MORE ON BMW'S BOXER
BMW's new flat-twin motorcycle engine marks a basic change from its predecessors: It's what BMW designers describe as an "industrial engine". By that, they mean it is intended to be built repeatedly and inexpensively with modern manufacturing techniques, something that simply wasn't possible with the old Boxer. Indeed, on a trip to BMW's Berlin motorcycle factory three years ago, I got one of BMW's engineers to admit something I had long suspected: Ignoring tooling amortization, it cost the German company more to build a flat-twin engine than a water-cooled K100 four-cylinder!
The old engine was based around a one-piece crankcase, with main bearings carried in plates that bolted to the front and back of the complex crankcase casting. The crankshaft couldn't be put into the case with connecting rods in place, and the crankcase itself was none too stiff: BMW tuners watched Boxer engines modified for racing flex their crankcases noticably during dyno runs, and, among other things, had to allow extra piston-to-head clearance to compensate.
The new engine uses vertically split crankcases that can be assembled around a built-up crank assembly. They're far stronger and stiffer than those used for the old engine, and they locate the oil sump so far below the crankshaft that this almost could be described as a dry sump engine. The deep sump also provides valuable crankcase volume to help minimize the pressurization that takes place when both big pistons come toward bottom-dead-center at the same time.
The valve gear is as simple as it can be for a four-valve boxer: A single camshaft mounted high in each head opens each pair of valves via a very short pushrod and a single rocker arm with an extension for each valve. Valve lash can easily be set through screw adjusters. The camshafts are chain driven from a jackshaft under the crank; the jackshaft also carries the oil pump.
The cylinders are shorter than on the older Boxer, and carried higher; this permits, according to BMW, a 49-degree lean angle before a cylinder head enounters the ground.
The new Boxer engine will be offered in at least 1100cc and 800cc displacements, powering an RS-type motorcycle to be released in the spring of 1993. It will be followed by a GS variant in the fall, with an RT touring version likely to be announced at the same time.
BMW insiders have hinted that the new bikes will be substantially heavier and more expensive than current BMW twins, as well as substantially more powerful. The 1100cc engine is claimed to make 90 horsepower, and the RS model that will carry it will certainly cost more than $10,000.