Till 1980 Four-Wheel-Drive (4WD) was almost exclusively used for off road vehicles. Today nearly every European or Japanese vehicle manufacturer has in his product program a passenger car with 4WD. The systems used reach from a manual engagement of the second axle to a computer controlled power train.The purpose of this paper is, to elaborate on the most important differences of the available 4WD systems especially regarding the torque split. Emphasis is laid on the application of the viscous unit for torque transfer and torque control. New calculation methods confirm test results, proving that a vehicle using a viscous coupling for torque transfer has, indeed, permanent 4WD.