The only missing model is a big coupe, something to compete with the likes of the BMW 3- and 6-series and the Mercedes CLK- and CL-classes. Presto: With the 2008 S5?Audi's first coupe with adult-size rear seats since the demise of the Coupe Quattro in 1991?that hole in its lineup has now been plugged.

And it has been plugged by a machine that delivers the sex appeal that is a coupe's major reason for being. The S5's combination of big grille, rakish roofline, and artfully sculpted character lines will turn heads with regularity. And inside, the S5 provides perhaps the most engaging experience since the original TT.

Our test car was finished in black and deep-red leather and brushed aluminum, and in the current world of mostly monochromatic interiors, the effect is striking and luxurious. A new dashboard arrays all the controls and displays toward the driver, enhancing comfort and usability. Details like the metallic vents and the subtle red stitching on the black leather shift boot and steering wheel enhance the posh atmosphere. Even Audi's control-everything MMI knob falls readily to hand and eye.

The S5's exterior appeal is more than skin-deep. The keen observer will notice that the front wheels seem closer to the car's nose than on most other Audis. This reduced overhang not only enhances the S5's looks but also reflects some major changes under the coupe's sexy skin.

The S5 is the first Audi to arise from the company's B8 architecture, which will also underpin the next-gen A4?scheduled to appear at the Frankfurt auto show in September'as well as future A6 and even A8 models. Besides the usual improvements in structural stiffness and crashworthiness, there are two major areas where the B8 differs from the current A4/A6 platform. That reduced overhang at the nose reflects a transaxle whose front differential is about six inches farther forward than in other Audis. The engine is still completely ahead of the axle, but only just. The front differential is now adjacent to the transmission bell housing (on the right), and the cross-shaft that feeds the left half-shaft barely clears the flywheel. In fact, the clutch is attached to a second flywheel behind this cross-shaft.

The second key change is the migration of the steering rack from behind the engine to a more conventional position below it, which helps lower the car's center of gravity. Otherwise, the front suspension is a sophisticated aluminum control-arm layout with two separate links forming the arms to achieve superior geometry. The rear wheels are attached to a multilink suspension that's similar to what is used on current A4s and A6s.

The S5 is the first Audi to arise from the company's B8 architecture, which will also underpin the next-gen A4?scheduled to appear at the Frankfurt auto show in September'as well as future A6 and even A8 models. Besides the usual improvements in structural stiffness and crashworthiness, there are two major areas where the B8 differs from the current A4/A6 platform. That reduced overhang at the nose reflects a transaxle whose front differential is about six inches farther forward than in other Audis. The engine is still completely ahead of the axle, but only just. The front differential is now adjacent to the transmission bell housing (on the right), and the cross-shaft that feeds the left half-shaft barely clears the flywheel. In fact, the clutch is attached to a second flywheel behind this cross-shaft.

The second key change is the migration of the steering rack from behind the engine to a more conventional position below it, which helps lower the car's center of gravity. Otherwise, the front suspension is a sophisticated aluminum control-arm layout with two separate links forming the arms to achieve superior geometry. The rear wheels are attached to a multilink suspension that's similar to what is used on current A4s and A6s.

The improvement over the old S4 is not, however, as pronounced as we would have expected from the four-percentage-point improvement in weight balance?although that reflects how well the S4 performed. In driving an S4 and the S5 back-to-back, the coupe turns in a bit more sharply and accurately, but even the S4 has decent cornering balance. It likely requires the elevated speeds of a visit to the racetrack to find the benefits of the new architecture.

What is immediately apparent is the tighter structure of the S5. It's rock solid, and even on our fractured Michigan roads, the suspension keeps body motions under control and the tires tightly pressed to the pavement at triple-digit speeds. We would prefer that the steering didn't become unnaturally stiff above 80 mph, and the 35-series, 19-inch tires don't have much compliance on rough roads. But the S5 is so refined and effortless that it's easy to lose sight of just how fast it is.

Motivated by the latest version of Audi's 4.2-liter direct-injection V-8, rated at 354 horsepower in this application, the S5 is a rocket, hitting 60 mph in 4.8 seconds and ripping through the quarter-mile in 13.4 seconds at 105 mph. That performance is a testament to the power of the V-8, the short gearing of the powertrain (75 mph in sixth gear has the engine spinning at 3000 rpm), and the quick shifting of the six-speed manual transmission.

But even those figures don't reveal the silkiness and captivating appeal of the S5's powertrain. When you drive with the windows down around town, you hear a terrific V-8 burble from the quad exhaust pipes. When feeling lazy, you can almost forget about shifting because the engine will take full throttle at 1000 rpm in sixth gear perfectly smoothly. And at any speed, the V-8 spins with an eager and refined hum.