BMW used a lot of tricks to squeeze a lot of power out of a small package
This article is part of an IEEE Spectrum special report: Top 10 Tech Cars of 2010.
As engineering achievements go, the world’s fastest SUV might seem on a par with a hydroelectric stapler or a self-buttoning cardigan: interesting, but pointless. Ignoring the howls of automotive purists and Sierra Club donors, BMW has bestowed on us the X6 M. Like BMW’s M3 and M5 sedans, this offshoot of the standard X6 crossover wears the ”M” badge that denotes the company’s explosive high-performance division. And while the X6 M looks more like Batman’s assault vehicle than a traditional sports sedan, there’s no disputing the engineering heroics that let this 2380-kilogram (5247-pound), 408-kilowatt (547-horsepower) beast thumb its Bavarian nose at the laws of physics.
The BMW imperils highway laws as well, which made track testing both welcome and enlightening. Over two separate weeks with the X6 M, I was impressed with the BMW’s Jekyll-and-Hyde personality: one part fleet, serene cruiser with room for four adults and decent cargo space, despite its slope-roofed styling; one part roadway killer that needs the curves at Pennsylvania’s Pocono Raceway or New York’s Monticello Motor Club to achieve complete and blissful self-actualization.
Detailing the BMW’s myriad technologies would require a dissertation, but let’s start with the Cliffs Notes: Using the 300-kW (402-hp), all-wheel-drive X6 as a starting point, the mad scientists at the M division drew up a new, direct-injection V-8 with pricey twin-scroll turbochargers and a patented, world’s-first ”crossover” exhaust manifold. A raft of computerized handling aids—including a sophisticated active rear differential—work magic in extreme cornering maneuvers. An automated launch control unit catapults the BMW to 100 kilometers per hour (62 miles per hour) in 4.7 seconds, keeping up with a standard Corvette that weighs a ton less.
Twenty-inch wheels with ultrahigh-performance tires are grappled by shockingly large brake rotors—the ones up front measure 39.6 centimeters (15.6 inches)—whose two-piece composite design trims about a kilogram of critical unsprung weight from each wheel, boosting ride and handling. The body gets its own M additions to boost style, aerodynamics, and cooling, including front air inlets whose gaping nostrils seem straight off a Lamborghini.
The cabin of this ”sports activity coupe,” as BMW calls it, is equally chockablock with features. Driver-selectable performance settings adjust the engine, throttle, six-speed paddle-shifted automatic gearbox, and electronic suspension—the latter reading information from the road surface to adjust shocks at up to 400 times per second (really, couldn’t 350 have sufficed?). Amenities include a head-up display and a 16-speaker audio system.
The engine is a 7000-revolution-per-minute masterwork. Dual turbochargers and their catalytic converters are actually nestled between the V-shaped cylinder banks; BMW’s ingeniously compact layout has been used on no other production V-8 in history. The challenging design required exhaust valves mounted inboard and intake valves outboard, the opposite of the tens of millions of standard V-8 engines produced over the last century.
That reverse layout isn’t just for show: Power-boosting exhaust gases travel a much shorter route to the turbochargers and catalytic converters, meaning less wasted energy and superior emissions performance. The crossover manifold is another BMW first: Spent gases are routed through four pairs of cylinders on opposite banks, with each pair’s ignition separated by 360 degrees of crankshaft revolution. Each pair then feeds into an individual exhaust runner and turbocharger scroll; essentially, the engine works like a four-turbo design. Four exhaust runners of identical length mean that the turbines receive perfectly consistent pulses of exhaust gas, boosting gas velocity and virtually eliminating dreaded ”turbo lag,” the brief lapse between stomping the gas pedal and feeling the love when the turbos kick in.
For drivers, the result is a delightfully broad and ferocious power curve, with all 680 newton meters (501 foot-pounds) of torque fully on tap between 1500 and 5650 rpm. The engine’s unusual architecture also produces a one-of-a-kind sound, a mellow, baritone blat as opposed to the chesty roar of a typical V-8.
Next, the BMW’s xDrive all-wheel-drive system adds a feature called Dynamic Performance Control—the active rear differential that helps maintain stability and even nudge the vehicle around turns. Dubbed Vector Drive by its maker, Germany’s ZF Friedrichshafen, this complex arrangement of multidisc clutches and planetary gear sets act on speed and g-force information gathered from sensors at all four wheels.
Combining that data with complex algorithms from the electronic stability control system, the unit can vector full engine torque from one rear wheel to the other—a differential of up to 1800 Nm (1328 ft-lb)—in as little as 100 milliseconds. During a right-hand corner, for example, the left rear wheel speeds up, and the right rear wheel rapidly slows down, creating a pivoting force called a yaw torque. Those lightning-quick adjustments also keep the vehicle on the driver’s intended path, neutralizing both understeer—the tendency of a vehicle’s front wheels to skid—and oversteer, when a car fishtails from the rear.
The system may sound arcane, but the results are noticeable on road and track alike. On a damp track at Monticello, I could feel the active differential help pivot the BMW’s rear through fast corners, like a helpful football coach twisting a lineman’s hips to improve his footwork.
The unfairness of the situation, in both accelerative force and tech-enhanced traction, wasn’t lost on a colleague driving a 217-kW (291-hp) Mitsubishi Evolution all-wheel-drive sedan on Monticello’s snaking course. That colleague saw the usually formidable Evo get eaten alive by the hulking Bimmer—to the shocking tune of 10 seconds per lap. BMW engineers have also seen their most powerful M model confound expectations: At the legendary Nurbürgring course in Germany—for decades a testing benchmark for the world’s production and racing cars—the X6 M covers the 20.8-km (12.9-mile) circuit faster than the previous-generation M3 sedan.
The X6 M’s downside, as you might expect, is pitiful fuel economy. This 2.4-metric-ton Bimmer slurped premium unleaded at 1.6 liters per 100 kilometers (14 miles per gallon) during my testing, with the Environmental Protection Agency pegging official mileage at 12 and 17 mpg in city and highway use, respectively.
Yet ultimately, type A drivers with the means to buy this US $89 000 mildly sociopathic monster will also be the type to brush off social critics, preferably by tromping on the gas. Questions of relevance aside, those owners will be rewarded with an SUV of which NASA might be proud—for both velocity and technology.
This article originally appeared in print as "BMW stuffs a big engine into a small package."
About the Author
Lawrence Ulrich, who test-drove two cars for “Top 10 Tech Cars”, came to auto journalism by an unorthodox route: rock music. The native Detroiter worked in the 1980s as a rock musician, playing keyboard as far afield as Europe before becoming a business writer in the early 1990s, then a car writer. He lives in Brooklyn, N.Y., and regularly writes for The New York Times and Automobile.
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