At Schwalbenschwanz, a horseshoe-shaped left-hander on the Nurburgring Nordschleife, probably named for a machine that suctions vomit off upholstery, it begins to rain. Hans Stuck just stomps the accelerator.

BMW has hired the stringy-haired, perpetually smiling German hotshoe to ferry writers around the 12.9-mile, 73-turn Nordschleife in a one-of-a-kind X5 sport-ute fitted with a V-12 engine from one of BMW's Le Mans-winning sports-prototype cars. Stuck, 50, himself a two-time winner at the Circuit de la Sarthe and a starter in 74 Grands Prix, is determined to hit 170 mph on the Nordschleife's looming straightaway, rain be damned. With the engine shrieking at 8000 rpm in fifth, Stuck calmly points to the speedometer and says, "Look -- 170, ja?"

Ja, indeed. The X5 LM ranks right up there with the ill-considered purchase of Rover as evidence that the boys in Bavaria have gone completely meshuga. Last spring, they plucked an X5 off the South Carolina assembly line and brought it back to the mothership in Munich. A 15-member surgical team was waiting there with one of BMW's cast-alloy, 6.0-liter V-12s selected from the stable of engines used to assault and conquer the 1999 24 Hours of Le Mans.

The compact 60-degree DOHC unit is narrower and shorter than the X5's stock 4.4-liter, 90-degree V-8, so it settled in without much persuasion. However, the V-12 weighs 176 more pounds than the V-8. It also needs 50 percent more cooling area, so the surgeons cut gaping holes in the bumper and hood to move cold air in and hot air out. Pending news that oxides of nitrogen are actually good for us, the exhaust is pure, untreated toxins.

It could have been worse. The original plan was to shove in a V-10 from Ralf Schumacher's Williams-BMW F1 car. But none of BMW's conventional transmissions can handle an 18,000-rpm redline, and X5 project leader Edward Walek wanted to keep the driveline and body as close to stock as possible.

"We wanted to find out what the absolute limitations are of the basic X5," Walek says. The other reason: Get people accustomed to the idea of a BMW super-ute. BMW will begin building a hot version of the 4900-pound X5 within 18 months. Prototypes dubbed "X5 HP" were on hand at the Nordschleife featuring the BMW V-8 bored and stroked a few millimeters to 4.6 liters and mated to a recalibrated GM five-speed automatic. A short test drive on public roads revealed some electrifying thrust from the 350-hp V-8. Expect runs to 60 mph in less than six seconds and a top speed of about 150 mph.

Meanwhile, the horribly mutated X5 LM is capable of reaching 186 mph, owing to a V-12 that makes 700 hp at 8000 rpm. That's about 200 more horsepower than the Le Mans car, because the X5 breathes through twin 80-millimeter conduits. (At Le Mans, the engines were restricted to 32mm apertures.) The power is transmitted via a twin-plate racing clutch to a ZF six-speed manual from an old 850CSi coupe. The gearbox is apparently hardy enough to stand up to the engine's 531 pound-feet of torque, but not in first or second gears, which are only allowed to see part throttle. The stock transfer case preserves the X5's torque split of 38 percent up front and 62 to the rear.

Apart from the engine and transmission, the rest of the X5 LM is largely stock. It squats lower by 1.9 inches in the front and 1.6 inches in the rear, sports firmer springs and bushings, and wears 20-inch wheels veneered with Euro-market 275/35ZR-20 Michelin tires in the front, 315/35ZR-20s behind. ABS is gone, as is the hill-descent feature used to give ordinary X5s a dram of off-road capability.

Fitted with slicks, the X5 LM did lap the Nordschleife in a blistering eight minutes, five seconds -- 15 seconds faster than the factory's Z8. On this soggy day, however, Stuck was sandbagging to preserve the car for all 14 scheduled laps (the last time this was tried, the X5 blew up after only two). He kept the rampant understeer in check by expertly playing the throttle and steering wheel against each other with insect-quick movements, and a few sideways slides kept the adrenaline flowing for his passenger. After 14 laps, the X5 was still running. And they didn't even need a Schwalbenschwanz.