Exploring what the 996 turbo PSM actually does

CAUTION, NERD CONTENT! I have a job to do for work to collect some brake testing data next week, in preparation I have been getting my car set up and whilst I was at it I thought I would update what data I am pulling from CAN and add a few more channels if I could. I am now getting enough data to have a look at what the traction control system does on the car when activated. As I know some of you are as inquisitive as I am, I thought I would share what I have found. 2nd gear, 4000rpm, cold but dry road, slightly uphill. Accelerating briskly and I managed to provoke the traction control. Top to bottom on the graphs are: Throttle position Front brake pressure (Front right) Rear brake pressure (Rear right) Manifold pressure Actual throttle angle Basic ignition timing Actual ignition timing Rear right wheel speed Front right wheel speed Actual engine load Ratio between average front and wheel speeds (<1 means rears are going faster) All data is from CAN bus except for the two brake pressures which are from pressure transducers in two brake lines. At this point on the graph boost is building, 0.9bar, actual and basic ignition angles are the same, throttle is wide open and the rear wheels are going about 3.5% faster than the fronts, all is fine and the traction control isn't doing anything. Here the right rear is at 90km/h and the right front is at 75km/h. The rears are going about 12% faster than the fronts and the traction control is just about to step in. I zoomed in slightly for this one, entire graph is around 2s of data here. Traction control has now intervened, boost level is dropping, rear brakes have been applied but the throttle is still open. Ignition is being pulled by up to 6 degrees. At this point the rear wheels have regained traction. By this point the rear wheels have well and truly gained traction but the throttle is being closed and the rear brakes are still being applied. For a short time afterwards there is also slight pressure on the front brakes. So, it appears that the traction control starts to activate at a slip ratio of somewhere around 10%, it works by pulling ignition timing and reducing boost, as well as throttle control. It also seems to overreact and keep the power off for much longer than is actually needed! As I was just on a public road I wasn't exactly going balls-out on this one. If I get a chance next week when doing more testing I'll push it a bit harder and see if I can see anything else interesting. MC

More empirical data. More empirical data. More empirical data. :food:

Nice work MC!!! This is interesting. I always thought that the PSM just cut the throttle when it spotted slip. That is just what my arse on the seat has told me. Interesting to know that it does add in some braking too! Mine is more or less permanently off though as it is so OTT. With it on it is more or less impossible for me to open the throttle without it going in to melt down!

I always thought it reduced power and applied the brakes to any particular wheel it needs to. I'm sure i've felt it braking the right front when taking a right hander too fast in order to help pull the car around the corner for example.. Whilst I'm sure its primitive compared to modern TC units, I think it was pretty good in its day. My old 4S used to eat pads if I left PSM on when on track, but didn't if i switched it off. Loving the data by the way MC, more of the same please! :thumb:

Yep, that definitely the clever way of testing the TC. You can do it this way too: https://youtu.be/ad2rForYHD0?t=1m28s Yes, was deliberate and no, I did not shift my foot off the throttle once after turn-in, all that was done by the systems. Don't try this at home etc. etc.

Ragpicker- that's the abs doing its job when the fronts brake on a corner. Its a bit of a flawed system if you ask me. You'd think applying throttle whilst braking the rears would be to power the fronts whilst restricting the reas, but thats not possible due to the design of the awd system. Interesting data mate. Nice one. :thumb:

I am either learning something new or confused (or both) :dont know: 'For a short time afterwards there is also slight pressure on the front brakes'. My understanding of traction control was management of the drive through the driving wheels. ie cutting the power or braking the rears in a two wheel drive car. If brakes are being applied to the front wheels (for traction control), what is the difference to this and Porsche Stability Management? I'm aware the PSM receives data from other sensors like Steering angle, so it controls stability rather than straightfowrad traction control. Is that the difference? Is PSM a form of traction control as it controls wheel slip? :?

PSM is traction control, but it monitors more variables and collects more data so that is can control the car better, ie. Stability Management. Don't forget, the car used here is awd. The operation of psm may differ slightly on a rwd car. Edit: maybe title should be exploring the PSM?

Very interesting MC, good work. I'm intrigued, does the car have a yaw sensor and if so, does this get taken into account when the TC is activated? I agree with your findings too, the intrusion is altogether too 'long', in fact if you were to get the car sliding around a corner by stamping the throttle to the floor, the throttle is closed completely until you lift off... Found this out a couple of years ago on a hoon. In other words, all power was cut to save the car from a slide because the throttle was wide open, but it would not reinstate any power until coming off the loud pedal. It would be interesting to see which of the many parameters triggered this scenario. Meanwhile, keep up the logging! :worship:

There is a yaw sensor, as the data from this available over CAN, no idea how this is taken in to account, I don't plan to try to get it out of shape on a bend on the road. Roll on Monday for some testing at Bruntingthorpe where I will have the full INS setup (roll, pitch and yaw rates and angles) and will be able to throw it around a lot more on slicks. I'll post an update after that. There is a lot less information for when the ABS is activated, all I can see is a single flag to show it is operating and the values from the pressure sensors and wheel speeds. MC

Excellent post! :thumbs:

:thumbs: thank you for sharing i have occasionally thought that it was a bit clumsy when it does intervene (2nd gear on a 180Deg hillclimb last month for example) in that it really bogs down (logically cuts power and also brakes the wheel(s) losing traction but maybe too much?) but conversely i have also been very grateful at times over the years Definitely wouldn't be without it :grin:

:agree: :thumb:

^^ +1. Excellent post. Bordering on being above my level of understanding but I'm with it so far. Well done on putting it into layman's terms. :thumb: I always find these systems too intrusive, although obviously as time goes on they become more and more advanced.

Sorry to necro but wondering if you had anything further to share on this from back then @MisterCorn