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Even though both are based on the 5 Series, the Gran Coupe is more a stretched 6 Series Coupe than a 5 Series sedan in terms of style and temperament.
Mmmm . . . Fast
Except for the different number of doors, the BMW M5 and M6 are nearly identical. The M6 is 0.5-in. shorter, 0.4-in. wider and 3.2-in. lower than the M5, sits on a 4.5-in. shorter wheelbase, and in all but transmission options is mechanically identical to the M5. Think of it as a coupe version of the 5 Series.
For both M cars, efficiency is all the rage. BMW’s 2013 M5 and M6 have ditched the Formula 1-inspired naturally aspirated 5.0-liter V10 and recalcitrant six-speed SMG automated manual gearbox in favor of a twin-turbocharged 4.4-liter V8 and seven-speed dual-clutch Getrag transmission. Power output is up, the torque curve is as flat as Kansas, and fuel efficiency is better, despite the reduction in the number of cylinders.
The S63Tü V8 is a variant of the direct-injection N63Tü V8 that first made its appearance as the N63 in the X6 sport crossover, and has since migrated to every BMW line save the 1 and 3 Series. It uses a “reverse flow” design that places a cross-bank exhaust manifold with integral twin-scroll turbochargers in the engine valley; the incoming charge is cooled through a pair of air-to-water intercoolers before it’s sent to the cylinders. A new addition to all BMW’s twin-turbo V8s is Valvetronic. It elimi-nates the need for a separate throttle, and controls engine speed and output through the valves themselves. This com-bines with Double-Vanos—a system for providing infinitely variable timing on both the intake and exhaust cams—and direct fuel injection for greater power and efficiency. In the standard 550 and 650 models, this is enough to pump out a respectable 400 hp at 5,500-6,400 rpm and 450 lb-ft from 1,750-4,500 rpm. Increasing the boost while keeping the compression ratio at 10.0:1, and upgrad-ing the internals for the greater in-cylinder pressure, is enough to obliterate these figures. In M guise, the twin-turbo V8 produces 560 hp at 6,000 rpm, and 501 lb-ft of torque from 1,500 to 5,800 rpm.
There are a six-speed manual and a seven-speed M-DCT dual-clutch gearbox; the latter replaces the eight-speed automatic fitted to non-M 5 and 6 Series cars. The M6 is available only with the M-DCT transmission. Using the seven-speed wet-clutch DCT gearbox found in the six-cylinder Z4 as its base, this transmission is re-engineered from top to bottom to cope with the greatly increased output of the V8, and is unique to BMW’s M division.
It sends power to a differential that, with the independent rear suspension, sits in an aluminum subframe mounted rigidly to the body. There are no bushings as BMW claims the car’s high-speed data bus allows near-instantaneous control over the computer-controlled dampers, and the lack of bushings keeps rear suspension geometry near-perfect under load. The Active M Differential Lock uses an electric actuator mounted on the differential case to give 0 to 100% lockup, based on information from the M-tuned dynamic stability control system. Thus, it can pre-lock the diff before boost spool-up when accelerating, and unlock it under braking or while braking and turning into a corner.
The standard brakes are vented and cross-drilled steel rotors with aluminum hats. Front rotors are 15.7 in. in diameter, while the rears are slightly smaller at 15.6 in. Blue six-piston calipers with the M logo are used up front, while single-piston units are employed in the rear. There is a carbon-ceramic brake option that reduces unsprung weight by 42 lb. Its six-piston front calipers are finished in matte gold, and the rotor size increases to 16.1 in.. As if to prove which end does the most braking, the rear rotors are downsized to 15.2 in. and use the same single-piston calipers as the steel/aluminum disc brakes.
More power and efficiency
The fifth-generation 7 Series debuted in 2009, so now it is a mid-term update. Beyond the new front and rear fascias, new lighting units, and other superficial updates sit the real changes. The N63 twin-turbo V8 found in the 750i/Li models is upgraded to N63Tü spec, which brings an output increase of 45 hp and 30 lb-ft of torque, as well as greater fuel efficiency. Engine stop-start has been added to all 740 and 750 models, as has an eight-speed Steptronic automatic transmission. However, the efficiency increase comes mainly from the addition of high-pressure direct fuel injection with coil injectors, the Valvetronic throttle-less intake system, and a variable-output water pump.
All-wheel drive (xDrive in BMW parlance) is added to the six-cylinder 740 Li, and all of the six-cylinder models have a single twin-scroll turbocharger, Valvetronic and direct injection along with Double-Vanos infinitely variable intake and exhaust valve timing. This engine also forms the base for the ActiveHybrid 7, replacing the V8 formerly used. The 315-hp turbocharged inline six combines with a hybrid-specific eight-speed automatic with integral 55-hp electric motor for a combined output of 348 hp and 367 lb-ft of torque. The trunk-mounted lithium-ion battery pack has enough capacity to push the car to 37 mph on electric power alone or cruise for 2.5 miles without engaging the engine. Also, the 3D navigation system relays topographical and speed information to the hybrid unit to prepare it for the most efficient use of this technology. If that’s not enough, in Coast Mode the car shuts off the engine when coasting. Non-hybrid 7 Series cars decouple the engine from the transmission from 30 to100 mph while decelerating.
Speed, style, subtlety
Although the 6 Series is derived from the more upright 5 Series sedan, the Gran Coupe is pulled from the two-door 6 Series Coupe. Longer, lower and wider than the 5 Series, it is a direct competitor for the Mercedes CLS and Audi A7, and uses the same engines, gearbox (seven-speed DCT only), steering, brakes and suspension found in the 5 and 6 Series cars. As with the M5 and M6, there are buttons to alter steering boost, damper settings, throttle response, transmission shift points and severity, and stability control. However, putting each on the “Comfort” setting introduces a subtlety those cars lack, without detracting from the car’s capability. The 640i’s inline six has 315 hp and 330 lb-ft of torque, and the V8 in the 650i models pumps out 445 hp and 480 lb-ft. That’s good enough to go from 0 to 60 mph in 4.5 seconds in the rear-drive 650i, and in 4.3 seconds with the slightly heavier xDrive model.