Just when you thought that BMW’s venerable flat-Twin had reached the logical conclusion of its evolution with the R1200′s cam-in-head design, the Germans go and throw us a wonderful curveball.
The first pitch was the dohc setup that debuted in 2007 on the limited-production (and expensive) HP2 Sport. We never thought those exotic parts would ever find their way to the rest of the Boxer lineup, but here they are!
Whether the present R1200GS, R1200GS Adventure and R1200RT flat-Twins needed dohc in place of successful and elegant cam-in-head setup is one of those questions that probably won’t ever be answered by the project engineers. They wanted it, they were able to do it, so they did it. And it is also likely that all Boxers will get this engine in due time.
The main challenge in applying dohc to this mill has been that, if a traditional design were used, the cam chains would get in the way of either the inlet or exhaust ports. In the 1950s, BMW solved that issue with a race-only version of the R50, a 500cc Boxer Twin featuring bevel-gear-driven dohc heads, very much in Norton Manx style.
Bevel-driven cams are fine on a racer, but chains are much more cost-effective and practical. So, to maintain the traditional longitudinal Boxer port arrangement with exhausts at front and intakes at rear, BMW developed camshafts carrying one inlet and one exhaust cam lobe each, rather than the more traditional setup grouping intake lobes on one cam and exhaust lobes on the other. To set the opposed inlet/exhaust valves at the proper included angle, the lobes are conically ground and actuate the valves through suitably inclined finger-type cam followers. This arrangement also allows a relatively small radial included valve angle of 22 degrees that yields a very compact and tidy combustion-chamber profile.
Benefits were plain on the HP2 Sport. That engine allegedly made 128 horsepower, a significant bump from the cam-in-head R1200S and its claimed 122 hp, with more power through the whole rev range.
The GS and RT use the same 39mm inlet valves (plus 3mm vs. the cih engine) and 33mm exhausts (plus 2mm). Compression is down half a point to a still-high 12.0:1, with a single sparkplug per head, rather than the twin-plug setup on the HP2 Sport. Throttle bodies are 50mm, 2mm smaller than those of the HP2 Sport but 3mm larger than the previous GS units. An electronically controlled butterfly valve is inserted in the exhaust system to obtain the best possible compromise between noise control and scavenging effect, as well as allowing the traditional BMW Boxer sound to come through. These changes result in a claimed output of 110 hp at 7750 rpm and 88 foot-pounds of torque at 6000 revs, an increase of 5 hp and 4 ft.-lb., respectively.
What the numbers don’t show is the substantial increase in torque available at just about any rpm and the ability of the engine to rev strongly to 8500 rpm, 500 revs higher than the old powerplant. A beefed-up gearbox handles the increased output. Contrary to the big powertrain changes, the chassis for the $14,950 GS and $17,000 GS Adventure chassis are essentially unaltered.
And so, the GS remains a big bike, its wheelbase spanning 59.3 inches and seat height variable between 33.5 and 34.3 inches. Wheel travel is 7.5 inches at the front and 7.9 at the rear. The Adventure takes that a step farther with 8.3/8.7 inches of travel, and consequently sets the seat at a stock height of 35.0 to 35.8 inches. The claimed dry weights are surprisingly low at 448 pounds for the R1200GS and 492 for the R1200GS Adventure. From my personal experience riding these machines, these claimed weights must be very, very dry, like James Bond’s favorite vodka martini!
The long list of optional equipment includes a lowered, reduced-travel suspension setup, as well as a cut-down seat, but those options do, of course limit off-highway versatility.
My first ride on the new R1200GS was on Italian backroads leading from the coast of Tuscany and through the beautiful, oak-wooded hills north of Grosseto to some incredible, ridge-top villages that appear to have been forgotten by time.
Overnight rain and thick morning fog meant the roads were quite treacherous. In other words, perfect to test the GS, a machine that has been continually honed over the years for exactly this kind of riding. The big BMW once again proved its competence with surprising agility, even if all the torque and power from the new engine was difficult to fully exploit on the tight, slippery corners. The dirt portion of the riding loop had been turned to mud by the rain, but chassis composure and control remained, thanks in part to the deeply grooved Metzeler Tourance EXP radial tires that provided good traction and steering response. We even threw in a stream-crossing.
The GS definitely proved its worth in these harsh conditions, but the ride to that point hadn’t exploited the new-found high-rpm power and revability. So I plotted my own test course on beautiful, winding roads through the same hills, but on a more open stretch with broad sweepers and longer straightaways, where the engine felt wonderfully strong and seemed to breathe more freely. Dry tarmac later in the day meant I could ride at full speed, blasting the bike easily up to 110 mph on the straights, then diving into the corners well cranked over. Riding the strong midrange torque is all you’d ever need to do, but it is nice to have the extra power and 500 rpm on top. With either choice, great engine response is there.
Ultimately, the versatility of the GS has been expanded once again, adding high-speed grand touring to its already broad capabilities.