The 4WD system used by the 964 Carrera 4 of 1989 was one of the most advanced designs for the time being. It was derived from the 911 Carrera 4x4 racer which won Paris-Dakar rally in 1984, but the experienced learned from the 959 supercar also helped. In fact, Porsche described it as a simplified version of 959’s PSK system.
The system employed 3 differentials plus a multi-plate clutch. The epicyclic gear centre differential normally sent 31% to the front axle and 69% to the rear. Although it didn’t have 959-style variable torque split, the torque split is not fixed either. A multi-plate clutch incorporated with the centre differential would intervene whenever ABS sensors detected wheelspin at the rear wheels, hence sending more torque to the front wheels. This made the Carrera 4 more sophisticated than the contemporary pure-mechanical Audi quattro system.
The rear differential also incorporated a similar clutch acting as a limited slip differential. Again, the clutch is controlled by computer which got information from ABS sensors in individual wheels. Therefore the rear end of the Carrera 4 could hardly loose grip.
However, the Carrera 4 was far from popular. It was generally criticised as "over-corrected" the flaws of the tail-happy 911, transforming the car into an understeerer. The understeer was particularly severe when pushing the car to the limit, thanks to the large amount of torque sending to the front axle. Therefore Porsche designed a completely different system in the 993 Carrera 4.
In contrast to the 964, the 993’s 4-wheel drive system was rather conventional and simple, but it actually performed far superior in real world. Instead of epicyclic differential and mult-plate clutch, it used a simple viscous-coupling LSD as centre differential. To most FF car, viscous-coupling means understeer, but for the rear-wheel-drive-based 911, it means very much loyal to the Carrera 2’s character yet provided superior grip when needed.
To make the viscous-coupling always engaged the front wheels, the rear tyres were made marginally smaller in diameter, enhance established a small speed difference between the drive shafts to front and rear. With the speed difference, the viscous liquid normally transferred 5-15% torque to the front axle, which was much less than the 964’s system. In abnormal conditions, that is, whenever one axle lost grip, the viscous-coupling LSD may send up to almost 100% torque to the other axle.
Both the center LSD and rear LSD were now pure mechanical, but clever electronics was used in the newly-added ABD (Automatic Brake Differential). Again, ABD was simple yet effective. It was just a program, sharing all the hardware with ABS. Whenever rear wheels spin, it braked the spinning wheel thus the rear differential would send more torque to the other wheel. It was particularly useful for extreme conditions such as on snow, while LSD covered most normal conditions.
The 993's system weighed only 50 kg, that’s just half of its predecessor. Energy loss was also halved. It made the 993 Carrera 4 nearly as quick as the RWD version. Production cost was reduced as well.
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