Clutch Issue

I have had my 02 C4s for almost a year, I love the car, it is everything I was looking for, a car I could drive on the streets comfortably and take to DE events at times. When I got the car, I had a list of items I new I would need to address, the clutch was one of them, that clutch was holding fine even during my first DE event. A few weeks after the DE it did show symptoms of slippage so I purchased a new clutch/pressure plate from a fellow board member and replaced it. This was a Sachs pressure plate and a Kevlar TF1600 clutch disk, I did dressed the flywheel before installing. This new clutch worked perfectly on the street, even during spirited driving, but during another DE event after a few sessions it slipped a bit when on the power on 5th gear, this only happened well into the session and I attributed this to possible over aggressive shifting and getting on the power aggressively after shifting. Since then the clutch worked with no issue while driving the car on the streets. Today I attended another DE event, the first session I had no issues, but after 20 minutes on the second session, the clutch started to slip, the strange part is, it only did this after shifting to 3rd or 4th, on a few corners were I downshift to 3rd and got out of the corner on that gear there was no slippage whatsoever. Then on the next session after the car had cooled down, everything worked ok, but again after 20mins or so the same with the clutch, this happened on 3 different sessions, all ok until 20mins or so into the session. On the drive home, 25 miles or so, no issues. Any ideas? I did flushed all the brake fluid of the car with racing fluid, but I have not flush the clutch fluid, could this be the problem? Also is this clutch now done for, since it has been abused this way?

Are you using heel-and-toe rev matching on downshifts? It is a great way to keep clutch temperatures down while tracking.( virtually a MUST when tracking to keep down clutch temperatures and save syncro wear) Also timed speed shifts instead of power shifts will keep clutch temperatures down. Save power shifts for hot laps or for passing to gain a tenth or so, otherwise use them sparingly. Every downshift without heel-and-toe rev matching or power shift upshift, will raise clutch temperature @100f, too many in a row without time to cool down, will cause clutch to fade (just like brakes when they get too hot), but will return to normal as long as threshold was not exceeded. I don't know if you have exceeded the threshold or if the clutch is "done"(Kevlar is very forgiving), but you should be able tell by driving around a bit and see if it performs properly, may just need to have a bit of glaze worn away. I think that with the clutch working smoothly and fine on the street and for 20 minutes of track time each session, if you can better manage the clutch temperature on the track by practicing some equipment saving techniques like heel-and-toe downshifting (rev match) and timed speed shifting you will be fine. Also contrary to popular belief, as the clutch disc wears(thinner), the clutch will actually get stronger. The diaphragm spring changes the fulcrum geometry as the disc gets thinner increasing the clamp load. That's why as the disc wears out , the pedal effort will increase.

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Thanks Porschetech3! Awesome and educational post! May I ask, because people use different terminology for things, how do you differentiate between "timed speed shifts" and "power shifts"?

My $0.02 Power shift is shifting and clutching as fast as you can and getting back on the power immediately without any regard. When the next gear is grabbed and you let the clutch out and hit the gas, the engine might still be spinning at near redline, so the clutch absorbs the rotational momentum to take the engine from redline to the synchronize RPM. It has to do this while at the same time, absorbing over 250HP because the throttle is applied. The above is the fastest way around the track but at the expense of damage. Timed shifts are when you lift off the throttle, shift carefully and slowly enough that you give the synchros time to do their job, and wait for the engine RPM to drop to the synchronized speed before letting out the clutch. The clutch absorbs zero heat and wear. Power is applied at the perfect moment when everything has completed. This isnt as fast around the track, but has virtually no wear. It�s also how you should drive on the street. Professional racers might find a balance between the two - long enough to win the race but don�t mind rebuilding after the race. PDK machines are doing a lot of this for us.

Thank you, I am sure I was not power shifting, but regardless I will be more conscious of how I shift on my next event and see how it goes. In the meantime I am going to flush all the clutch fluid and hope the clutch didn't experience to much wear do to this. I guess this will show up while driving it.

So, the reason for the slippage on the track is heat. However, we don�t know if heat is thinning the oil and oil is getting past the RMS and onto the pressure plate/disc. Did you see evidence of oil when you changed your clutch? Or if heat is boiling the fluid in your hydrolics, or if heat is weakening the springs in the pressure plate to allow the disc to slip. Did you replace the flywheel and throw-out bearing when replacing the clutch?

Yes, throw-out bearing was replaced as well as the RMS, this was done as a preventive measure, there is no evidence of oil then nor now. The pressure plate is also new. I did not replace the flywheel since it was within parameters, but I did cleaned up, roughed up all the mating surface. Mechanically, the hydraulics was not checked or replaced, I am going to flush the systems next. Also I will be conscious of my shifting stile to be sure I do not over abuse the clutch.

Murphyslaw1978 is spot on with his assessment of power shifting and timed speed shifting.Some people even use the rev limiter to limit max rpm during power shifts and never let off accelerator.( for real, while this is the fastest way, it is the most abusive) As Murphyslaw1978 correctly states, timed speed shifts allow the rpm to drop the necessary amount so that smooth transfer of power to the next gear with the least amount of work by the syncro's or the clutch. The time will vary depending on gear ratio differential of the shift.On a 996 G96 gear box the differential is 1.62 for 2nd gear shift, .68 for 3rd, .3 for 4th, .2 for 5th,.18 for 6th. So as you can see, the shift can be made faster in the higher gears as there is less differential and the rpm doen't need to drop as much. Also a light weight flywheel will drop rpm faster allowing a quicker timed speed shift than a stock 13lb heavier DMFW. The reason for the clutch slippage after too much heat during track usage, is if the clutch disc temperature exceeds 750F, the binding resins in the clutch material will start to melt and act as a lubricant. After cooling they re-solidify and leave a glaze on the surface. This glaze can be worn away by use,(or in the case of brake pads easily removed, can be sanded off).. An indication that "too much" resin has been melted or burned is by a cloud of smoke or an obnoxious smell. Kevlar is very good at withstanding high temp and still being very smooth engagement for the street.

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Wow, I did not know that. Great info.

Thanks for the clarification! Great info!

I just wanted to comment to say this thread is full of great info. I'd like a way to "thumbs up" a comment/thread to show appreciation.

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There is a lot of good information on this thread. I am planning on reinstalling my stock dual mass flywheel in my 3.6L. The flywheel was pulled out at 49,000 miles. It looks like it has no issues. Is there any suggestions on any prep before reinstalling?

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