The S4 is, at its core, a modified A4. Thus it retains the four-door configuration and general dimensions of the smallest sedan Audi offers in the U.S. Differences begin under the hood. Like all Audi engines sold in the U.S., the S4's cylinder heads have five-valve-per-cylinder architecture for superior breathing. Two small turbochargers nestle out of sight under each bank of the vee on either side. Two small turbos can deliver the same boost as one larger turbine because, with less mass, they spool up faster. It virtually eliminates annoying the lag, or delay in throttle response, associated with big turbos.Audi did more than hang twin turbos on its V6, however. The engineers also made the cylinder bore slightly smaller -- and thicker -- to handle the added pressure from turbocharging; that reduced displacement from 2.8 to 2.7 liters. Carefully shaped intake tracts cause the intake charge to tumble in the combustion chamber for more efficient burning, and the intake valve timing is variable. The engine is tuned for responsive torque and develops 258 foot-pounds from 1850 to 3600 rpm; it reaches its horsepower peak at a relatively low 5800 rpm. Audi's quattro system features a TORSEN differential that mechanically redistributes torque up to 66 percent to whichever axle has more traction. At both front and rear the S4 has Electronic Differential Locking, which detects and limits wheel spin from side to side up to 45 mph. Audi brags that with the current quattro system, its cars can get underway with traction at only one wheel. Audi revised the suspension of the S4 with extensive use of aluminum. Through the magic of geometry, the patented four-link front suspension sharpens steering response. All the control arms of the front suspension, the ball joints, front suspension wheel carriers and double wishbone rear suspension are made of forged aluminum. The lighter suspension pieces mean a smoother ride and improved tracking of wheels over bumpy pavement for better grip. The sport suspension tuning lowers the S4 and includes firmer springs and shock absorbers. The S4 sports new brakes, with massive 16-inch discs with 4-piston 4-pad calipers inside 17-inch Avus alloy wheels mounted with 225/45 performance radials. The S4 doesn't shout its presence (unless one chooses the Imola Yellow or Nogaro Blue pearl effect paint). Visual differences from the A4 models are limited to the wheels and tires, larger air intakes at the front of the car, standard xenon headlamps with clear glass covers and subtle badging.
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