If you're looking for a rational reason to buy a 2011 BMW X6 M, you won't find one here. That's because there isn't one, as the bonkers-performance version of the BMW X6 crossover takes rational car-buying, sticks it in a burlap sack, beats it with crowbars and tosses it off a 14-story bridge into the gaping maws of ill-tempered sharks. This high-performance crossover is not practical transportation but instead an automotive novelty item to add to your eight-car garage.

That said, the 2011 BMW X6 M represents an impressive feat of automotive engineering. Like the X6, this is a coupe-style crossover, meant to carry passengers rather than cargo. It weighs nearly 5,300 pounds, yet the X6 M motivates the mass with a 555-horsepower twin-turbo V8, so 60 mph arrives in a mind-blowing 4.3 seconds. That's quicker than every SUV we've ever tested (the new 2011 Porsche Cayenne Turbo will likely be very close) and quicker than quite a few dedicated high-performance cars -- including those from BMW.

This crossover from the BMW M division is more than just quick in a straight line. This hatchback utility vehicle employs every technological and suspension-tuning weapon in BMW's extensive arsenal to produce such responsive handling that you'll never give a thought to its weight. Of course, a tight road will quickly instill a sense of paint-scraping anxiety as the bulky X6 passes perilously close to shrubs and canyon walls, but the capability for extreme cornering is there.

All things considered, we can't get around the sheer nonsense of the 2011 BMW X6 M. It seats only four people, and those in back must contend with limited headroom and a backrest that doesn't recline. Its cargo capacity is far less than the mechanically similar 2011 BMW X5 M. It might cost less than a 2011 Porsche Cayenne Turbo, but at nearly $100,000, there are a lot of sports cars and high-performance sedans that generate as many thrills.

So there's no rational reason to buy a 2011 BMW X6 M, but should you have an eight-car garage and be willing to toss rationality to the sharks, there's rarely been a more thrilling case of engineering overkill to enjoy.

The 2011 BMW X6 M is a four-passenger SUV available in one trim level, although there are less performance-oriented X6 models available.

Standard equipment includes 20-inch alloy wheels, a self-leveling sport-tuned suspension, Adaptive Drive (includes electronic damping control and active roll stabilization), hill descent control, adaptive xenon headlights with auto-leveling and washers, auto-dimming mirrors, automatic wipers, a power tailgate, a sunroof, front and rear parking sensors, heated 10-way power front seats with four-way lumbar and driver memory settings, a power tilt-and-telescoping steering wheel, Bluetooth, BMW Assist emergency telematics, a navigation system with real-time traffic and voice controls, BMW's iDrive electronics interface, and a 16-speaker sound system with CD player, auxiliary audio jack and HD radio.

The Driver Assistance package adds a rearview camera, a top-view camera, sideview cameras, automatic high beams and a head-up display (available as a stand-alone option). The Cold Weather package adds a heated steering wheel, heated rear seats and a rear seat pass-through ski bag. The Active Ventilated Seat package adds ventilated 14-way power front seats with adjustable and active side bolsters. The Premium Sound package adds an enhanced sound system, satellite radio and an iPod/USB adapter (the latter two items can be had as stand-alone items). The Rear Climate package adds four-zone climate control and manual rear side window shades.

Other options include keyless ignition/entry, soft-close automatic doors, a cross-traffic sideview camera, a rear-seat entertainment system, a six-DVD changer and extensive leather upholstery.

The 2011 BMW X6 M is powered by a twin-turbo 4.4-liter V8 that produces 555 hp and 501 pound-feet of torque that is sent to all four wheels. A six-speed automatic with manual shift paddles is the only available transmission.

In performance testing, the BMW X6 M reaches 60 mph from a standstill in just 4.3 seconds. This makes it not only the quickest SUV we've ever tested, but also quicker than a BMW M3. The BMW X6 M can also tow 6,600 pounds. The trade-off for so much power is fuel economy, and the X6 M has an EPA-estimated fuel economy rating of 12 mpg city/17 mpg highway and 14 mpg combined.

The 2011 BMW X6 M comes standard with ABS (with brake assist), stability control, front seat side-impact airbags, side curtain airbags and whiplash-reducing front head restraints. In Edmunds brake testing, the X6 M came to a stop from 60 mph in an exceptionally short 107 feet.

Crash tests have not been conducted, but the structurally similar BMW X5 scored a perfect five stars in government crash tests for frontal-impact driver protection, four stars for frontal-impact passenger protection and five stars for front and rear side protection. Also, the Insurance Institute for Highway Safety gave the X5 the highest rating of "Good" in its frontal-offset and side-impact crash tests.

The 2011 BMW X6 M weighs 5,256 pounds. It's also a crossover SUV, so its center of gravity isn't exactly like a sports car's. So how does this beast manage to handle like a sports car? Technology, lots of technology.

There's "Active" this and "Dynamic" that in just about every area of the X6 M's operation, various high-tech electronic aids designed to keep the laws of physics at bay. Our seasoned professional test-drivers are amazed at this massive BMW's athletic character. On a tight road, however, no amount of athleticism will be able to hide just how big the X6 M is.

Then there's the power. Turbo lag is nonexistent and acceleration is at once awe-inspiring and giggle-inducing. Press the M button on the steering wheel and you can program the throttle to become more responsive and the transmission to shift more rapidly. Even the exhaust can be set to let out a gnarly bark as it shifts at full throttle. The X6 M might not be a classic performance machine, but it's certainly a riot.

Like the regular X6, the M version can seat only four people, and its maximum cargo capacity of 60 cubic feet is smaller than many compact crossovers. As such, you get all the weight and size of a big SUV with only a portion of the utility.

Of course, take away the "M" badges sprinkled liberally throughout the X6 M's interior as well as the special M sport seats, and you've got the same excellent interior that comes with the regular X6 crossover. The instrumentation is comprehensive and intuitive to understand, the materials are premium and the construction quality is superb. And as you'd expect, the M sport seats provide superb support and comfort.

The X6 M has BMW's latest iDrive interface, which includes more user-friendly physical buttons next to the control knob and a more sensible menu structure. It also has BMW's now-familiar joystick-like gear selector, which drivers may find confusing due to the separate button that engages Park.