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.
Audi A4
The quattro permanent four-wheel drive system uses the latest Torsen T-3 centre differential, with a default 40:60 front to rear asymmetric torque distribution ratio (used first on the B7 RS4) as standard. (Previous A4 quattro models split torque with a default front:rear 50:50). The additional torque bias applied to the rear wheels helps mimic the driving dynamics of rear wheel drive cars.