Audi S4
Audi's quattro four-wheel-drive system was standard on the C4 S4/S6; a first-generation Torsen-based system which used the "T-1" automatically biasing centre differential (ATB) was used on vehicles equipped with manual transmissions, while a transfer box based system was used on vehicles equipped with an automatic transmission.
Quattro (four-wheel-drive system)
Audi's new sport differential, debuted 'torque vectoring' to quattro generation V. The Audi sport differential allowed the dynamic allocation of torque across the rear axle of the debut vehicle: the B8 (2008) S4, and is now an optional addition to all quattro vehicles, which continue to use the 40:60 asymmetric Torsen (Type "C") centre differential. The sport differential replaces the normal open rear differential while the front axle still rely on an open differential with EDL.