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 didnít 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 993ís 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 2ís 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 964ís 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, thatís 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.
Porsche's Tiptronic was the earliest semi-automatic transmission offered by a major car maker. First appeared in 1990 in the 911 as an option, it became available in 968, Boxster and was licensed to Audi and Mitsubishi for production.
Based on an automatic transmission with torque converter, besides conventional auto mode it offers a manual override allowing the driver to shift by pushing the gear lever forward and backward. Faster reaction it may not had, but more fun to the driver was assured.
The auto mode had 5 different programs to suit different driving style, something like the "Sport", "Economy" and "Winter" mode in traditional autoboxes. The computer determined shifting according to driving style. For instance, frequent full-throttle operation and brisk release of throttle indicated a sporting driving mode, thus "fast" program would be selected.
Even in manual mode, the computer might intervene under harmful conditions, e.g. it would shift up without the driver's command if the engine speed had reached the upper limit.
Tiptronic was developed in
conjunction by Porsche, ZF and Bosch. Porsche originated the idea, ZF made
the gearbox and Bosch was responsible for the electronic control. Little
had changed since its introduction :