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 A6
Upon introducing the C4 series A6, Audi 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). It was available as both a saloon/sedan ("typ" 4A5), and an "Avant" ("typ" 4A9) (estate/wagon) to European and United States customers, but only as a saloon in Canada, Asia and Australia.