The third generation Audi S4 quattro was based on the latest Volkswagen Group B6 platform. Despite having moved to an entirely new platform, one of the S4's most publicized new features was its new internal combustion engine - a 4.2 liter V8 engine.
Replacing the 2.7 liter 'biturbo' V6 engine of the previous generation S4 gained an all-aluminum alloy 4.2 liter, 40 valve V8 engine. The new V8 engine produced an output of 340 hp, making it more potent than the RS2 Avant, and nearly as powerful as the B5 RS4.
An unusual sight in the engine compartment was the addition of water cooling for the 190 amp alternator, along with two additional thermostatically-controlled side-mounted radiators.
Discrete lower door trims, along with the now trademark Audi 'S' model vertical-emphasis line grille, and solid aluminum, exterior door mirrors, and S4 badging front and rear are all that visually distinguish the S4 from its related A4. In the interior, front and rear Recaro seats (fronts fully electric), standard carbon fiber trims, as well as S4 logos in the instrument panel and door sills complete the detail.
Source: Wikipedia, 2011