PSM question for the technical porsche insider guys

I have a question about PSM I understand there are two aspects of the PSM. One is for stability (YAW) and the other is for traction. How is traction defined? How does it relate to the LSD function of the car? I understand that when the PSM switch is turned off, that once the brake pedal is pressed whiles its "safety zone" is being exceeded it automatically turns it's self back on. Does the LSD function continue to operate while the PSM is in the off mode? I.E. will it continue to apply braking to the spinning wheel in-order to transfer power to the wheel with more traction? How much braking force will it apply attempting to get traction? If both the wheels don't ever get traction will it allow me a smoky burnout or will it eventually apply brakes until the car starts moving forward without wheel spin? My question arises from this. In my other cars with a torsen sensing LSD, I could charge into a corner and use the gas to "step out" the back end, inducing oversteer to get me through the corner. The LSD made sure that both wheels kept going. (not doing the one legged chicken scratch). With the 996 with PSM on of course it limits the amount of sliding, hits the brakes and flashes the little orange light at me. Now if I turn off PSM, will it allow me to get the sliding/wheel spin in the corner but STILL modulate the left/right rear brakes to assure that both wheels are spinning and trying to get the power down? OR will I just get that chicken scratching going? Porsche of course doesn't publish such detailed data on how the PSM works. I just don't see it as a good replacement for a LSD. Thanks! Andy

LSD is mechanical... one purpose is to give traction to both driven wheels... simple.. never turns off unless it is broken. PSM is stability control..... electronic gizmos trying to keep the car going where the front wheels are pointed and overriding your Throttle and Brake inputs to accomplish that goal. I assume you have a 40th anniv car in order to have both PSM and an LSD?

Paul, I am very familiar with how LSDs (both torque sensing and clutch pack type) work. Porsche claims that their PSM replaces the need for a mechanical LSD in the non-LSD equipped cars. Theoretically applying brakes to the slipping wheel and the open differential then transfers power to the other. This "traction control" is different than the stability control that keeps the car going where is is pointed as you mention above. As I said PSM has two fuctions, stability AND traction. My questions are to how the traction feature manifests it's self in the the 'on' and 'off' position. My car (2002 Carrera coupe) does not have a LSD. The 2004 40th anniversary car was the only MK II car that came with an LSD (clutch type) as you mentioned. (With the acception of the GT1 based cars Turbo, GT3, GT2). The 1999 Mk I 996 also had a mechanical (clutch type) LSD as an option.

Excellent question that I actually had also wondered, because my BMW has LSD and DSC, so I know exactly what you are referring to. I WILL say that far and away I find Porsches PSM far less intrusive than BMW DSC, where you REALLY feel the throttle go and the brakes apply..it KILLS you powering out of a turn. I haven't realy pushed the 996 much at all, but on a rainy day I was really impressed with the difference. Honestly, though I'd rather replace pads & rotors regularly than LSD clutch packs if the result is the same!! From what I read when researching getting the MKII 996, the traction part is still ON with PSM OFF..but the proof is in the pudding, need to try it on a safe slick road..

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Yes, DSC in the M3 is much, much more intrusive than PSM. Even when I was straightline monkey racing it in 30-40 degree weather I felt like my 2nd gear was just gone cuz of all the wheel spin DSC was trying to stave off so the CLK 430 ended up getting way ahead. PSM > DSC for me.

I tried my best. My responses are in bold. "I have a question about PSM I understand there are two aspects of the PSM. One is for stability (YAW) and the other is for traction. How is traction defined? How does it relate to the LSD function of the car?. As you probably already know, the LSD function of PSM will keep the rotational difference between the driven wheels within a particular range. If one wheel rotates faster, the brakes will be applied to that particular wheel. I don't believe that the actual number, ie. percentage difference, is published anywhere. I understand that when the PSM switch is turned off, that once the brake pedal is pressed whiles its "safety zone" is being exceeded it automatically turns it's self back on. Yes. I feels like an anchor has been thrown. Throttle is cut and brakes differentially applied Does the LSD function continue to operate while the PSM is in the off mode? I.E. will it continue to apply braking to the spinning wheel in-order to transfer power to the wheel with more traction? Yes How much braking force will it apply attempting to get traction? Theoretically up to full lock on the spinning wheel. Understandably, it will be somewhere in between as the open diff will fight the braking wheel. If both the wheels don't ever get traction will it allow me a smoky burnout or will it eventually apply brakes until the car starts moving forward without wheel spin? Although difficult to do because of the rear weight bias and the wide rear tires going against a relatively weaker clutch, I've seen people do burn-outs. If the ground is dry and traction is good, more likely than not you'll burn the clutch and not the tires. My question arises from this. In my other cars with a torsen sensing LSD, I could charge into a corner and use the gas to "step out" the back end, inducing oversteer to get me through the corner. The LSD made sure that both wheels kept going. (not doing the one legged chicken scratch). With the 996 with PSM on of course it limits the amount of sliding, hits the brakes and flashes the little orange light at me. Now if I turn off PSM, will it allow me to get the sliding/wheel spin in the corner but STILL modulate the left/right rear brakes to assure that both wheels are spinning and trying to get the power down? OR will I just get that chicken scratching going? Again, due to the rear weight bias and the wide tires, it's hard to get the rear to step out. I'm not saying it can't be done. It's just harder to do than, say, a Z06. Moderation in trail braking and trailing throttle will get that rear out. Too aggressive and PSM comes back to life. Once you get that rear to step out, progressive throttle will work with the LSD function of the PSM to transition you from oversteer to neutral drift to understeer. Stomp on the gas and those wide tires will dig in and you go directly to understeer. It takes time and practice, playing with tire pressures, maybe a suspension upgrade. That's part of the fun, and the feeling of accomplishment, in driving a 911. Porsche of course doesn't publish such detailed data on how the PSM works. I just don't see it as a good replacement for a LSD. That's what some people say. I disagree. Thanks! Andy"

Andy, I was trying to help The PSM is a poor substitute for a real LSD. 2nd hand info is that when you turn off the PSM it allows you a bit more slip angle (ie tail out) before intruding. I have a 99 with the LSD and the TC that you can turn off all the way.... still have to hold the button down for 10 seconds like in the M3's otherwise it still comes back on above a certain limit.

As far as I know the LSD function only operates below certain speeds. I believe it was designed to help you get out of snow, sand etc and not as a substitute for a mechanical LSD. When PSM is off, yaw control is still active. Yaw control uses a steering wheel position sensor and a lateral acceleration sensor. If the inputs from the two are out of range the system intervenes. This can be felt if one trail brakes too enthusiastically. Yaw control will bang (not a tremble like ABS, a real hard hit) the brake pedal hard - to get your foot off it and PSM will diagonal brake to correct the (perceived) over-rotation. You don't get this much with street tires since the system is calibrated for N rated rubber. On R compounds it can be a surprise. My experience with a Cayman S with PSM is that on or off, there is no noticeable LSD function exiting corners, acceleration is held back by the transfer of torque to the lightly loaded wheel. Perhaps the system does apply braking to the inside wheel, I can't tell from behind the wheel, but the result is the same, the car does not pull as well. This is the reason that trackies like to install a real LSD on a Cayman S. I also endorse Palting's comment. If you try a burn out on sticky tires and dry pavement, the smoke will be from the clutch, not the tires. Ask me how I know <grin>. Best,

Now that I think about it again, I think Bob is right. LSD in PSM is only for slow speeds. No LSD at high speeds. OTOH, I have not had "chicken scratching" despite aggressive track driving. Try it yourself, on track. Smooth and fast, I doubt you'll find the need for LSD in a 996 or end up "chicken scratching". Now, racing, well, that's a different kettle of fish altogether. Racing suspension, racing tires, racing alignment...... yeah, you'll probably look for the real mechanical LSD.

When you turn off PSM. It is off, the sensors are still active but it won't come on ever. Not unless you hit the brakes, then it takes over but if your not on the brakes it won't come on. Also pulling the ABS fuse will disable PSM for good.