Loosely translated from the original German

High-tech and intelligent lightweight construction

Tradition is a commitment. In the mid-1980s, the technology carrier Porsche 959 demonstrated an absolute cutting edge of development with its highly effective and complex all-wheel drive system. This undoubtedly benefited the parallel-designed and equally convincing concept of the previous Porsche 911 Carrera 4, which received top marks from customers and professional test drivers. For the new Porsche 911 Carrera, all-wheel drive was also taken into account in the first planning phase in order to be able to offer customers an even higher level of driving dynamics, combined with optimal traction under all conditions.

The specifications called for intelligent lightweight construction with sophisticated components, maximum efficiency and the greatest possible cost-effectiveness, retention of the vehicle's character and adaptation to today's environmental conditions - targets that could only be met with the extensive all-wheel drive know-how of the Weissach engineers and a high-tech solution.

The consistent implementation of the contents of the specifications resulted in a drive system in the new 911 Carrera 4 that combines the agility and driving pleasure of the rear-wheel drive 911 Carrera with the superiority of an all-wheel drive concept. High-quality technology and the design and coordination of the components, which have been tried and tested in long series of tests, lead to significantly lower weight through the use of less material. The secret of the Porsche all-wheel drive concept, however, clearly lies in the balance of all the necessary components.

The low weight of the complete all-wheel drive for the new Porsche 911 Carrera 4 is remarkable; it is one of the lightest systems on the market. This is particularly evident when compared to its predecessor: previously the additional weight was 100 kilograms, the new solution manages with an exemplary 50 kilograms. The weight saving is the result of intelligent lightweight construction and has a positive effect on fuel consumption and driving dynamics. This effect is reinforced by the fact that the mechanical drag losses could also be reduced by 50 percent compared to the old drive train.

The key components of the new Porsche all-wheel drive system are:

• The all-wheel drive via a shaft with a central tube (transaxle) and a viscous coupling between the front and rear axles;
• a dynamic limited-slip differential for the rear axle;
• the automatic brake differential (ABD).

This is how the all-wheel drive works

The viscous coupling is located at the transmission output in front of the rear axle and ensures that the drive torque is automatically distributed between the axles depending on the slip of the wheels. It is connected to the compact and weight-optimized front axle differential via a hollow shaft. The maintenance-free viscous coupling works depending on the force, speed and temperature due to the interaction between the disk packs and the silicone oil filling. The viscous coupling also takes care of the speed compensation between the two axles when cornering.

If the rear axle tends to spin, the viscous coupling responds in a fraction of a second by increasing the drive torque on the front axle. Thanks to a viscous map that is specifically tailored to the vehicle, the viscous coupling actually always allocates the ideal drive torque to the front axle and ensures driving stability under all conditions.

The dynamic rear axle limited-slip differential is designed asymmetrically. This means that when pulling (engine drive) there is 25 percent locking effect between the two rear wheels, but when coasting it is 40 percent. The lock therefore generates a yaw moment (understeering tendency) that is almost twice as high under engine braking force as under drive force. This achieves a stabilizing effect when changing load (abruptly releasing the accelerator) in curves, which keeps the vehicle more precisely on the chosen curve radius. When accelerating, however, according to the development goal and the result of careful system tuning, the new Porsche 911 Carrera 4 shows the agile and corner-happy driving behavior that contributes significantly to the fascination of the 911 and has always been appreciated.

The standard automatic brake differential, or ABD for short, - offered as an option on the rear-wheel drive 911 Carrera - ideally complements the function and effect of the all-wheel drive with viscous coupling and the rear axle limited-slip differential. The ABD detects the drive slip of individual wheels via the ABS sensors; the control unit then ensures an appropriate braking torque on the wheel in question in order to restore optimal power transmission.

If the friction values differ on the left and right, the rear axle limited-slip differential is activated first and continuously allocates the driving force to the wheel with the higher traction. If this is not sufficient and increased slip continues to occur on a rear wheel, the spinning wheel is braked via the ABD control, with this braking torque acting as a driving torque on the opposite wheel. This function is effective up to 70 kilometers per hour and is particularly helpful when starting off is problematic due to weather conditions. The complete system with all-wheel drive via viscous coupling, rear axle limited-slip differential and ABD automatically offers maximum traction in any road condition.

Character of the rear-wheel drive 911 retained

The new Porsche 911 Carrera 4 has an all-wheel drive concept that meets the high demands of this high-performance sports car and is therefore optimized in terms of driving dynamics in terms of design and tuning. The character of the rear-wheel drive 911 Carrera was deliberately retained unchanged. The stepless and slip-dependent power distribution without intervention by the driver provides even more driving stability under all circumstances. In addition, the 911 Carrera 4 offers maximum traction in wet conditions and in winter.

The ideal combination of drive components, the consistent lightweight construction and the high level of efficiency correspond to the latest state of development and set standards. Above all, however, a vehicle has been created that has practically unchanged data for performance and consumption compared to the rear-wheel drive variant - an absolutely unusual finding when comparing other all-wheel drive vehicles with the corresponding two-wheel drive models.

The curb weight of the 911 Carrera 4 is 1420 kilograms, only 50 kilograms higher than that of the rear-wheel drive model, so the vehicle still weighs 30 kilograms less than the Carrera 4 predecessor. The measured values are another indication that the Weissach engineers have brilliantly achieved their targets with the high-tech solution they have implemented for the all-wheel drive of the new Porsche 911 Carrera 4.

Comfort included

In terms of dimensions, equipment and technology, the 911 Carrera 4 - apart from the drive concept - corresponds to the rear-wheel drive version. Even the trunk dimensions at the front remained unchanged due to the compact design of the front axle differential. The powerful six-cylinder boxer engine in the rear with a displacement of 3.6 liters also delivers a whopping 200 kW/272 hp in the 911 Carrera 4. Power is transmitted via a six-speed gearbox. The safety-oriented yet comfortable chassis with the complex LSA multi-link rear axle was adopted from the Carrera. The braking system with hydraulic brake booster and the ABS system were adapted to the all-wheel drive system.

Even with all-wheel drive, the new Porsche 911 Carrera 4 conveys the agile handling typical of the rear-wheel drive vehicle without restriction. In addition, it offers superior driving dynamics with drastic traction advantages on wet roads. The automatically controlled, variable drive power distribution ensures well-controllable driving characteristics. Even in city traffic, the all-wheel drive driver does not have to sacrifice comfort. Maneuverability when maneuvering remains unchanged compared to the rear-wheel drive vehicle, and there are no drive influences on the steering.