Audi was re-configuring its lineup during the early 1990s, and introduced the S4 in 1991 (1992 in North America) as the performance side of the re-styled 100 automobile. regardless of being based on the C4 Platform and sharing its central makeup with the 100, the S4 successfully took the place of the 200 Turbo-Quattro sedan, which had been discontinued the year earlier.

The S4 was sold universal with a turbocharged version of Audi’s 2.2 L (2226 cc), 20-valve inline-5 engine increasing 230 PS (227 hp/169 kW) and 350 N·m (258 ft·lbf) of torque. A 280 PS (276 hp/206 kW) 4.2 V8 became available to European customers as an option. Quattro all-wheel-drive and a manual transmission (six-speed in Europe, five-speed in North America) were set equipment, even though a 4 speed automatic transmission was obtainable as an alternative outside North America. The most frequent configuration 2.2L 5 speed was no slouch for its day, achieving 100km/h (60mph) in just 6.2 seconds despite a curb mass of 3384 lbs. After Audi dropped the 100 nameplate and re-badged the vehicle as A6 in 1994 (in 1995 in North America), the S4 became the S6, although the only real changes to the model were insignificant cosmetic updates and the adding (outside North America) of a more powerful S6 Plus model to the lineup again using the V8 engine. After the first generation A6 was replaced in 1998 by an updated version based on C5 Chassis, the S4 and S6 became distinctly separate from one another.

Being the first in the S4 lineup, it is usually referred to as the UrS4, coming either from the German words Ursprünglich (original) or Uralt (ancient) depending on belief, and even 15 yrs after its introduction continues to attract a strong enthusiast following due to its extremely robust engine, advanced design, superb interior (even more luxury, space and conveniences than the later model), and very capable Turbocharger system, which can be readily and cost successfully modified. Changes to the engine management system only (commonly called Stage 1 or 2), can yield an extra 25 ~ 50hp making it faster than later model B5 S4′s. While simply swapping the Turbo, and fuel delivery components (commonly called Stage 3) with the (same Inline 5 engine) Audi RS2 can yield upwards of 380hp, making it even faster than latest model S4′s. Very high mileage Stage 1~3 Cars with over 320,000km (200,000 miles) and original engine & drivetrain are not uncommon, and a testament to the enormously solid foundation. Continued development on the Turbocharged Inline 5 engine has lead to kits with alternatives to Audi/KKK Turbos and over 450hp, still using only bolt-on upgrades outside the engine.
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