The New 911 Carrera 4 The Carrera 4 continues the successful tradition of four-wheel-drive Porsche 911 models. Its drive technology is a further development of Porsche's proven and highly successful concept, the entire process of developing the new Carrara 4 once again confirming the experience that the best way to divide power between the two axles on a rear-engined sports car is with a viscous clutch. A viscous clutch is a fully encapsulated multiple-plate unit filled with highly viscous silicone fluid. Part of this multiple-plate clutch unit is connected directly to the rear-wheel drive, the other section is connected with the front axle. Whenever wheel slip results in a difference in speed between the axles there is also slip between the plates within the viscous clutch. In that case, depending on the degree of slip, drive power is transmitted to the front axle by the viscous silicon fluid. Short Propeller Shaft Within the Frame Tunnel A number of fundamental modifications have been introduced between the gearbox and the front axle in the process of changing over to the new model: The first point is that the drive shaft no longer runs through a rigid tube as with the transaxle principle. Instead, the new Carrera 4 features a short propeller shaft running freely within the frame tunnel. The first advantage is that this solution is about 4 kilos lighter, the second point is that it provides space for the coolant pipes now running through the tunnel for the water-cooled flat-six power unit. Yet a further point is that the viscous clutch is no longer within the transmission itself, as it was in the past. Rather, it is now housed on the 911 Carrera 4 in the front differential casing moved far back to the rear of the car, a modification offering the big advantage that the larger differential housing and the viscous clutch apply a bit more weight on the front drive axle. This means even better front-to-rear weight distribution beneficial to the car's driving characteristics and traction. Four-wheel drive on the new Carrera 4 ensures that a certain amount of torque is always able to flow to the front wheels at all times. This share of power conveyed to the front varies from 5-40 per cent, depending directly on drive slip on the rear wheels. Driving Behavior Even Better Than Before The primary objective in this case is of course to perfect the characteristic driving behavior and the superior driving safety of the Porsche 911, thus reaching a higher standard than ever before. Four-wheel drive conveys the supreme power of the engine to the road even more smoothly and steadily than one drive axle. Installing four-wheel drive in the floor pan of the Porsche 911 required a number of modifications. The first point is that various body components had to be modified in the region of the front axle in order to accommodate the front differential. This called for a new fuel tank shaped like a saddle and offering approximately the same volume as before. An inevitable result of the more sophisticated drive technology is a slight loss of space in the luggage compartment, which is however kept to a minimum by the folding wheel in flat arrangement on the floor. Luggage compartment capacity therefore remains adequate at 100 litres or 3.5 cu ft. And despite its more sophisticated technology the Carrera 4 remains a light and agile vehicle from the start, the coupé weighing in at 1375 kg or 3032 lb, which makes it only 55 kg or 121 lb heavier than its rear-wheel-drive Carrera counterpart.