But it is the Turbo’s Porsche Traction Management (PTM) system that significantly expands the Carrera 4’s performance envelope. Unlike the viscous clutch that handled torque-apportioning duties in the 996 Turbo and the outgoing C4, PTM is built around an electromagnetic multi-plate unit. In the old AWD system, any differential in axle speed would activate the viscous coupling, providing a maximum torque transfer of 30 percent to the front wheels. Here, the multi-plate clutch takes inputs from all kinds of sensors — steering-wheel angle, yaw, longitudinal acceleration, wheel speed — drives them through a control unit, which tells an annular magnet to act upon a small mechanical booster, which in turn directs the eight sets of friction plates in the main clutch. All this happens in a maximum of 100 milliseconds, and the system can direct 100 percent of the car’s torque to either its front or rear axle.
Porsche’s rationale for this arrangement is two-fold: 1) It offers better traction on low-friction surfaces, such as steep, snowy driveways, and 2) it delivers better driving dynamics. PTM is able to shift whatever amount of moment you need, wherever and whenever you need it. In full-on cornering, the system sends all engine torque to the rear (where an LSD manages lateral flow), so that the front wheels can devote their full attention to cornering. If, while in that corner, you induce a large amount of understeer, the car sends the appropriate dose of power rearward to neutralize slip angles.
Source: Motive Mag