BMW is dishing new details on the newly developed but throwback-style 2-cylinder boxer engine that features in four Motorrad concept bikes unveiled over the past year that look increasingly likely to presage production motorcycles.
Dubbed the “Big Boxer” by BMW, the 1,802 cc (1.8-liter) flat-twin features in the Concept R18/2, shown earlier this month at EICMA, the Concept R18, and the Motorrad-suppported concepts the Departed by ZON and Revival Cycles’ Birdcage. It’s said to be the most powerful BMW boxer engine, making 91 horsepower and 116 pound-feet of torque and is said to balance high pulling power and running smoothness.
BMW says the engine harkens to the first air-cooled Motorrad boxer engines that debuted in 1923 and stayed in production for about 70 years, with the same overhead valve drive and separate engine and transmission housings and built to be reliable and easy to maintain. but the new Big Boxer is air- and oil-cooled, of course, and the quenched- and tempered-steel crankshaft has an additional main bearing at the center to prevent against unwanted bending vibrations in the large-volume cylinders. It also has a vertically split aluminum engine housing. A wet sump lubrication system supplies the lubricating and cooling oil via a two-stage oil pump and a sleeve-type chain driven by the crankshaft.
It was also inspired by the 2-cylinder engines of the R5 and R51, from 1936 to 1941, and the R51/2, from 1950-51, that featured two camshaft driven by the crankshaft via a sleeve-type chain and similarly positioned to the left and right above the crankshaft. That makes for shorter pushrods and reduced moving masses, among other advantages, plus improved precision and higher speed stability. Rather than employ modern hydraulic elements for valve clearance, the new engine also borrows the legacy Motorrad boxer method of employing adjusting one screw and a lock nut for each steel valve.
The transmission is a constant mesh six-speed, with a reverse gear available as an option, and torque transmitted to the rear wheel via a propeller-shaft or universal-shaft drive. Both the propeller shaft and the universal joint are nickel-plated and open in another wink to Motorrad history on models through 1955.