Instructors at PSDS were very clear about the subject of turning off PASM. The car can be driven at its limit without intervention of any control system. it is only when the car is beyond coordinated speed and balance parameters that the system(s) intervenes. It is not advisable for most of us to turn off PASM. I suppose another way to look at this is whether one wants to maybe preserve rotors and pads by defeating PASM at the risk of running off the track and damaging much more. Pros may be able to coax a bit lower track times out with PASM off, but it is an academic question for most of us who are not professional drivers or very experienced amateur racers.

 

I'm not really looking forward to turning PSM off. I'd rather have warped disks than a warped car or body ;-)

 

Turning off PSM also turns off rev match if you have a 7 speed. Sucks once you are used to rev match...

I can attest that having PSM on does eat rear pads/rotors if you are an aggressive driver...

 

First, it should be clarified that "warped rotors" is a misnomer. Although it is physically possible to warp a rotor, it would be next to impossible while driving, even on the track. First, they are made of iron, which ain't exactly tin foil. Second, those fins that separate the 2 sides of the disc not only allow the rotors to vent heat but also serve to dramatically increase the torsional & compressional strength of the rotors. You would need Vulcan's furnace and Thor's hammer to actually warp them. One of the most likely culprits of brake chatter (i.e. warping) is the transfer of a thin deposit of pad material onto the surface of the rotor. This can happen when the brake surfaces are hot by setting your parking brake (as has been mentioned previously) or by simply putting pressure on the brake pedal when you come to a stop, such as white listening to your instructor's post-session ramblings at the grid. If you have to stop while the brakes are still hot, assuming you did a proper cool down lap, then it's better to turn the car off & put it in gear than to hold the brake pedal down.
The other likely cause of brake chatter is insufficient bedding of the brake material prior to hard use. Cars that experience hard brake use, such as on the track, require the brake pads to be properly bedded, which is the process of applying a UNIFORM layer of pad material into the entire surface of the rotor, as opposed to the nonuniform patch that is on there now. This allows for better braking since the pad will be making contact primarily with its own material rather than slick iron. Road cars that don't see hard brake use do not require this to brake because the brake systems are rarely if ever required to brake as hard or as suddenly as on track. However, even for a road car it is advantageous to bed brake pads. There is a wealth of info & videos on the interwebs on how to properly bed pads.

But, you still have the brake chatter to contend with. There are 2 basic ways to remove this pad material from the rotor surface. First, you can have the rotors resurfaced on a lathe (not a problem for cross-drilled rotors, but almost impossible for channeled rotors, & totally impossible for floating rotors). Second, you could install pads with a harder compound (i.e. race pads) and find a long empty road where you can do some repeated hard braking to accomplish the same thing.

Now, for the future, if you are looking to upgrade some equipment for DEs, start with the pads, rotors, & brake fluid. OEM pads and rotors are terrific, but they are designed for street use and are not up to the task of repeated high temp use required on track. They will fade because the pad compound is soft and not thermally as resistant as race pads. I recommend Pagid yellow or orange pads, or a comparable set from another brand. These are much harder and more heat tolerant than OEM. If you do install race pads, you really should upgrade the rotors too because the hard compound of the race pads will really eat into the stock discs. Move up to 2-piece floating rotors like GyroDisc or StopTech, or something comparable. I also recommend channeled rotors rather than cross drilled, as they are more efficient at removing heat and water from the rotor and can't get clogged with brake dust (which reduces heat efficiency and negates the existence of the holes). They are also perfectly acceptable for street use, but you will definitely want to use soft pads for street driving because race pads squeal like Mariah Carey caught in a bear trap and WILL garner the attention of anyone not listening to Mariah Carey at 130 decibels. Or you just get the bumper sticker that reads "Brakes squeal cuz Race Car". Also get some better brake fluid. That DOT crap that's in there now ain't gonna cut it. Get some Motul in there. Stat.

I definitely recommend against you running racing slicks or even mild R comps like Nitto NT01s yet. If you are fighting to stay in your seat now (as evidenced by the bruising on the outside of your left knee the Monday after a DE event), then you will wish you had put super glue on the seat of your pants if you run R comps. Save those for when you are ready to install harnesses, a roll bar, & fixed back seats. You will get there, but it doesn't sound like you're there yet.

As for whether or not to deactivate PASM, it depends on your skill level. Turning it off doesn't imply that you will immediately lose control of the car and donate your quarter
panels to the barrier in turn 6. Why? Because the safety nannies have made sure that you cannot fully disengage PASM. Even when it's off it's on. That said, when it's turned off you will notice a difference. The back end suddenly is a able to break loose when you lift in a turn, which can be good or bad, again depending on your skill level. Personally I would wait until you have upgraded the disposable parts of the brake system and feel you have mastered control of the car with the nannies turned on, then turn PASM off and put it into sport mode (if you have it), and slowly test the new limits of the car and your skill. You will discover the absolute joy of throttle steering through a turn, letting the rear end slip loose just enough to rotate the nose of the car (without having to brake or turn the wheel) just before getting in the juice again. Because of the unique engine placement of our cars means that they, even the 991, can throttle steer like none other.

There are likely other opinions to be offered in this and a multitude of other threads that could work for you as well or better than what I rambled about here. This is just my $0.02, FWIW.

 

Car is going to dealer next week. They will be measuring the disks first to see if they're really warped or not.
But as I said before, I had to end one session without a cooldown lap and was stupid enough to stop the car in the parking lot (without handbrake obviously) instead of drive it on the public road for a cooldown. People told me the part of the disk that's under the calliper stays hot when the rest cools down, and that can warp the disk. I don't know.
Allthough I'm confident in my driving and the handling of the car with PSM on, I'm not turning PSM off as I'm not skilled enough to be comfortable with a loose rear end, lift off oversteer and throttle steer. I'd love to get to know the cars reactions with PSM off on a big open strip of tarmac first before I learn the hard way at Francorchamps !

 

Yep, sounds like you know exactly what the cause was. Also, kudos to you for knowing your limits.

Sorry for the rambling, but like Mark Twain said, I would have written something shorter but I didn't have time.

Good luck at dealer. Let us know what happens.

 

Ushgeo, I've been speculating and trying to analyze, that's about it, I know absolutely nothing :-(
But I do thank you for the long answer, I enjoyed it, still digesting it (allready read it several times over), trying to learn from it. So please don't make your answers shorter in the future ! The more info I get the more I can learn ! But the more confused I'll probably get at first, the problem every non savant rookie has I presume ;-)
I allready shared some of your thoughts and findings with the dealers lead mechanic ! I'll keep you posted ...

 

Track pads, Motul RBF 600, PSM on, and you're good to go.

 

Back from the dealer. Rotors were measured. Rear ones were warped, front ones were OK. Rear rotors replaced, as were brake pads, vibration is gone. Everything still OEM though. Next time I'll blow out the brakedust from my rotors between sessions, and make sure I get a cooldown if need be on the public road if I can't on track. If they warp again I will be pissed and will be replacing OEM.

 

Rotors *can* warp though its pretty rare, usually as a heatsink issue what you experiences. They get pretty hot sometimes. More commonly there is pad debris buildup as ushgeo wrote about that will give the same 'feel' as warped rotors. I've only warped one set of car and one set of motorcycle rotors of all the many vehicles I've owned, rest have been pad buildup which is why I suggest running dedicated track rotors and pads and swapping them out before the event (its quicker to do so than you might think, about 1 hour). Keep a track set and a street set and the pads stay with those given rotors.

A cool down lap is IMPORTANT, though 90% of my students at the track still drive it too aggressively. I remind them to take it easy and let the car cool down and the brakes rest - they still go around too briskly.

Porsche OEM rotors are pretty good, doubt you will find better aftermarket. Ideally you'd go to a PCCB setup, but that's a costly retrofit.

 

Great info ushgeo! Thanks for posting!