TechTime with Scott M

Porsche Stability Management & Limited Slip Differential

By Scott McIlvain

My personal favorite Porsche technology has to Porsche Stability Management (PSM). This may be due to all the times it has saved me while instructing in the passenger seat while on track. The little PSM switch on the dash has saved countless number of Porsche and their drivers. Personally, I feel that this safety technology is almost as important as air bags. Starting with the first gen 996 Porsche introduced PSM to help keep drivers on the road. Like any car Porsche cars have a limit on what they are capable on the street and on the track. If the driver pushed passed these limits of traction due to a mistake or weather conditions PSM comes in and most times will save the day. PSM looks and many different sensors hundred times a second to see what the car is doing and finding if there is a problem. One of the more important sensors is the wheel speed for each wheel and determining if there is any wheel slippage. As the driver turns the steering wheel the car can see the steering angle. PSM also looks at the throttle position sensor to see what the driver is commanding for throttle. I say commanding because really with throttle pedals that are not connected by a cable to the throttle body the driver is only requesting acceleration, the car actual determines how much to open the throttle body. So if while coming out of a corner with too much acceleration for the given traction PSM will take away throttle to keep the car on the road. PSM can also determine the angle of the car by a yaw sensor. With all of these sensors and more PSM can determine if there is going to be a “problem” before it even happens and correct it. These corrections can be done by the brakes with PSM determining if the car is understeering/oversteering by applying one of the rear brakes to get the car to “pivot” around the brake caliper that is being applied. PSM will cut throttle, turn on traction control (more on this later) and even activate the ABS if needed. All of this keeps the car pointed where the driver wants. It is a great system with improvements with every generation, but it is not 100% full proof. I have spun many times on the track with PSM. So you shouldn’t drive as hard as you can and think that PSM will save you every time. Also for those who like to turn PSM off it will automatically turn back on when the brakes or applied in most PSM systems. Traction control is another safety system that Porsche uses. If the car senses that one of the tires are losing traction and brakes the spinning wheel transferring power to the wheel with more grip. So in my 996 Turbo 2wd with 600WHP the only thing keeping the car on the road while on full boost is the traction control. Unfortunately, my car does not have limited slip differential. A limited slip differential (LSD) have been around for a long time. Many factory Porsches were and are equipped with a LSD from the factory.

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