At 20,000 euro, or nearly $25,600, the price for MTM's K540 supercharger conversion may seem a bit steep for most RS4 owners. This is why MTM boss, Roland Mayer, does not expect to sell more than a handful. In the context of the $130,000 Audi R8, however, the tag seems more reasonable, giving Audi's new supercar the kind of grunt that will fend off advances from pushy Ferrari and Porsche drivers. "I fully expect the greatest response to come from R8 owners, and that is the car we will be concentrating on with this supercharger conversion," says Mayer.

Given that the V10-powered R8 is expected to follow a few months behind the V8 model, MTM has a good chance of fulfilling this prophecy. And for some, the fact that the lighter and more compact V8 makes a nicer noise while promising better handling balance makes it far more than just the entry-level model.

Even in standard form, the RS4 is nothing short of inspirational-one of our all-time favorite performance cars. The idea of giving it another 120 bhp and 96 lb-ft of torque is tremendously appealing. MTM offers more than one flavor of RS4. The example with a sport exhaust, uprated chassis and lightweight carbon fiber bonnet is just a bit harder-edged than the factory car. And then there's this.

The engine conversion uses a variation of the ultra-efficient Swedish-made Lysholm Technologies Twin Screw Compressor. Boosting at just 5.8 psi, this is a bolt-on conversion that does not require the engine's compression ratio to be lowered. Cooling arrangements are similarly efficient with a Swedish-made Laminova intercooler providing the charge-air temperature control. This is a self-contained water-cooled system built into the intake manifold between the supercharger and cylinder heads. This configuration's advantage is in an extremely short flow length with no long pipes, so the chance of pressure drop and loss of charge pressure are minimized. Importantly, mounting the kit is also less of a challenge in an engine bay already short on space.

One of the benefits of the Opcon/Lysholm supercharger is its low mechanical drag. This means both the sharp throttle response and free-revving character of the RS4's engine remain uncorrupted, its explosive power delivery simply enhanced. It is easy to make a tuned engine go well on full throttle, but the trick is to make it civilized in all other situations, especially on part throttle or in heavy traffic. A drive along the country roads from MTM's Wettstetten base to the nearest autobahn is useful for assessing a car's behavior. While carefully warming vital fluids, the engine's smooth running is impressive. It sounds strong, with a deep, menacing V8 rumble from the four 3-inch outlets of its free-flow sport exhaust. No sign of hunting or misbehavior beneath the vented carbon fiber hood.

Joining an autobahn is always easier with a fast car and a heavy right foot. Holding the accelerator to the carpet in third and fourth gears provides the sort of manic acceleration you would expect from a major-league supercar. Although the RS4 weighs a hefty 3600 pounds, the mountain of torque from the blown motor feels fully capable of upending physical laws. The only problem with this car is that it's addictive.

MTM's claimed performance numbers are 3.9 seconds for the 0-100 kph (0-62 mph) sprint, 0-200 kph in 11.3 seconds and a top speed of 320 kph (199 mph), junking the speed limiter. That's close enough to 200 mph, making this the fastest street-legal four-door Audi currently in production. The rest of the car can be beefed up by MTM as much or as little as the customer desires. Many will go for the complete conversion (MTM's race suspension kit replaces the factory springs and dampers), even though the standard RS4's brakes and handling are superb.

Developed at Hockenheim and the Nürburgring with KW, MTM's fully adjustable system uses dampers with race-style external fluid reservoirs. For road use, the wishbone bushings remain stock. But if you are a dedicated track-day enthusiast, these can be replaced with uniball joints. In either case, new top mounts are used that allow more negative camber to be dialed in.For normal road use, MTM suggests an inch drop from standard ride height. For dedicated track use, the maximum 2.4-inch (60mm) drop is optimum. Damper tubes are specially designed with a shorter housing so that spring travel is compromised less, even with the suspension at its the lowest setting. With ride height set an inch lower and the dampers backed off to 'fast road' position, secondary ride at low speeds is still impressive. It is firmer than stock, but smoothes off bumps nicely. At high speeds, body control is noticeably better. With the lower ride height and tighter damper rebound control, body roll through fast corners is minimized. Importantly, the tendency for the standard car's tail to weave slightly under hard braking is banished.

MTM's massive 15-inch cross-drilled vented floating rotors and enormous eight-pot calipers replace the 14.2-inch Audi front brakes, while 12.6-inch rotors with four-pot calipers replace the factory 12.3-inch rear discs and provide balanced retardation. MTM Bimoto alloys (9.5x19 with 265/30 Michelin Pilot Sport Cup tires), using spacers to achieve the correct offset, create both a slightly wider track and significantly more mechanical grip.With the 40/60 torque split dialed into the RS4's Quattro-equipped chassis and the sticky tires clinging to the tarmac, it is easy to believe this car will never run out of grip, at least on a dry road. It may not hang out like an M3, but with so much power and torque, regaining speed after slowing for a bend is easy. And since the experience of accelerating is so much fun, you can sometimes find yourself slowing down just to accelerate again for the sheer hell of it.

In the early '80s, when the only superchargers used by European tuners were heavy, inefficient and noisy Roots-type units, a blower conversion was a compromise. Now superchargers are lighter, more efficient, noise-free and, thanks to advanced electronics, seamless in the way they integrate with other systems. MTM's K540 conversion is so good, it's difficult to tell while driving normally (apart from the rorty exhaust) whetheranything has been done to the engine. But once the open road beckons, it's a whole other story.