Technology explained: Porsche Stability Management
With the unveiling of the 996 Carrera 4 in 1998, Porsche introduced PSM, an electronic stability control system aimed at making sure your 911 headed in the right direction even when you overstepped the mark.
Since becoming a universally adopted standard feature in 2003, PSM has been steadily developing, resulting in the system we now experience in the latest 991s.
Rather than single system, PSM is actually an umbrella term for a range of electronic controls, all working together to ensure maximum stability. Myriad sensors are monitored, with wheel speed measured courtesy of the ABS sensors.
Engine speed, throttle position, lateral acceleration, yaw (the car spinning around a central axis) and steering wheel position measurements are also recorded, enabling the PSM system to calculate the actual direction of travel at all times.
From this data, the PSM system can undertake preventative action should the car start understeering or oversteering. If the front end starts to slide, the PSM light will start to flash and the inside rear wheel is braked automatically to help the car turn tighter.
Conversely, if the rear end steps out, the outside front wheel is selectively braked to provide rear end stability. In Sport Plus mode, the PSM threshold is increased, allowing a greater slip angle to be achieved.
PSM also makes use of an Automatic Brake Differential (ABD) to improve traction. This brakes the spinning wheel if traction is lost, transferring power to the wheel with more grip.
Traction can also be controlled via a standard traction control system, where the engine’s ignition is retarded to lower the power output momentarily. On PDK models, the PSM system can also influence gear changes to maintain stability.
The ABS system is also controlled under the PSM umbrella and remains active, even when PSM is turned off via the centre console button (as does the ABD).
When turned off, the PSM controls will automatically re-engage if one of the front wheels required ABS assistance (in Sport Plus mode, both front wheels require ABS assistance before PSM is re-engaged). This ensures that, even during exuberant driving, the PSM is also primed to keep you out of trouble.
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