My 2001 C2 has PSM. I know that it is the "Porsche Stability Management" and is supposed to control the angle of the car by braking individual wheels and/or killing power.

I autocrossed this car for the first time on Sunday and was quite surprised that the PSM was quite unobtusive (compared to some other cars with traction control that I have autocrossed). I manually turned it off before my first run and noticed that the light at the PSM button was still indicating that it was off after the run. I thought that it automatically turned on when you hit the brakes. After the first run I left it on for my final six runs and only saw the PSM light in the dash come on once when I really screwed up.

Now my questions.
1. Does the PSM act as an electronic limited slip?

2. Would a change in tire sizes other than stock width and profile adversely affect the PSM? I am thinking of trying to get wider rubber especially up front but there don't seem to be any alternative combinations that give you close to the same overall diameter.

 

after (1) autocross I would recommend that you do not make any changes - simple go again and again and learn to control the animal that you have...the understeer (which is my guess as to why you want bigger front tires) is something you need to learn to control thru throttle management. braking, speed, and propper apex cornering. You can gain a lot of advantage thru simply changing tire pressure too...play with wht you have before spending money and energy fixing what may not even be a problem...

my $02

 

+1 on that.

have not had many events under my belt yet, my third year.

only been the last four autoX's that I have been turning PSM off (when I remember too) FWIW have found that in AutoX it can slow you down when you get it wrong, the throttle cut's out, brakes do what they do when some rotation is called for, ie, getting the rear to step out. is frustrating for sure, putting your foot down and the nanny says, no, not yet....
thing is the courses are so tight you need the rotation to make the corners and from what I gather PSM is there to prevent over steer, it does nothing for understeer.

on the other hand, some of my smoothest, fastest runs have been with it on and the nanny said nothing, so like folks say if your getting it wrong you will know it with the nanny but the nanny will not say anything when ya get it right.

make sense guys?

mind, this last event I had my first 360 on the pad where it is safe (psm off of course) and there is something to be said for the rush and the experience you get from that.

having said all that, the coil overs, sway bars alignment have dialed some of the inherent under steer out.

the OP is thinking right though I think, more traction on the front is called for.
play with tire pressure with that in mind, you want the rear to be lose.

one more thing to think about is weight transfer, you want to load up the front wheels, so a quick jab of the brakes moves some weight to the front, giving more traction on the front wheels and unloading the rears.
left foot braking is something to think about as well.

main thing is to have fun and continue to do it in a safe controlled setting.

one more one more thing, have found the folks at auto crosses are most helpfull, find a veteran for ride alongs and tips.

maybe more then you asked for but that my 0.02cnd, and thats more then the green back today....

 

its all in the acronym.... Please Save Me

 

+1 even on a track day. Learned this the hard way. First event I could drive with it on (most likely inexperience), second event I had it one and cursed until I pressed the button.

 

It's pretty much the "Oh **** I effed up big time and need the car to prevent itself from flying off the road" button"
It's also a thrill killer, but hey.. better than a totalled car right? Disable it at your peril.

 

PSM will stay off until you invoke ABS in which case on your car it turns back on. Later cars do not do this.

PSM does no act like a limited slip diff.

Wider tires are not an issue but tire diameter is! You need to keep the front and rear rolling diameter of the wheels and tires within 3 percent of each other. If wider wheels make the fronts 1 inch higher than the rears, PSM and ABS will be upset and you won't like the consequences.

To reduce understeer, get more negative camber in front. On a 2001 you can do this by elongating the adjustment slots at the top of the front shock mounts. That should give you around 1.7 degrees negative camber which will make turn in more responsive.

 

treat - search is your friend:

From memory, PSM consists of 5 components

1 ABD or Automatic Brake differential

2 Yaw Stability under Braking

3 Electronic Throttle

4 Traction Control

5 Stability management.

I forget the Porsche name for item 5, but it is the part that catches the car and starightens it up.

When PSM is selected OFF, components 3, 4, 5 are disabled. Items 1 and 2
remain in effect. PSM does NOT get re-enabled under braking as you will often hear people try to misinform you. Item 2 simply is always in effect on a PSM equipped car (barring fuses blowing or system faults).

Item 1 is Porsche's method of providing a less expensive LSD when all the PSM technology is there.

Item 2 really really wants you to brake in a straight line. I really want my students to do this also. I am personaly happy to trail brake and let the tail of the car step 5 to 10 degrees out of line. It's not what I teach, but with 4 years of forest rallying experience it doesn't make me uncomfortable. What instructors need to know is that if you try to do this, the Yaw Stability under braking program will not only try to straighten the car with differential braking, but will also push back on the brake pedal with a force that I would estimate at about 50 lbs. If you are threshold braking the first few times this happens to you, it will be quite disconcerting! My experience is that it is effective to push back on the brake pedal if you are able.

Item 3, the E-Throttle can be turned off to the extent that it will not cut power to the engine when the stability or traction control system would otherwise demand it.

However what the owner's manual does not tell you is not to bother asking your 2000 and later (E-throttle equipped, PSM or not) 996 students to left foot brake. It appears at first attempt that if the brake is applied when you are on the gas, then power to the engine is cut by the E-throttle. This is true, and so you can't left foot brake. However you can do a perfectly good heel and toe, and so further research shows that if the brake is applied first, then the engine will respond to the gas pedal. So you can heel and toe. If the gas pedal is applied first, then anything over light brake pressure will entirely cut engine power. So I wouldn't advise trying left foot braking.

One other thing you won't find in the owner's manual is that the PSM requires that the steering wheel be pointed straight ahead when the car is tracking in a straight line. Of course they all come that way, but I might think twice about climbing in to a PSM car with the wheel pointing significantly off straight.

These days, I consider PSM to be a very good system and not especially intrusive. If people suggest that it is intrusive, I suggest to them that it enforces what could be considered excessive smoothness. This is no bad thing. That being said, when I was getting used to the car, I would frequently turn it off, because either I wasn't smooth enough and it got in my way, or the alignment wasn't quite right and intruded a little soon. I have had people who consider themselves smooth drivers get into a fight with PSM while driving my car. Oddly enough these are usually 944 or M3 drivers. I have the impression that those cars are more forgiving of a more abrubt driving style.

 

Bob R, thanks for the info. This is exactly what I was looking for. I should clarify to the other posters that I have been autocrossing for some time. My previous autocross car was a Toyota MR2 Turbo. I ran this in BS with Hoosiers. I do believe I am familiar with weight transfer techniques but I did find the front of the 996 to feel much lighter. I am not sure if it is a camber issue, tire traction issue, the difference between a mid engine car and a rear engine car or some combination of all of these. These comments are made upon my first impressions. I am sure that I will learn how to drive this car better at future events. I was just curious as to what the PSM really did and glad that Bob R clarified some of the misconceptions.

So if I stay within 3% of tire diameters from front to back I can theoretically put 265/35s on the front? Practically with 18x8 rims I can get 245/40 tires that give me a 1.6% difference. Does this seem reasonable?

 

I've autocrossed my C4 a few times now and I've tried it both ways. On one hand, it's annoying when it cuts the throttle. But when it does its thing, it seems to mean I'm doing something wrong in terms of driving. On the other, I like the fact it helps keeps the car pointed the right way when I make a mistake.

 

Keeps your insurance rates stable.

 

If the ABS re-engages the PSM and my PSM was still off when I finished my run, does this mean that I wasn't braking hard enough?

I do believe that I may not have been exploiting the braking limits since I my approach speed was likely higher than with my previous car and I would brake earlier.

 

errrrr uhhhh isn't ABS just to keep the braking from skidding the tires and PSM goes off direction the fronts are pointed to manage brakes to help direct the car in the direction pointed?

Once again in this world I do not like the ABS on most other cars. Although I do thing porsche's ABS isn't an electronic automatic suicide switch to make you keep going when you want to brake.