By John Matras

Audi's A4 is a svelte and sophisticated sports sedan with power and handling to make quick work of most twisty roads. But for some die-hard performance enthusiasts, the basic version is not enough.

Enter the S4, a bulked-up bruiser equipped to take on all comers. Armed with a twin-turbo version of the 2.8-liter V6 used in the larger A6, plus a stiffer suspension and bigger brakes, the S4 is a world-class athlete. Yet true to its German roots, styling cues are subtle, meaning this sedan can go about its business quietly until its driver is ready to rumble.

Model Lineup

Audi S4 comes as either a sedan, or, new for 2001, an S4 Avant wagon model priced at. Both versions are available with a choice of transmissions: 6-speed manual or, as a no-cost option, 5-speed Tiptronic automatic.

All S4s come with Audi's quattro all-wheel-drive system, a 250-horsepower twin-turbo V6, massive 225/45R17 tires and leather upholstery.

Walkaround

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.

The S4 doesn't shout its presence unless one chooses the Imola Yellow or Nogaro Blue pearl effect paint. (The Nogaro blue paint with some guy standing in the picture is shown above). Visual differences from the A4 models are limited to the wheels and tires, larger air intakes at the front of the car, and xenon headlamps with clear glass covers. Aluminum mirror housings are optional. S4 Avant gets aluminum roof rails.

Interior Features

The S4 interior shares its interior design with the rest of the A4 line, which is to say the days of dark and foreboding German interiors are over. Buyers can choose from a variety of interior treatments called "Atmospheres." Our test S4 had faux carbon fiber accents, which look right at home in this high-performance sedan.

Special to the S4 are sport seats with deep side bolsters designed to keep driver and passenger in place during vigorous driving. The seats are multi-adjustable and everyone should be able to get comfortable. Pearl nappa leather covers seats, armrests, and door panel inserts. The leather is available in either a dark onyx or silver; Alcantara suede inserts are available at no extra cost in either silver or a striking blue.

Audi paid particular attention to the driver's office, with large round easy-to-read instruments under a blister on the dash. The steering wheel is contoured with thumb grips at the proper nine and three o'clock positions, but the surface is harder and slipperier than we'd like.

The back seat is comfortable. This is a compact car, however, so rear legroom is more limited than in bigger sedans. Although there are three seatbelts in back, three adults will find shoulder room inadequate. This is a four-grownup car.

The S4 Avant offers 63.7 cubic feet of storage space with the rear seats folded down.

Driving Impressions

Until you get aggressive with the throttle, you'd never know this was such a serious machine. The S4 idles quietly and smoothly, making casual around-town driving a cinch. But give it some gas and the Audi surges ahead, even with revs as low as 2000 rpm. There's a solid push throughout the rev range, with neither turbo lag nor turbo whistle, just a muted throaty roar from the V6. Motorheads who are most likely to buy this car might actually wish the engine was a bit louder under full throttle. The twin-turbo makes quick work out of passing on a two-lane road and it catapults the car out of corners. Without attention to the speedometer, one could easily become radar fodder. In this case, you'll appreciate that conservative exterior.

Like all Audi engines sold in the U.S., the S4's cylinder heads have five-valve-per-cylinder architecture for superior breathing. But the S4 adds two small turbos, which can deliver the same boost as one big turbocharger because, with less mass, they spool up faster. The smaller twin turbos virtually eliminate the turbo lag (delay in throttle response) that's 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 liters 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.

Although the S4 is available with a 5-speed Tiptronic automatic, we drove only the 6-speed manual transmission. Some 6-speed gearboxes are difficult to use, but there was no such problem with the S4. The only possible complaint is that the fore-and-aft shift throws are somewhat long. Clutch engagement and disengagement are abrupt, not surprising given the amount of power available. Also, as on most Audis, we found the initial brake-pedal action to be extremely sensitive. Practice will be required for smoothness.

Suspension is pleasantly firm. The S4 has the kind of ride that traditional luxury car buyers would call hard. Compared to the A6 2.7 T, with which the S4 shares its engine, the S4 rides much more firmly. Road seams, blemishes and flaws that the A6 would absorb, the S4 telegraphs to the occupants. On an undulating road the S4 feels like a speedboat on a light chop. The low-profile 17-inch tires relay a lot of road noise from coarse pavement, seams and bots dots.

The benefit comes in the crispness with which the S4 attacks corners. The steering feel has a nice Germanic weight to it, providing precise, one-to-one correspondence with the road. The S4 rotates well under hard cornering. Where other cars would drift wide through a corner, the S4 will continue to turn, thanks to the tremendous grip offered by its quattro four-wheel drive. Enthusiasts will want to arrange some track time.

Audi revised the suspension of the S4 with extensive use of aluminum. The lighter suspension pieces mean a smoother ride and improved tracking of wheels over bumpy pavement for better grip. Furthermore, revised geometry sharpens steering response. The sports suspension tuning lowers the S4 and includes firmer springs and shock absorbers. The S4 also boasts race-track-worthy brakes, with massive 16-inch discs with 4-piston 4-pad calipers inside 17-inch Avus alloy wheels mounted with 225/45 performance radials.

Audi's quattro system, celebrating its 20th anniversary this year, 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.

New for 2001 as standard on the S4 is Audi's Electronic Stabilization Program. ESP monitors driver inputs and vehicle dynamics and intervenes if something goes seriously wrong by using the anti-lock brake system and traction control to keep the car pointed straight. While such a system can't defeat the laws of physics, it comes pretty close. If the driver of the S4 gets carried away and overcooks it into a corner, ESP can help.

A racetrack would be necessary to fully test the massive brakes on the S4. We were unable to elicit any fade from the four big stoppers even when making repeated stops from high speed. If anything, they improved with a little heat in them. We suspect that even track testing would find these brakes unflappable.

Final Word

This car is clearly intended for enthusiasts. The $7000 price premium of an S4 over an A4 2.8 would not return equal extra enjoyment to someone not interested in driving aggressively. In fact, the harder ride would probably make the S4 less rewarding. To get the most out of this car, you have to be willing to use it. This machine is not for the faint of heart.

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