An "S" on the rear end of an Audi signifies a pumped-up version of a given model. The 2007 Audi S4, packing a 340-horsepower V8 within its relatively petite dimensions, shows that the gentlemen from Ingolstadt mean business in the compact sport sedan segment. Aimed at the BMW M3, and to a lesser degree, the considerably more costly Mercedes-Benz C55, the 2007 S4 is a serious enthusiast's machine.Somehow, Audi has managed to shoehorn its 4.2-liter V8 from the A6 and A8 into an engine bay best served by a 4-cylinder -- and cramped even with a 6. And "shoehorned" is not the least bit exaggerated. Audi literally had to redesign the engine to shorten it some 2 inches; chains mounted in the rear now drive camshafts once run by a forward-facing rubber belt. Both the oil and water pumps have been relocated lower and to the side. Total output for this normally aspirated V8 is an impressive 340 horsepower and 302 pound-feet of torque, and Audi fills out the car's abilities with a sport-tuned suspension, big wheels, sticky tires and more powerful brakes.In addition, the S4 also offers a few things the competition doesn't: all-wheel drive (AWD) and a variety of body styles to choose from. The benefits of the increased grip that Quattro provides on slippery roads won't be lost on enthusiasts who live in rainy or snowy climates. In addition to the expected sedan body style, Audi offers convertible and wagon variants of the S4. Buyers also have a choice of transmissions, manual or automatic, whereas some rivals offer an automatic only.Apart from the performance potential, the Audi S4 maintains the qualities we've admired for a long time in Audi's jewel of a small car -- a stylish and functional cabin, very supportive seats (in this case, Recaro sport seats) and excellent build quality. On the periphery are competitors such as the Cadillac CTS-V (which is fast but lacks the nimble feel of the smaller Audi) and the Volvo S60 R (which offers sedan and wagon body styles with AWD, but not the same level of synergy between car and driver). Compared to its closest peer, the M3, the Audi is not quite as visceral, but in the real world of rush-hour traffic, potholes and precipitation, the S4 looms as the more practical choice for a compact super sport sedan.

The 2007 Audi S4 is available as a sedan, wagon ("Avant") or convertible ("Cabriolet") in one well-equipped trim level. Standard features include 18-inch wheels and tires, sunroof, xenon HID headlights, 12-way power-adjustable Recaro seats, Silk napa leather upholstery, dual-zone automatic climate control, heated mirrors and a 10-speaker audio system with an in-dash CD changer. The updated-for-'07 Cabriolet is equipped with more conventional leather-upholstered sport seats in place of the Recaros, along with a power-operated "Acoustic" soft top that promises a quieter cabin, especially at freeway speeds. Options for the S4 include an upgraded 215-watt Bose stereo, a navigation system and, for the sedan only, a "DTM" exterior package, which adds unique front and rear fascias, multispoke wheels and carbon-fiber spoilers.

All Audi S4s are fitted with a 4.2-liter 340-horsepower V8 that sends the power through either a 6-speed manual gearbox or a 6-speed Tiptronic automatic. The latter transmission allows manual-style shifting via steering wheel-mounted paddles. The Quattro all-wheel-drive system, which is standard on the S4, is unlike most other Audi Quattro offerings. It has a 40/60 front-to-rear power split in normal situations to give the car a more dynamic attitude when cornering. Audi claims that an S4 sedan equipped with the manual tranny can sprint from zero to 60 mph in 5.3 seconds.

Stability control, antilock disc brakes (with brake assist and EBD) and a multitude of airbags (including front seat-mounted side and full-length side curtain) are standard on the S4. Rear side airbags are optional. In NHTSA crash-testing, the A4 sedan (on which the S4 is based) received four stars (out of five) for frontal crash tests, while side-impact testing resulted in five stars for the front seat and four stars for the rear seat. In IIHS tests, the Audi garnered ratings of "Good" (the highest possible) for both frontal-offset and side-impact categories.

As one would expect with a 340-horse V8, performance is thrilling, and on winding back roads, the S4's top-notch underpinnings result in excellent handling while still providing a supple ride. While the Audi S4 isn't quite as razor-sharp as BMW's M3, the extra grip afforded by its all-wheel drive could justify the tradeoff for many enthusiasts, particularly those who live in parts of the country where rain- or snow-slicked roads are a part of life.

Audi interiors are some of the finest in the luxury car arena; one look at the S4's cabin and the high quality of the materials used is quickly apparent. Tasteful accents abound, such as brushed aluminum trim and gray instrument facings. The S4's serious performance intentions are reinforced by standard Recaro front seats in the sedan and wagon, which feature aggressive bolstering to hold one in place during spirited runs through the twisties. The new S4 Cabriolet features an optional "Acoustic Top" that has more sound insulation than the prior soft top and thus promises a quieter ride, especially at freeway speeds. Although the rear seat is fine for small children, adults seated back there will likely complain about a shortage of legroom. The Avant wagon provides up to 61 cubic feet of cargo space with the rear seats folded.