1988 Porsche 911 Carrera 4 964 dynamic all-wheel control
All-wheel-drive has been a part of the 911’s repertoire since the 964 of 1988. Total 911 explains how the first AWD system worked.
Four-wheel-drive has been with the 911 for over 30 years now, but the idea was tested by Ferdinand Porsche in the 1947 Type 360/Cisitalia. That used an all-wheel-drivetrain to meter the power from a supercharged 12-cylinder engine, but the idea had been set in motion from the off. Porsche’s first car, the Lohner-Porsche of 1900, was four-wheel-drive thanks to an electronic motor for each wheel.
The first four-wheel-drive 911 appeared in the 1988 964 Carrera 4, off the back of the success of the 953 and 959. The race cars used centre differential locks and a rear differential lock, whereas the 964 system was simplified, using two multi-plate clutches: one acting between front and rear axle, and another between rear wheels.
The system relies on a manual lock that’s hydraulically operated and electronically controlled. In normal driving, PDAS splits torque in the central power transfer unit mounted between gearbox and front diff, assigning 31 per cent torque forwards and 69 per cent rearwards. PDAS adds to that drive split by using the ABS wheel sensors to detect a difference in wheel speed, where it then hydraulically locks the clutch plates in the PTU to push more drive forwards. When traction is reached, pressure on the PTU clutch is then backed off in stages. In this manner, extra torque is variably transferred forwards as PDAS detects a wheel speed difference, front to rear.
Separate to this, in the case of an oversteer event, the wheel speed sensors enable the PDAS to detect and then counter it by applying hydraulic pressure to lock the rear diff. Again, as the wheel sensors detect a difference in wheel speeds, locking torque is variably increased, then stepped down as required. In this manner, the 964’s drivetrain constantly provides four-wheel drive. However, if required the PDAS can assign extra torque front to back, or side to side (at the rear) to transfer traction to the wheel with the most grip – in other words, the one that isn’t spinning.
When driving on wet grass, ice or snow, the PDAS system can also be manually engaged by turning the dashboard knob when the vehicle is stationary. This fully locks both centre and rear diff locks until the vehicle reaches 40kph, but should only be utilised in low-traction circumstances, so as to avoid transmission damage and tyre scrub.