The Audi S4 is Audi's entry into the sport sedan market. It is marketed to compete directly with other German sport sedans, such as the BMW M3 and the Mercedes-Benz CLK Series as examples. The two most prevalent variations of the S4 one will find on the road today are the B5 and B6, indicating the chassis each car is built around respectively.
The first iteration of the B5 S4 was shown off at Audi of America's headquarters in Auburn Hills, MI. The S4's general theme, is that of a sport-luxury car that combines capability with maturity. The S4 looks identical to its less potent but still capable base design, the Audi A4, save the red and silver S4 badge which serves as the only distinguishing mark. Hiding inside this deceptive exterior is a 2.7 litre, twin turbo-charged V6. This is delivered to Audi's renowned quattro drive train through a tightly geared 6 speed transmission. The result is a car that speeds from 0-60 in 5.5 seconds.
This 1998 model was representative of the B5 generation of S4s. Under the stock configuration the turbos run at a reserved boost level of 10.2 psi, each turbo is responsibile for providing the boost to half of the cylinder ports in the engine. (Compare with the Subaru Impreza WRX's 14.2 psi or Volvo S60's almost 15psi). These small turbos have less inertia to overcome and a low peak duty, thus resulting in a negligible amount of turbo lag, which means that all the brawn the S4 musters is available at a moment's notice. The car's peak torque of 258 foot pounds is achieved at only 1850 rpm and maintained through 3600 rpm. The peak 250 horsepower is achieved at 5800 rpm, and the engine redlines at 7000. Throughout the engine's duty it maintains a reserved yet aggressive character, an intimidating growl that revs quickly but never reaches the whining of many Japanese or Italian sports cars.
The interior strikes a fine balance between spartan sport simplicity, and the luxury one would expect for the sticker. It maintains it's purpose as a sedan, holding onto all 4 doors instead of discarding them for the weight savings found in the M3's 2 door configuration. Like much of the S4 the compromise between the demands on the car usually results in Audi erring on the side of adding weight. The B5 S4's weighed in at around 3700 pounds, not a light car by any means. This has a somewhat negative impact on the S4's handling capacity, it clocks in at 0.86 g on the skidpad. Much of this weight is sound deadening materials which make the car more tolerable to drive on long drives.
The B6 has done away with the B5's engine, replacing it with a 4.2 litre v8. This new engine makes 350 horsepower peak and provides more low end torque. If you enjoy turbo'd cars however, the B5 is still a blast to drive. Additionally the B6 reportedly has clutch problems with clutch discs shattering. I'll remain blissfully tied to my antiquated vehicle for the time being!