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 S6
Audi was restructuring their model lineup in early and mid-1990s, and in late 1994, began selling the fifth generation (C4) Audi 100 model, re-badged as the Audi A6. Wanting to keep a sports-saloon in their lineup, the company made small revisions to what was previously known as the Audi S4, renaming it the Audi S6; the S4 name would eventually be re-used for a completely different model derived from Audi's smaller Volkswagen Group B platform-based Audi A4. As this was the first S6 model from Audi, it is commonly referred to as the "Ur-S6", derived from the German augmentive, "Ursprünglich" (meaning: original).