mds

How does the GT3 limited-slip differential work and what maintenance is required? Is it a clutch based design where the clutch can wear out and eventually needs to be replaced? Or is it a gear based system with no clutch? Are oil changes required? Is overheating ever an issue? Mechanically how does the 40/60 locking factor change from under power to overrun? And why did Porsche pick the these particular locking factors?

ldw

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...

mds

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.

ldw

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/)

enthusiast

Bringing this thread forward since we are having so many current discussions on the LSD. When this was written we did not know much about the 996 GT3 clutch pack life span.

christallon

Thank You for digging this out. Very good explanation there. So what are the current opinions on clutch pack life, and how many have performed this service out of warranty? What is the cost, and what is entailed to accomplish this work?

Thanks in advance..........

LVDell

I was quoted from my local dealer that parts were approx $2K and the labor was, sit down for this one......26.5 hours! That is book rate. They offered to goodwill the parts only but with still paying labor I was still paying over $3K. I opted to go ahead and pass. Instead my shop has sent my LSD off to Copans for a cup rebuild

Greygt3

Dell you might want to shop around. I looked into this and was told that it was a fairly simple procedure. 4-6 hours to remove and replace the LSD. I was also quoted 750-800 to rebuild the LSD clutch pack once it was out of the car. This quote came from a Porsche transmission race shop. If you want their number let me know.

Dean

LVDell

Thanks Dean. But already one step ahead See my previous post as I already decided to pass and instead had my shop remove and send off to Copans for the rebuild. Rates sound fairly close to what I was quoted.

997gt3north

Dell,

Listen to this guy ^^^
I just paid $2700 for a new Guard Transmission LSD and my total labor bill for an entire tranny gear box tear down / rebuild for the cup 4.0 r&p install (this was the big part) + LSD install + new rear sway bar install was $1900. There is no possible way that 26 hours are required for just the LSD - max, max, max would be 10hours (tranny/out/down/apart, lsd/off/open/new clutch pack in/close/test/reattach, reinstall tranny/fluids/etc, test everything take for test drive)


Paul

roberga

putting in a new LSD is a higher labor cost than a rebuild. The new requires shim alinement and can be time consuming.

Rickard 993 Turbo

changing clutch pack doesn´t need the tranny to be removed, just remove the LSD and change cluch pack, my shop charge 700 for this+ parts

AndyT

exactly we just rebuilt mine and we took off the diff side cover and out it came. be sure to use the factory torque specs when reassembling.

Paul 996

Glad you pointed that out. I thought that I recalled the same thing but wasn't 100% sure so I kept my mouth shut.

It is the exact same on the 996 gearboxes, drain the diff, drop the axles, remove diff housing cover, extract, upgrade/repair/replace.. plus a few other odds/ends. But the trans stays in.

christallon

I spoke to Peter at TRG this afternoon and learned that the factory probably set the spring rate for the clutch pack at a relatively light setting. It is posible to adjust the preload to a heavier setting, from what I gathered. I am planning to do more research on this. It is acurate that one can remove the cluth pack without pulling the trans down. That is so cool