BMW’s six-cylinder motorcycle concept aims at production

  • 25-Nov-2009 04:38 EST
BMW Concept 6 powertrain.jpg
BMW's Concept 6 powertrain, showing the new compact 1.6-L inline six engine and shaft final drive. The twin mufflers shown are production-type units; their designs differ from those on the Milan show bike.  

BMW the automaker is best known for its inline six-cylinder engines that combine high specific output with excellent NVH characteristics. BMW the motorcycle maker has offered ever-sophisticated three- and four-cylinder inline bike engines since 1983, while it is still most closely associated with the horizontally opposed twin-cylinder “boxer” engine that helped launch the company and remains popular today.

At the 2009 EICMA Show in Milan, BMW Motorrad surprised the industry with its Concept 6 motorcycle, the name denoting the engine’s cylinder count. The impressive prototype is a glimpse at BMW’s next-generation super-sports touring machine, which BMW officials admitted is headed for production.

Six-cylinder engines have been rare in the production bike world. Honda currently is the only maker offering the configuration, its Gold Wing touring model featuring a 1.8-L boxer. Six-cylinder power first appeared on the 500-cm³ motorcycle Grand Prix scene in 1958, with Italy’s MV Agusta briefly testing (but not racing) a DOHC six in response to Moto Guzzi’s Grand Prix V8.

During the mid-1960s Honda's 250-cm³ RC166 delighted Grand Prix fans with its shrieking six-megaphone exhaust and 17,000-rpm redline.

Sixes took to the street in 1972, when Benelli launched a sohc 750-cm³ air-cooled I6 sports roadster, soon followed by a 900-cm³ version. Honda and Kawasaki followed shortly after with I6 street bikes, the latter differing from its Japanese and Italian counterparts by being liquid cooled and using a shaft final drive rather than chain drive.

The early six-cylinder street bikes were fast, smooth, and exotic, but their wide engines and high curb weights were rapidly eclipsed by lighter, more powerful and better handling machines with fewer cylinders. But BMW engineers and product planners decided the market is again ripe for the charismatic look, feel, sound, and prestige of a high-performance inline six bike, inspiring them to develop the Concept 6.

BMW offered minimum technical information at the bike’s Milan debut, and officials would not comment on specific start-of-production timing. The engine architecture is derived from the current K-series liquid-cooled inline four-cylinder, with the cylinder bank inclined 55° from vertical to allow a sufficiently short wheelbase and place mass closer to the front axle. The canted cylinders also help optimize airflow into the bike’s upper fairing and into the intake airbox and heat exchangers.

The liquid-cooled, fuel-injected engine will be approximately four inches (101.6-mm) narrower than any previous inline-six-cylinder motorcycle engine and only a few millimeters wider than the company’s K1300 four, according to BMW. The engine’s bore/stroke ratio is claimed to be slightly oversquare, and the compact cylinder bank features thin-wall die castings. Coolant passage design is optimized for maximum flow and overall compactness of the cylinder block.

The new six features a dry-sump lubrication system, with a chamber in the rear section of the crankcase serving as the oil reservoir. BMW engineers wanted to avoid using a separate oil tank, in order to keep the center of gravity low and make the power unit more compact.

Engine and reciprocating mass also are minimized by use of hollow camshafts and lightweight connecting rods. BMW officials also point out that, unlike an inline four-cylinder, the I6’s perfect primary balance negates the need for heavy, power-robbing balance shafts.

The dry-sump arrangement also allowed chassis engineers to package the large, complex exhaust system which in concept form features a sextet of outlet pipes on each side and a large-volume muffler with closed-loop catalyst underneath the machine. The system will likely revert to a conventional six-into-two arrangement for production.

To further aid compactness, the alternator, six-speed gearbox, and output to the shaft final drive are located “piggyback” style behind the cylinder bank.

Engine displacement on the Concept 6 is 1.6 L and is expected to be in the 1.6-1.7-L range for production. Claimed power levels are equivalent to the current K1300S four-cylinder bike but with significantly greater peak torque and a much broader torque curve—officials quoted 143 N·m (96 lb·ft) at 2000 rpm. The engine is so torquey, the development team did not see the need to fit a tachometer. Peak horsepower will arrive at approximately 9000 rpm.

Like the K1300, the Concept 6 features a rigid chassis comprised of aluminum castings and extrusions. Suspension is provided by BMW’s Duolever and Paralever setups front and rear, respectively. Front foundation brakes feature Nissin six-piston calipers, and the 17-in forged wheels are sourced from HP.

The production version of the Concept 6 is expected to launch as a 2011 model.

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