaife) use an extremely clever gear-only arrangement to accomplish the same thing. The clutch-pack LSDs are probably more durable and are easier to modify for varying torque splits, but they wear out and need to be rebuilt every once in a while. The Quaifes contain a whole bunch of little gears, so they may be somewhat fragile, but on the other hand, they never go out of adjustment.
Some racers like to use a completely LOCKED differential, in which NO slip is allowed between the rear wheels. You can spot these guys in the paddock pretty easily -- they're the ones who can't make U-turns. However, the tendency of locker-equipped cars to just drag their inside rear wheel in a turn causes really bad corner-entry understeer.
2. Discussion on Porsche LSD in GT3 street and Cup Cars
By Rennlist member ‘ldw’, 8 August 2003
1) It is an involved discussion as to exactly how it works, but i will try to use an example or two for practical purposes. Assume performance driving effort for the examples as many street cars do not have LSD and these events do not happen.
If you had no LSD and you accelerated around a corner and lifted the inside rear tire, the power would be applied to that tire and hence it would spin and the car would not put power to the road and subsequently the car might oversteer.
If you had a "spool" or locked diff(100%), then you would go around that corner and both wheels would be applying power because they are "locked" and the car would tend to "push" or understeer.
In the case of the LSD in the GT3, it is a 40% lock factor under acceleration. On decel, in a braking zone for example, with an open diff,...if you turned the wheel, one wheel would spin faster than the other as the friction of the road dictated and the car might oversteer relatively.
With a locked diff, the car would feel very stable under braking and would tend to track straight,....subsequently, it may not want to turn so well. In the case of the GT3 diff, the factor is 60% on the decel side. These are compromises and have been developed from extensive testing and R and D at Porsche in the 996 race car platform. The diff is able to determine accel or decel mode based on differences in frictional force application.
2) It should be generally maintenance free if the transmission is not mistreated. Consult owners manual for factory oil recommendations.
3) It has clutches. Ramp angles can be changed to change the lock factors. It is commonly done in the race cars depending on driver style, track conditions, and tires.
4) The clutches can wear. Eventually, like any clutch, after enough time, they will wear out.
5) This gear box has a very highly developed internal oiling system directly inherited from the cup car and GT3RS race cars. If all is working properly, overheating should not be an issue.
6) These factors were selected based on extensive race testing. They are the factors as delivered in the GT3 cup car and GT3RS/with 65% also seen in as delivered GT3RS diffs.
7) As an aside, in the 993 era, ABD acted in a limited slip capacity to assist with getting power to the road on accel, and helping with handling on decel. The RSR 993 race cars had actual LSDs of course. The first generation 996 was delivered with a limited slip. The LSD was not utilized once PSM was incorporated in the later generations.
Hope that is helpful...
lynn Wilson 08/08/2003
By Rennlist member ‘mds’, 9 Aug 2003
Thanks lynn. I had not realized that inside rear wheel spin on a no-LSD car could cause oversteer. But this makes sense to me now that I think about it. As the rear wheel starts to spin, you loose acceleration, which is effectively a lift, and so it is possibe get oversteer as weight shifts rearward. And also following that same reasoning, as the rear wheel regains traction coming out of the corner, understeer is possible.
It also occured to me that an LSD might reduce oversteer when you are forced to lift, say in a safety situation. With no LSD all of the engine braking torque goes to the inside rear wheel. But the inside wheel is light and so that torque will have relatively little effect. With an LSD, some of that torque is transfered to the outside, heavy rear wheel. This braking torque on the outside rear wheel will tend to counteract the car's tendency to oversteer due to the weight transfer from the lift. Its sort of like the selective braking PSM system. Also, this may be one reason why for the larger 60% ratio. Engine braking torque is less than power torque and so transfering more of it in this safety situation makes sense.
By Rennlist member ‘ldw’, 9 Aug 2003
That exactly right....... The inside rear wheel on a non LSD car (which is the case in many street cars) rarely spins though because most road going cars are so soft suspension wise, that by the time the car had enough lateral force to lift a wheel ever so slightly, it would have likely spun out of control already with the average driver. In a car like the Porsche, since it is set up for excellent handling and is relatively stiff, the inside rear tire can loose grip and hence effective acceleration is lost, and weight is actually transferred to the front end not rear, and rear grip is lost-effectively like lifting in a corner. The diff though, will assist in keeping steady flow of power to the rear wheels. It will get you out of the corner, AND help prevent the scenario similar to lifting where the grip that was there is suddenly lost because of a weight transfer toward the front during the weight tranfer-so it helps keep power to the raod, and with balance indirectly. Also, in braking, it is much more stable to have a LSD. Depending on ramp set up, in a cup car, I could make the car have great difficulty turning into a corner or have it feel as though it has tremendous oversteer going into that same corner simply by changing the ramp angles and subsequently the lock factor. It is an excellent tool when properly incorporated into a handling package. Porsche has decided 40/60 is an excellent compromise. ALMS and Grand Am teams may change differential lock ups several times in a given weekend in the search for perfect handling. The advantage of the diff is underestimated by many,....
Many of the leading Porsche race drivers, particularly drivers in the supercup and carrera cup series-who are exceptionally talented-, use the differential to great advantage....it seems like a paradox, but if one is on the power slightly in a corner on the way out-feeling as though further application of power will induce more sliding or cause a spin, additional application of throttle(which intuitively seems like would not be tolerated) in some circumstances allow the car to have grip through engagement of the diff and additional power application is tolerated and the car under control. The GT3 cup cars do not have the grip in the rear end that a GT3RS does for many reasons so the diff is a key tool for the supercup driver. Go to the farnbacher website and see this in action in the video section within gallery(http://www.farnbacher-racing.com/)
lynn Wilson 08/09/2003
By Rennlist member 'NJ-GT', 16 September 2005
The street US GT3 uses the crappy brass made LSD. Most of the GT3 that have seen track time on courses with tight turns, using grippy tires and with some mileage most likely have an open differential by now. The stock LSD has very low torque, and it wears quickly.
I was told by Copans that our brass LSD was good for 2,000 miles. I trust this person on GT3 transmissions as I trust nobody else. He was recommended to me by two rennlisters as a tranny expert, and he definitely surpassed my expectations. All the Rolex GrandAm GT3 transmissions are Copans with the exception of Alex Job Racing.
When Brian upgraded my ring and pinion gear (8:32 from a reinforced 2005 RSR version), he found that my diff was gone (again). This is my second transmission, my first one broke with just 2,000 miles on the odometer, and I got a new one under warranty.
Brian recommended me to install a 04 GT3Cup Steel Differential. He told me that the car was going to understeer a lot, and not be driveable on the rain, it was the first time he did that for a street car (with a big warning). Fortunately, the car works great on the rain (better than before), no understeer. I called to give him feedback on my street car, and he was pleased to know he had a solution for the street transmissions.
The 04GT3Cup has more than twice the torque of the stock differential, and it will last way longer. The 05GT3Cup has even more torque, but that one definitely won't work on the street as good as mine does, because it has over 135 ft/lb of torque.
It's very easy to verify the conditions on the LSD, with just a floojack, a torque wrench and 10 minutes.
Rennlist Member #: NJ-GT
By Rennlist member 'Phokaioglaukos', 2 Nov 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?
By Rennlist member 'mikymu', 18 December 2009, from his LSD rebuild article, 2007 997 GT3 RS
Side view of OEM LSD unit with ring gear
OEM used plate
OEM used friction plate
OEM ramp piece
Guard brand GT2 ramp pieces
By Rennlist member ‘JR944’, 3 February 2010, working with Chris Cervelli to rebuild his 2004 996 GT3 street LSD and gearbox after 83,902 miles
Used LSD without ring gear
lower bellville washer which is the only part of the diff that showed significant obvious wear
OEM GT3 street car bellville washer on the left next to the Cup washer we installed (on right). These are the top and bottom of the diff "sandwich" and create the preload. Note that the Cup washer is much more cone shaped vs. the flatter street car washer. This will obviously result in greater preload.
Used OEM friction plate on the left and the replacement (Guard) one on the right
By Guard Transmission, 26 February 2010, Planet-9.com discussion thread The Cayman factory LSD dissected… Rennlist user ‘webbie’ suggested I add it. Focus on Cayman LSD but GT3 discussed as well.
I think part of what we are seeing here is Porsche's continued push away from really high performance automobiles and more towards GT cars that are safe and comfortable, that still perform well, but not stellar. I don't think when they put together the specification on this LSD that they ever intended it to see the track. There's a number of street drivers who are perfectly satisfied with the performance of this differential, especially when you leave all the various electronic traction control items engaged. It's being developed as an integrated system and the practical reality is the LSD isn't expected to do as much as it once did because the brakes are automatically more involved than they were in the past.
But then you take the car to the track. And you turn off the electric babysitters so that you can really feel the car and drive it at the limit without the computers stepping in and saving your **** when you push it too far. It's just too much for this little LSD. Or in some instance they get in your way because you and the car can go closer to the edge than they will let you and they step in and pull you back, making you slower but safer. Heck, it's too much for the street GT3 LSDs which are having a catastrophical failure rate when taken to the track. Even Porsche's flagship trackday special is ill equipped for use on the track in this department. And if the new TT model (only offered with a PDK and electronic differential) is any indication they are just going to go further to the side of electronics in the road cars going forward. Technology is a double edged sword here. The cars keep getting safer and more comfortable, but at the same time we keep getting further removed from them and the real control of the driving experience. Just look at how they are advertising the newest GT3R. They are openly calling it a racecar that's easier to drive for a gentleman racer that doesn't need the skills of professional.
Porsche has been systematically making their LSDs less aggressive on all their cars for a while now. IIRC, the current GT3 is 28/40 locking versus the 40/60 they used on the 996 variant. The GT2s have used the lower locking percentages since the 996 version of that car. It stands to reason that they would go low on the Cayman as well. It makes the cars more forgiving and basically makes it harder for the street driver to catastrophically fail and do something really stupid that kills or maims people because they pushed it too far. But for the people racing and tracking the cars, it disables them. This is why there will always be a healthy aftermarket for Porsches, because people will always want to race them no matter what Porsche does with the product line.
If you read the end of my second post, you will see that I did suggest that you could restack them in the traditional configuration. But given how the ramps are set up, I don't think it will actually double the locking percentages like it did on the old 911/915/930 ZF LSDs. You can see what those ramps look like compared to the Cayman ones. This is just a guess, but I would say that restacking it might get you to something like 32/40 but not much more.
Other than the ramps, there is the very small area of the friction surfaces. And beyond that, they are so dang thin I don't see them having a very long service life. Those friction discs have a total thickness of 1.6mm. The pads are .3mm each. And since it's got no preload and very limited ramp movement I just don't see them being able to take more than .1mm of total wear on them before they stop locking. The GT3 LSDs, which have low preload are failing in about .1mm of wear. For comparison, our LSDs will continue to lock with up to .4 or .5mm of wear on them. In practice what we see is our diffferentials will go 3-4 seasons of PCA club racing between rebuilds, compared to GT3, and now Cayman S, LSDs that last less than one whole season. So, in short, I don't think restacking them is a solution that will work for very long in a racing environment.
Guard Transmission LLC
3. Testing the LSD
By Rennlist member 'Hans GT3', 2 Nov 2005
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.
By Rennlist member 'Phokaioglaukos', 2 Nov 2005
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.
By Rennlist member 'mds', 2 Nov 2005
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.
Question by Rennlist member 'elh0102', 25 February 2010
After reading the posts on testing the differential, I did the test of jacking up one rear corner and testing for wheel movement. It moved...
Answer by 'Erik@GBox', 25 February 2010
The normal test that you have done to check the differential for wear may not quite apply to your particular car. We have noticed that Porsche has lowered the Pre-Load on a lot of their differentials, relying on the ramps to generate the needed pressure for lock-up. Your differential could be quite healthy. I would simply inspect the internals to confirm that they are in good shape, and then make the call from there. Be careful taking the differential out and putting it back in. If you ding the ring gear against the pinion head you might find that you have a whine after reinstallation.
Personally though based on what you have described, I would not worry about it. Sounds like it is working just fine for a street application.
Hope that this helps,
GBox Performance Transaxles
(303) 440-8899 work
(303) 895-4828 cell
By Rennlist member 'enthusiast', 29 March 2008
Please remember that this test is probably not valid on the lubricated LSD while fully assembled in the GT3, but may be a good wear indicator. Here is the process I used if you want to try it. (torque range is listed in the illustrations below)
1. remove rear wheels
2. gear box in neutral and parking brake off
3. use a 32mm socket on the axle nut to rotate axle and LSD
4. use an appropriate ranged torque wrench (very low torque here, 5-15 NM range for 996 LSD)
5. secure one of the rear brake disks so the axle won't turn
6. apply turning force on other axle very slowly (normal direction of rotation), begin with a very low torque setting on wrench (3 Nm)
7. repeat Step #6, incrementally increasing the torque wrench setting until the axle rotates
8. that is your break away torque value of the combined assembly (friction of bearings, brakes, axle constant velocity joints, gears, and LSD with lubricant)
Workshop manual pages for the Porsche 996 LSD Bench Test posted on Rennlist by 'v12man'.
4. Transaxle Lubricant
By Rennlist Member ‘enthusiast’, 13 February 2009
I read a lot of notes from various web sites and Porsche AG documents about the proper transaxle / gear box lubricant for the GT3 models. I decided not to experiment and I bought one container of the Porsche AG specified Mobilube PTX 75 W 90.
- concensus is that PTX is made just for Porsche AG and only available from Porsche
- only packaged in a 20 liter container at a current list price of $620.62.
- Porsche part number 000 043 204 20
- specified for 1999-2005 996 GT3, 2001-2005 996 Turbo & GT2, 2006-2007 997 GT3, 2006-2007 997 Turbo, and probably current year of same models
- no alternate oil is specified by Porsche AG
- most of these Porsche models require between 3 - 4 liters
- many owners are concerned about gear box & limited slip materials and lubricant compatibility
I recommend that if you have the Porsche dealership replace the oil be very specific as to exactly what lubricant you want. They have several 75 W 90.
If you are a 'do it yourself' type you might be able to buy just the quantity you need from a dealership.
The Porsche parts catalog shows that for all of these cars some of the LSD internal components are the same; axles, ring set, gear set, vanes, and the selection of adjusting rings (they vary by thickness).
6. Upgrades (to be posted)
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