In the Audi A8, a driver can use the Driver Information Display to set the optional Adaptive Cruise Control, which minds tailgating and maintains a safe, pre-determined distance to the car ahead. The Electronic Stabilization Program can help control the car when the driver can't. Electronic Brake-force Distribution keeps the car balanced in a panic stop, and Brake Assist slams the binders harder if the driver doesn't press as hard he or she should. Adaptive Air Suspension keeps the ride smooth and tires planted no matter the surface. There are moisture-sensing wipers, high-intensity headlamps and ten airbags. Yet all these advanced systems, identified by a confusing array of acronyms, don't mask one crucial point. The A8 can be a complete joy to drive, reminding all but the sensory deprived how pleasant gobbling miles in a big, fast luxury sedan can be.

The first impression at the wheel of an A8 is its smoothness. There's nothing remotely resembling a squeak or rattle, and almost no vibration in the cabin.

The 4.2-liter V8 delivers powerful acceleration, but its power delivery is sophisticated, not crude. The V8 responds with a muted roar to every poke at the gas pedal. No matter how fast the A8 4.2 is already going, the driver can tap into a deep well of acceleration-producing torque. Audi claims the 350-hp A8 4.2 can sprint from 0 to 60 mph in just 5.9 seconds. A sub-six-second 0-60 is quick. Top speed is electronically governed at 130 mph. In short, the A8 is fast traffic. The base engine is also the easiest on gas, with EPA fuel economy numbers of 16 mpg City and 23 mpg Highway. Those numbers aren't bad for a big, V8-powered luxury car, and they're bested only by the base engine in the Jaguar XJ.

The 6.0-liter W12, rated at 450 horsepower, is quite remarkable. The A8 L W12 is a blast to gas. Throttle response is immediate, and the 12-cylinder engine delivers acceleration-producing torque in a wide, flexible power band befitting a luxury carmaker's flagship sedan. The W12 pulls hard up to its 6200-rpm power peak, and it feels like there's still more power coming when it hits the rev limiter. Moreover, the revs translate to executive-class thrust. Audi reports a 0-60 mph time of 5.0 seconds, very quick, indeed, with top speed governed at 130. The W12 is remarkably refined, docile, and tractable, particularly given 450 horsepower, but it does have a hint of an overly sensitive throttle so you might have to recalibrate your foot to avoid lurching from a standstill.

The six-speed automatic that comes with either engine shifts up or down according to the driver's wishes, deftly sensing how quickly and how hard the throttle is mashed. The transmission features what Audi calls DSP (for Dynamic Shift Program), a form of smart software that selects from over 200 possible shifting programs to adjust to any individual's driving style. Upshifts are silky smooth in full automatic mode; in some instances, downshifts could come quicker, but the reserve of torque in either engine more than compensates for any shift lag. The automatic features Porsche's Tiptronic system, allowing the driver to slide it into a manually controlled mode and use either the shifter or available steering wheel paddles to choose gears. Manual shifting is never necessary, because the transmission is quite responsive in the automatic mode, but it can be fun. Here, however, we lodge a small complaint. Even in manual mode, the transmission will shift up at high rpm, rather than holding the selected gear, which seems to defeat the purpose of giving the driver manual control, but it's a good thing if you forget to shift. If you want an even sportier shift program but don't want to select the gears yourself, you can put the gearshift in S. This mode holds gears longer and downshifts quicker to keep power more readily on tap. It also hurts fuel economy, so use it sparingly.

The quattro all-wheel-drive system offers excellent traction in slippery conditions, but also improves stability when cornering, whether under full-throttle acceleration or when the driver lifts off the gas suddenly in the middle of a turn. Quattro also eliminates torque steer, that pulling sensation on the steering wheel that powerful front-drive cars often exhibit under acceleration.

An adaptive air suspension is used at all four corners on the A8, and it's a lot more sophisticated than the rear air shocks that could be inflated on 1970-vintage American station wagons. Four settings are available, selected electronically with the MMI. There are genuine differences in ride and handling with the basic Comfort and Dynamic settings, but neither is uncomfortably firm nor disappointingly mushy.

In the Comfort mode, the A8 rides at the normal ride height (120 millimeters or 4.7 inches). Comfort might suggest a cushy, mushy ride, but that's not the case. Even on a narrow, undulating Kentucky backcountry road, we found the suspension well controlled with Comfort selected, yet still smooth, compliant and comfortable. Switching to the Dynamic mode lowers the suspension by 20 mm (about three-quarters of an inch). You might think Dynamic is buckboard firm, but we found it quite comfortable and compliant, though tuned for sporty handling and more aggressive driving. Both modes operate at all speeds, or you can switch to the Automatic mode. Here the system tailors the suspension damping to conditions and the way you're driving, automatically lowering the car at 75 mph. This is usually the best setting, as the system continuously matches the ride and handling to the situation, and does a good job of it. The ride is smooth and supple, without the slightest sensation of floating or wallowing. Lastly, there is the Lift mode, which raises the suspension 25 mm (about an inch) above the normal ride height. Lift is a good setting for gravel roads, snow, a nasty driveway or an abrupt transition, any situation that calls for a raised ride height. Exceed 62 mph in Lift mode and the suspension automatically lowers to the normal ride height.

Despite its length and substantial weight, the A8 is impressively agile, and bears up well under aggressive driving. The steering is sharp and precise, providing excellent communication between the tires and the driver. One key to this big sedan's excellent handling and ride quality is its rigid aluminum space frame. The frame resists flexing and lets the suspension do all the work, which is how it's supposed to be. That's why the A8 delivers such a nice balance of fine handling and ride comfort. Driven to the limit in a corner, it understeers a bit, tending to push toward the outside edge of the pavement. To counter this, the driver simply lifts a little from the throttle, and the front tucks in and tracks through the turn. It works beautifully.

Turn the wheel sharply at night and cornering lights automatically come on to illuminate the inside of the corner, a nice feature.

The brakes are easy to modulate for smooth stops, and powerful enough for repeated hard braking from high speeds without fading. Of course, the A8 has every imaginable braking feature, including Electronic Brake-force Distribution, Brake Assist and ABS. The A8 exhibits very little nose-dive during braking and absolutely no drama in the hardest stops. It simply stops straight and true, allowing the driver to maintain steering control in virtually all circumstances. In an emergency situation, just remember to stand on the brakes, don't relieve pedal pressure, and look and steer where you want to go.

The front and rear differential locks help assure stability even while turning under hard acceleration. An electronic stability program (ESP) compares vehicle behavior against driver input, and uses the antilock brakes and traction control to correct a skid or slide. Add quattro all-wheel drive, and the A8 will do everything physically possible to keep you heading where you want to go.

Audi Side Assist uses radar technology to scan the area next to and behind the car. If a vehicle occupies that space or is moving up rapidly, the system turns on yellow lights in the side mirrors to warn the driver. If the driver puts on his signal and a vehicle occupies that space, the lights flash. We found Audi Side Assist worked as advertised and, while subtle, the lights were visible even in bright sunlight.

Audi Lane Assist uses a camera mounted above the rearview mirror to monitor the road ahead. If the system detects that the vehicle is traveling above 40 mph and crosses a lane line without the use of a turn signal, it vibrates the steering wheel to warn the driver. The system can be set to three levels of sensitivity, one that triggers the warning when the vehicle is about to cross the lane line, one when the vehicle actually crosses the line, and one adaptive mode. We liked Audi's approach to this system. Other systems sound a warning that can become annoying because, in normal driving, lane lines are crossed on a regular basis, especially in curves. The vibrating steering wheel isn't as distracting, and the system can be shut off. In the most sensitive mode, however, we got false readings when the pavement changed, such as over bridges on the freeway.

The S8 packs a 40-valve V10 with 450 horsepower, the same output as the W12 but at a higher 7000 rpm. The S8 boasts 398 pound-feet of torque at 3500 rpm vs. the W12's 428 pound-feet at 4000-4700 rpm. Digest those numbers a minute and you'll see that the V10 is really the performance powerhouse in the A8 series, playing the rogue racer against the W12's executive express. The shorter wheelbase and 10-cylinder engine save weight, improving acceleration performance: Audi claims 0-60 mph in just 4.9 seconds, with top speed electronically limited to 155 mph. EPA estimated fuel consumption is the same as for the W12: 13/19 mpg City/Highway.

The S8 runs with the same six-speed automatic transmission as the A8 and A8 L W12, but it's programmed for more aggressive action. Paddle shifters on the steering wheel promise full manual control. The final drive ratio in the S8 is significantly shorter than in the other models (about 4.0:1 for the S8, vs. 3.3:1 for other A8s), for snappier acceleration in higher gears.

The S8's air suspension is adapted from the A8 Sport Package, but the bushings, shock absorbers and air springs are firmer, to reduce roll and pitch. Steering is variable-ratio as in the A8, but geared about 10 percent faster around the straight-ahead position. Ride height is the same as with the Sport suspension, reaching a minimum of 3.7 inches at freeway speeds. Brakes are vented discs, 15.2 inches in front and 13.2 inches in the rear; standard tires are 265/35 on 20-inch wheels. The Bosch 5.7 electronic stability control on the S8 is programmed to interfere later and more briefly, leaving more control in the hands of the driver. It also keeps the disc brakes dry by lightly applying them, just enough to generate heat but not enough for the driver to notice.

With all these modifications, it's no surprise that the S8 has sharper moves than the other models. Steering is quick and precise, and the sportier suspension bites better around turns. Still, this is a big car and the S8 is prone to understeer just like the other models. The S8's 20-inch wheels make the ride firmer, but not harsh. Even with the suspension in Sport mode, the S8 was comfortable on pockmarked Chicago streets. Discerning luxury buyers may find the ride too hard on occasion, but those with a sportier driving style will appreciate the S8's enhanced athleticism. Some of our test drivers find throttle response to be too sensitive, causing the car to lurch, but others felt it was just fine.