Conical-profile camshafts key to BMW bike’s valve geometry

  • 24-Jun-2010 04:18 EDT
BMW boxer DOHC head .jpg
BMW's new "boxer" DOHC cylinder heads for the R1200GS were developed for the high-output HP2 Sport. The 1.2-L engine is air/oil-cooled. Note short cam followers, which pivot on a shaft, and timing chain system.

Motorcycle engineers are a small but highly respected bunch within BMW’s product-development organization, and their esprit de corps proves it. The company’s Motorrad division traces its heritage back to 1923, preceding the car guys, and its first bike (the 500 cm3 R32) set the horizontally opposed, air-cooled, twin-cylinder layout which decades of BMW two-wheelers have followed.

Despite branching out into inline three- and four-cylinder machines in the early 1980s and collaborating with Italy’s Aprilia on vertical single and twin-cylinder models later, BMW never stopped evolving and improving its classic "boxer" twins.

This year BMW shocked the motorcycle industry with two developments—an inline six-cylinder concept bike that is being developed (with input from Ricardo) for production, and the new S1000RR, a 1000-cm3 inline four-cylinder sportbike rated at 200 hp (150 kW) and featuring a sophisticated traction control system.

But while the Concept6 and RR have dominated the news, the company didn’t neglect a major technology upgrade for its best-selling "boxer" model, the R1200GS.

For 2010, the dual-sport R1200GS trades its single-high-camshaft heads—technically not a true overhead-cam design—for the compact DOHC cylinder heads introduced in 2008 on the high-performance HP2 Sport. Both cylinder head types feature air/oil cooling and four valves per cylinder.

To maintain the 1.2-L engine’s traditional front-facing exhaust layout, the Motorrad design engineers developed unique camshafts, each carrying a single inlet and exhaust cam lobe.

To achieve the proper included valve angles, the cam lobes are conically ground and operate the valves via inclined finger-type followers. This configuration provides a 22° radial included valve angle, ideal for a compact, fast-burn combustion chamber.

With larger valve diameters, 12:1 compression ratio, and larger (50-mm) throttle bodies, the new twin-cam heads help boost the GS model’s peak output by 5 hp, to 110 hp (82 kW) at 7750 rpm. Peak torque is increased to 88 lb•ft (119 N•m), a 4-lb•ft gain.

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