Super Boxer
SUPER BOXER
FINALLY, BMW SHOWS ITS HAND
THE FACT THAT BMW HAS
been working on an all-new
opposed-Twin engine has
been a not very carefully
kept scrept swiring around
the company’s corporate skirts for nearly two years. Now, finally, BMW has let its neo-Boxer out of the bag, choosing the Cologne Show to debut the R259 engine, which will be installed in a new series of Beemers beginning in 1993.
The R259 represents the continuing development of a BMW tradition that began in 1923 with the introduction of the 500cc, 8.5horsepower, Max Friz-designed R32, the first BMW opposed-Twin, at that year’s Paris Motor Show.
This new engine may seem somewhat similar to BMW’s previous Boxers, but in reality it is as much a departure from the old-style engine as the inline-Four “FlyingBrick” K-motor was.
Its use of four valves per cylin-
der, a cam-in-head design and oilcooling all help to vault the new
motor into the era of contemporary design. The search for performance, and for chassis lean angles required of a modern motorcycle, were what led to these features, BMW says. Lean angles of up to 49 degrees were one of the project’s targets, and that goal magnified the importance of a narrow engine. For that reason, traditional sohc or dohc designs, which would have added considerable width to the alreadywide engine, were discarded in favor of a cam-in-head design not unlike that used by the Moto Guzzi Daytona 1000. The cams, located in the heads’ undersides, are chaindriven from a half-speed layshaft situated directly under the crankshaft. The four valves in each cylinder—a number chosen, BMW says, when its tests indicated no advantage to threeand five-valve designs-are opened by rocker arms. These are operated by short pushrods and cup tappets actuated by the cams.
The engine relies only partly upon air-cooling. Cooled oil is circulated through the heads and around the 31mm exhaust valves (intakes measure 36mm) by a dedicated coolingoil pump situated adjacent to the lubricating-oil pump at the front of the engine and driven by the layshaft.
Instead of the old-style Boxer’s single-piece tunnel casting, the R259 uses as its core a pair of vertically split aluminum-alloy engine cases that carry the two-main-bearing crankshaft, which drives a single-plate clutch, and through that, a five-speed gearbox derived from the company’s K-bikes. The crank operates its pistons, which work in nickel-silicone-lined bores, through a pair of sintered rods that BMW says are much more precisely manufactured than the rods of the oldstyle R-motor. Rods and pistons for the new engine weigh less than those of the old-style engine, and the result, BMW says, is considerably less vibration than was present in the old-style Boxers.
Fuel and spark are controlled by electronic engine-management systems that oversee ignition timing, throttle butterfly angle, rpm, intake air temperature, ambient air pressure and oil temperature. The result is an engine that displaces 1085cc from a bore and stroke of 90 x 70.5mm, produces a claimed 90 horsepower at 7250 rpm and pumps out 70 footpounds of torque at 5500 rpm-all clear improvements over the power production of the old-style Boxer, which last year in 980cc R100R form produced a claimed 60 horsepower at 6500 rpm and 56 foot-pounds of torque at 3750 rpm.
This engine, which will power a new alternative-front-suspension bike likely to be called the R1100, apparently is but the first of a family of neoBoxers under development at BMW. Also coming are smaller-capacity engines that use this same design.
Has the engine been worth the wait? Time, and the engine’s installation into a chassis that is worthy of it, will tell that story. Jon F. Thompson
