Hot 6: after a long absence, BMW returns to the sport GT market.

Numerically perched between BMW's 5 and 7 Series sedans, the new 645Ci--BMW's first new 6 Series since 1989--is meant to cover large distances at high speed in comfort, and is avail-able as either a coupe or convertible. It uses parts from its sedan siblings, but isn't a cut-down four-door by any means. A 4.4-liter V8 is the only engine thus far (a high-performance M6 with a 500 hp 90[degrees] V10 is just over the horizon); the body is an eclectic mixture of steel, aluminum, thermoplastics and SMC; and there are enough electronics under the skin to run the space station. Obviously, BMW has been busy over the past 15 years.

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POWERTRAIN

The N62 4.4-liter V8 is shared with both the 5 and 7 Series sedans, and produces 325 hp at 6,100 rpm and 330 lb-ft at 3,600. A Motronic ME9 engine management unit oversees knock control, variable lift and timing, engine cooling, the electronic throttle, and the variable intake system. Variable camshaft timing on both the intake and exhaust cams rotate the cams between the adjustment extremes in a stepless fashion. It works in conjunction with the Valvetronic mechanism that sits atop the intake valves. Valvetronic places an intermediate follower between the cam lobe and rocker arm, and is held in place by an eccentric shaft that is rotated by a small servo motor. This determines the follower's pivot point, and varies valve lift. The intake manifold has a pair of intertwined helical elements that are rotated by a servo motor to seamlessly vary the effective length of the intake passage. Unlike the 5 and 7 Series, the 645Ci's exhaust is fitted with a movable element in the right resonator to enhance the engine note without exceeding the exterior noise limit of 74 dBA. Three six-speed gearboxes--shared with the 5 Series--are available: a manual, an automatic, and an electro-hydraulically shifted sequential manual. The two-piece friction welded aluminum driveshaft eliminates 6.6 lb, and employs steel yokes.

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SUSPENSION

Like the bulk of the chassis, the suspension is pulled directly from the 5 Series and modified to suit the new car. The double-pivot aluminum front suspension has a lower ride height, firmer springs and dampers, and the steering is tuned for stronger on-center feel. The multilink rear suspension has a 0.4-in wider rear track, more negative camber (2[degrees] vs. 1.5[degrees]), and a lower roll center and ride height. Active roll stabilization is standard. Variable ratio ZF Servotronic steering from the 7 Series also is standard, while its active steering system is available as part of the optional sport package. The brakes are the same as those used for the V8-powered 545i, and utilize brake discs with a high-carbon cast-iron rotor affixed to an aluminum hat and aluminum calipers. Standard tires are four 245/45R-18s, while the sport package increases this to 245/40-19s in front and 275/35-19s in the rear. All are a run-flat design. The Dynamic Stability Control (DSC) has been upgraded to version 8.0 on the 645Ci, and includes Dynamic Traction Control (DTC). Pressing the DSC switch briefly disables the engine intervention, and uses the wheel brakes to control wheel spin. Normal traction control operation returns above 40 mph, though DTC reengages below this threshold until disengaged.

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BODY AND CHASSIS

The 109.4-in. wheelbase is 4.3-in. shorter than the 5 Series' but the composition of the unibody is similar. From the A-pillar forward, the structure is aluminum, and mated to a conventional steel structure via a full-contact adhesive and self-piercing rivets. The hood and doors are aluminum, but the front fenders are thermoplastic--a first for a volume BMW model--and the rear decklid is SMC. The thermoplastic fenders give nothing away to aluminum in terms of weight, but are easier to form with the requisite shallow curve and sharp character lines. The steatopygous profile of the SMC decklid increases trunk room (13.0 [ft.sup.3] in the coupe, 12.4 [ft.sup.3]--top up--in the convertible), and places the upper surface where it can do the most good aerodynamically while saving weight. BMW claims it didn't break in a 50-mph rear crash test. Weight of the body-in-white fitted with its closure panels is 728 lb, 34 lb lighter than the structure of a 330Ci coupe.

The convertible adds diagonal front and rear bracing, unique front subframe, a 3.0-mm aluminum front thrust plate, reinforced side sills, a stronger B-pillar lower section, reinforced diagonal rear subframe attachments, and a lateral lower body reinforcement. "Inside high-pressure forming," uses high heat and oil inside the windshield frame to ensure a consistent wall thickness. Without the windshield, torsional rigidity is 15,000 Nm/degree. The top is fully automatic, and the glass rear window can be raised to act as a wind break. The coupe's large sunroof has a power sunshade and tilts for ventilation, but otherwise does not open.

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By Christopher A. Sawyer, Executive Editor