Fichier PDF BMWGS.pdf

BMWGS.pdf

Nom du fichier: BMWGS.pdf
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Model Evaluation BMW’s New R1200GS DOHC Radial Valve Heads Boost the World’s Most Popular Adventure Bike C The flattened spherical shape of the new heads also makes room for larger valves in the same 101.0mm bore; increased from 36mm to 39mm on the intake side and from 31mm to 33mm on the exhaust, while the compression ratio remains the same at 12.0:1. Cam lift is also increased, now 10.80mm on both sides, vs. 10.54mm on the intakes and 9.27mm on the exhausts previously. Note that the by Dave Searle engine’s stroke is unchanged at 73.0mm, for the same true displacement of 1070cc. To refine the power delivery, the intake tracts are also larger—now 50mm in diameter, vs. 47mm previously and are painted black (vs. silver) for easy identification. To take full advantage of ELEBRATING ITS 30TH anniversary, BMW’s R1200GS has the freer breathing heads, the intake manifolds are reshaped and become its most popular model, accounting for 30% of its the air filter element now provides greater flow capacity. worldwide sales. For 2010, the GS/Adventure has received But from the rider’s point of view, the most significant change the most significant performance upgrade since the lighter is the way the new DOHC engine sounds. Not only do the new R1200GS replaced the R1150GS in 2004. Utilizing essentially the heads make for sharper combustion events, a new exhaust system same dual overhead cam, four radial valve cylinder head design design highlights this fact. In the collector, a new backpressure created for the limited-production HP2 Sport (the $25,000, 130hp, control valve now uses an electrically controlled servo (visible carbon fiber clad superboxer introduced in 2008), the new DOHC behind the engine’s left side) and the muffler is internally reconGS claims 5% more hp (from 105 to 110 hp), greater low-end figured although its external dimensions are unchanged. The torque to enhance its go-anywhere capability and 500 more rpm result is a deep throaty exhaust note that generates a snarling fury added to its rev range for greater power “under the curve.” at high rpm, missing from the earlier GS. Capping the new heads We’re told the new heads are not physically larger or heavier are revised magnesium valve covers—perhaps the easiest way to than the high-cam overhead valve heads they replace, and the tell the new motor from its predecessors—that now use two vs. advantage of the new radial valve combustion chambers is that four hold-down bolts. Although super light, magnesium is also they now better approximate the ideal “lens” shape to permit brittle and, as before, the new valve covers need guards to protect faster more efficient combustion, driving the pistons more them from damage in off-road use. strongly—the key to greater performance. Unlike the HP2 Sport, While the engine’s personality backs up its claims of stronger the DOHC GS heads continue to use two sparkplugs per cylinder, low end torque, our dyno tests revealed the true differences are which is said to significantly improve low-rpm combustion subtle. Compared to a clean, well broken-in 2009 GS, for instance, efficiency (whereas the racey HP2 Sport needs/uses only a at 2500 rpm, the difference is significant: 5.99 hp (29.51 vs. 23.52 single centrally located plug). hp); while at 4750 rpm the improvement is greatest: 6.17 hp The details of the new head design are shown in the illustration. (62.87 to 56.70 hp), with another strong showing at 6500 rpm: Note that both cams are now driven by a single chain at the rear 5.56 more hp (87.36 to 81.80 hp). But at other speeds, the 2009 of each head, using an unusually deep tensioner shoe to control motor is very close and it’s actually stronger from 5400–6100 slack. The design also uses BMW’s F1-derived finger followers rpm (by as much as 3.91 hp at 5750 rpm), and their peak power between the lobes and valves (also employed on BMW’s high pernumbers are very close, again alternating the advantage dependformance, four cylinder K-bikes) adjusted with half-spherical ing on the exact rpm: 94.99 hp @ 7750 rpm from the 2010 model, “shims” that are held by matching cups in the followers. Although vs. 93.8 from our 2008 GS and 92.45 @ 7500 rpm from our 2009 recommended inspection intervals are not changed, the DOHC example. Also, the dyno finds the rev limiter on the new machine heads are said to maintain proper clearances almost indefinitely, is at 8400, not the claimed 8500 rpm. which should mitigate the additional time they would require to A small change made to the GS’s already excellent 6-speed adjust vs. the easy screw-and-locknut equipped valves of the old transmission is a revised shift cam profile, using a OHV design. To achieve the tipped valve angles, the individual “power/travel” formula to reduce gearshift effort. The result is intake and exhaust lobes are tapered as well as cam ground, with noticeably crisper shifting action together with slightly shorter the finger followers’ pivots angled to match.