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BMW 7-series: Seven times four equals the new flagship sedan

Wed, 24 Jun 2009 00:00:00 -0700

BMW 7-series owners are among the world's most loyal. BMW sells 10,000 to 15,000 copies of its largest sedan during a good year in the United States. It makes you wonder why the company might muck with what has often been called the world's best-handling large sedan by adding an all-wheel-drive package no one has been clamoring for.

It's opportunity, friends, in a region known vaguely as the Snowbelt. As BMW has introduced its xDrive all-wheel-drive system in other sedans, including the 3-series and the 5-series, sales typically have picked up nationally. Significantly, the take-rate for all-wheel drive has risen to 80 percent or more along a wide swath stretching from Minneapolis through Chicago, across the northern tier of the United States to Massachusetts and then down through the tristate gold mine of Connecticut/New York/New Jersey. That, as well as anything, explains the pending arrival of the first 7-series with all-wheel drive.

The current 750i and 750iL debuted just six months ago, restyled and powered by a new twin-turbo 4.4-liter V8 generating 400 hp and 450 lb-ft feet of torque. The new 7-series appeared with BMW's usual array of technical wizardry, including an Integral Active Steering option that now includes rear-wheel steering and a revamped iDrive interface that works a bit better.

By the end of this year, the 7-series xDrive will follow. It won't be available with the active steering option, but the all-wheel-drive 7-series could potentially change the market for full-size luxury cars by wooing buyers who might otherwise choose an Audi A8 or Mercedes S-class 4Matic.

The xDrive in the 7-series starts with BMW's familiar single-range center differential, with a clutch that opens or closes fully in 100 milliseconds. Enhancements start by more thoroughly integrating xDrive with other electronic systems, including stability control and the 7-series's Active Roll Stabilization antiroll bars. Yet the biggest change lies in the xDrive's basic operating parameters.

In all current applications, from the X5 SUV to the 3-series sedan and coupe, xDrive's default 40 percent front/60 percent rear torque split adjusts only according to friction under the vehicle's wheels. If either the front or rear wheels slip, the center diff shifts torque to the other pair of wheels.

For the first time, xDrive in the new 7-series takes several other performance parameters--including steering angle, lateral g load and yaw rate--and uses them to adjust the all-wheel-drive system's power delivery front to rear.

The xDrive also works with what essentially is a pseudo limited-slip Performance Control rear differential used on all 7-series sedans. The rear differential's control system can brake the inside rear wheel in a curve, slowing that wheel's rotation and sending more torque to the outside wheel, which in turn helps rotate the car in the direction of the turn.

Bottom line: The xDrive for the 7-series is tuned more to deliver the sporting character of a typical BMW sedan and less for the enhanced safety and security of more even traction at all four wheels.

Like other xDrive-equipped BMWs, the 7-series starts with that 40/60 default torque split. But when the driver applies the throttle more aggressively, especially through bends, the 7-series xDrive adjusts torque distribution to maintain the familiar sporting dynamics of rear-wheel drive. Through a hard bend, its control system tries to maintain a steady torque split of 20/80 to optimize handling.

On the road, regardless of drive type, the 7-series is remarkably well balanced for a car of its heft. Standard or xDrive--it's almost is a toss up. With xDrive, the steering feels a bit heavier than in rear-drive cars with the active steering, and we like that. But once the driver gets used to its lighter steering touch, the rear-drive 7-series is the livelier car. It almost feels smaller. Still, the xDrive version is no more inclined to understeer than the rear-drive 7-series. It may be slightly less inclined to push when you turn aggressively into a bend.

Distinctions are much easier to find on a track--especially when the track is dampened to lower handling limits. Even with the antiskid electronics switched off, the 750i xDrive does a lot more of the work for the driver than the standard 750i, balancing itself more readily with less need to be really delicate or active with the throttle. The xDrive 7 also delivers a much more pleasant experience than the current 5-series sedan equipped with AWD. The 5-series xDrive understeers more, almost excessively in comparison, and it won't do much of anything else even if you try to overdrive it for effect.

The rear-drive 7-series requires a lot more work, and it asks more of its driver. While that may be exactly what you want for track day, it may not be the preferred setup in a snowstorm. And who brings a 7-series for track day, anyway? BMW and those buyers in the Snowbelt may be onto something here.

When the first all-wheel-drive 7-series reaches U.S. showrooms late this year, xDrive should add $2,000 to $3,000 to the base price. Figure roughly $86,000 for the standard wheelbase 750i xDrive, and $89,000 for the long-wheelbase iL.

By J. P. Vettraino