BMW X5 4.6is

BMW X5 4.6is BMW X5 4.6is
Short Take Road Test

As far as we're concerned, the base BMW X5 wedges its figurative boot into the fanny of the base Mercedes-Benz M-class. Certainly it does so in the three categories we favor: going, turning, and stopping. It looks better, too.

So why does the X5 even need a special model that features a supersize 340-hp, 4.6-liter V-8, plastic wind foils, and 20-inch wheels with openings big enough to swap the brake pads through. The answer is the existence of the Mercedes-Benz ML55 AMG, an itch in BMW's Y-fronts since it hit the market late in 1999.

Competitive antagonism curdles the air between Munich and Stuttgart, and until now the ML55 has been the top turkey among off-roaders that shed any pretense of being useful off-road. However, that changed when the first examples of the 4.6is reached dealers last December.

With a 6.5-second run to 60 mph, the $66,845 base price of the X5 4.6is (a $15,200 premium over a 4.4i with a Sport package) buys you 0.3 second more of leisure time than you get with a $66,545 ML55. The BMW also trims 0.2 second off the ML's quarter-mile with a 15.1-second warp at 92 mph and sticks to the skidpad against 0.84 g of vigorous lateral force, a full 10th of a g better than the ML.

Some credit goes to the fleshy 315/35WR-20 Michelin Diamaris tires (a European Michelin model making its U.S. debut) that make the 4.6is look as if it sat on a Viper. The front wheels wear 275/40WR-20s and pack 14-inch rotors that appear capable of receiving the Minsk morning show. Just 164 feet are required to bring this 5127-pound 4.6is to rest from 70 mph, beating our one-ton-lighter long-term 1999 Porsche 911 Carrera by four feet.

BMW considered using the M5's 394-hp S62 V-8 but didn't because of its peaky torque delivery and the fact that no stock slushboxes will handle the power. Hence, no M badge, either. Instead, a new 4.6-liter V-8 was engineered with some inspiration from Alpina, BMW's factory-anointed aftermarket tuner in Buchloe, Germany.

Like Alpina's own 4.6-liter B10, the factory's DOHC 32-valve variant is a riff on the current M62 V-8 found in the X5 4.4i and 540i. The crucial difference is that each pot now displaces 577cc instead of 550cc, due to 1.0mm more of bore and 2.3mm more of stroke. A compression-ratio increase of 0.3 to 10.5:1, a looser exhaust trimmed with chrome hoods, and lumpier intake cams backed up by a more aggressive mapping of the engine's cam-phasing mechanism account for a rating of 340 horsepower, 50 more than in the 2002 X5 4.4i, which went to 290 from 282 horsepower last year.

The crankshaft also turns another 26 pound-feet, for 350 total. That and the lower 3.91 rear-axle ratio help the 4.6is lop almost a half-second off the 0-to-60-mph sprint of the 282-hp 4.4i. Expect a gluttonous 14 mpg.

Running fast toward the horizon with enough steel around you to patch up the sunken Edmund Fitzgerald is the real reason to acquire a 4.6is. As for turning, the 36 feet of skeletal seam welding contribute to a chassis that feels rigid and stable. But the higher center of gravity and the distances separating driver, roll center, and road mean the 4.6is becomes uncertain above brisk touring speeds.

The mass and the 38 percent of engine torque on the front tires urge them to plow forward, and the rear end only rotates if you whisper a Hail Mary and turn down—it won't completely turn off—the stability control. Suspiciously, BMW forbade journalists from doing that while they were in their company, perhaps concerned that a two-and-a-half-ton truck in power oversteer might overtax the average driver's abilities.

Even with its standard X5 Sport package suspension, which includes thicker anti-roll bars and stiffer springs and shocks that keep the body composed in turns (but with the penalty of a concussive ride over rotten surfaces), the 4.6is still doesn't make you feel as though you're in a sports sedan. More as if you're sitting on the roof of one.

If, like us, you prefer to actually sit inside a car, buy a 540i wagon with the Sport package, save eight grand, haul more stuff in the larger interior, and kick everyone's fanny.