Inside BMW’s Mighty 110-Cubic-Inch “Big Boxer”

Building market share with the R 1800 cruiser while the economy is disturbed.

pushrod boxer twin
New BMW R 1800, an air-/oil-cooled, eight-valve, pushrod boxer twin, will power the company’s forthcoming R 18 cruiser platform. Several custom design studies have been teased.BMW

BMW has built a long, low cruiser whose 1,802cc boxer engine cranks 116 pound-feet of torque at 3,000 revs, giving 90 hp at a low 4,750 rpm. This is something new with a completely “other” look.

What would you do if product planning came to you and asked for a cruiser that combined the appeal of deep tradition with night-rider style plus instant twist-and-go acceleration? After researching that market, you’d see that it has declined in recent times and that there has been no shortage of competently engineered V-twin “metric” cruisers. Haven’t they already copied every obvious feature yet somehow missed the mark?

Then you might reconsider by comparing the two styles in which bikes are marketed. First is the dominant commodity approach, to produce a standard market lookalike in every category. But always more effective is to create a powerful story around your product, of which riders yearn to become a part. This motorcycle is going where you are destined to go. Only a very few brands have made this appeal work. BMW, which has been building shaft-driven bikes with boxer engines since 1923, is one of them.

R 1800 engine
Ghosted view of the R 1800 engine shows twin camshafts, valvetrain, crankshaft, connecting rods, and numerous other internal parts.BMW

The proven path to instant acceleration is big displacement. Sportbike engines can accelerate hard but not by roll-on, only after you tap down two or three gears to where their torque lives. But with big cylinders, turning the throttle instantly fills cylinders and combustion pressure hurls you forward—no shifting, no waiting.

This engine is big, weighing 244 pounds and being 21-1/2 inches high and 24-3/4 inches long with its in-unit gearbox.

I attended a tech day at BMW’s permanent car-and-bike testing facility, Autodrome de Miramas, in the south of France, far from the gray North Sea. There, we were shown a large beer stein as inspiration for the new R 1800’s 107.1mm bore (4.22 inches) and 100.0mm stroke (3.94 inches). Why not the traditional longer stroke and smaller bore? Three good reasons: 1) this engine is not a me-too copy of anything; 2) the engine rpm of peak torque is determined not by stroke but by valve timing together with valve and port sizes; and 3) with its cylinders out to the sides where aircraft-engine designer Max Friz put them in 1923, the longer the stroke, the less you can lean the bike in turning. As is usual in large-bore engines with a substantial compression ratio (R 1800’s is 9.6:1), mandated fuel is 95–98 RON (Research Octane Number).

For our inspection, a disassembled R 1800 engine was placed on the floor of a well-lit work bay, and we were able to pick up parts and examine them. This engine had done 13,000 kilometers (8,078 miles) in a testbike and 87,000 on a programmed cycle dynamometer. Visual condition of parts—camshaft lobes, crankshaft- and rod-bearing shells in particular—was excellent. As one engineer put it, “Everything here is modern technology, modern materials.”

This engine is big, weighing 244 pounds and being 21-1/2 inches high and 24-3/4 inches long with its in-unit gearbox. Each massive cylinder head is 11-1/2 inches long and 8 inches wide. End to end, there are 18 deep cooling fins on each head-and-cylinder combination.

100,000-kilometer engine
This was exhibit A at Autodrome de Miramas, BMW’s French testing facility, the 100,000-kilometer engine in pieces, with a bodywork-off prototype next to it.BMW

Next question: How many valves? Tradition says two but tough Euro 5 emissions limits arriving January 1 dictate basically no valve overlap. Valve overlap is that period around Top Dead Center (TDC) as the exhaust stroke ends, when the exhausts have not yet completely closed yet the intakes have begun to lift. Valve overlap is valuable in making power, but at idle and low throttle, it can allow unburned fuel to escape into the exhaust. With four valves per cylinder, even short, low-overlap valve timings can supply plenty of air, while the smaller flow capacity of two valves cannot. The two 41.2mm intake valves in each cylinder head have the area of a single 58mm valve (2.28 inches).

BMW has met Euro 5 in the R 1250 GS with ShiftCam, a system that at low- and medium-rpm ranges shifts to a different set of cam lobes that both increase intake-air velocity and create rotary swirl as a means of speeding the completion of combustion.

This R 1800 has been given a pushrod valve train. Instead of ShiftCam, it employs a secondary inlet system that supplies the engine’s charge in those ranges. Two small, jet-like ports emerge into the intake valve ports, small enough to flow at high velocity and aimed to generate rotary charge swirl. With the main 48mm intake butterfly throttles closed, throttle movement at first sends more and more air through this secondary air system. Even more throttle opening then begins to open the main throttles. Further help in meeting Euro 5 takes the form of a second spark plug in each cylinder, located to assist in prompt light-up.

R 1800 cylinders
Big! You can put your fist right through the R 1800 cylinders. Note the oil-cooling channel surrounding TDC. These fins are not faux.BMW

Closed-loop fueling and normal three-way exhaust catalyst complete the emissions package. The early rumor was that this would be a gasoline direct injection (GDI) engine, but that idea may have been sparked by the location of each cylinder head’s single fuel injector in the head rather than upstream in the throttle body. It sprays into the port, located as close as possible to the valves.

Why so close to the valves? Think about it: To obtain Euro 5-level completeness of combustion, slower-burning rich and lean zones must be avoided. But the farther upstream the injector is located, the larger the slug of pure air that may initially enter the cylinder—a classic lean zone.

Why two cams, one for each cylinder, rather than one as in earlier flat twins? The engineers told us, “The financial guys are not very happy about this, but with two cams we could make the pushrods shorter and lighter.”

Compact needle-bearing roller tappets are used and, like the cylinders themselves, the pushrods are aluminum; same material means both expand together, tending to keep valve clearance constant. Each pair of valves is operated by a single rocker with screw-and-nut clearance adjusters at each valve-stem contact. Engine service interval is 10,000 kilometers or 6,200 miles.

The classic problem of big air-cooled engines, especially those with four valves, is that of keeping exhaust-valve seats round, centered and in place, and of preventing cracking of the narrow exhaust bridge between the paired exhaust valves, which is heated from both sides. BMW tackled this problem in earlier models and the big boxer carries its solution: extensive oil cooling via the chin-mounted 40-square-inch oil cooler, a coolant oil pump, and cored oil passages in the head and surrounding the top of each cylinder. We were told, “Engine temperature is not an issue.” It was marvelous to handle the parts and trace out the flows; 4.1 liters (4.3 quarts) of oil are carried in a conventional sump below the crankshaft.

Joseph Miritsch
This is Joseph Miritsch, head of BMW’s boxer line. Customizers will think of alternative designs for the smooth valve covers and “breastplate,” the engine’s front cover.BMW

Pistons are cast and carry three rings in modern thin dimensions. The top ring is located 5mm down from the piston crown, and the oil scraper is of the long-wearing and conformable three-piece design. Each piston crown takes the form of a smoothly shaped shallow bowl, giving an open combustion volume that should burn quickly because there’s room for vigorous turbulence from the intake event to persist all the way to TDC.

Big pistons like these handle big forces; peak combustion pressure sends more than five tons of force down each fracture-split steel con-rod to the crank. Smaller boxers have two main bearings, but evidently when engineers mapped the stresses for an engine this big there was too much “red” in the false-color stress maps. Therefore, there is a third main bearing, located between the 180-degree crankpins, increasing cylinder offset from the approximately 45mm of the smaller engines to more like 62mm. This offset—the left cylinder is offset forward—causes the engine to oscillate around a vertical axis to the left as pistons decelerate to TDC and to the right as they approach BDC. In BMW’s early 500cc engines, a moderate peak rpm of 5,800 plus the light weight of small pistons made this oscillation minor. But as the flat motors grew to 1,000 and 1,100cc, turning higher revs with bigger pistons, a balance shaft became necessary.

When more than one journalist objected that the cylinders are in the way of the traditional cruiser’s “pelvic pegs,” they were told, “A boxer is a boxer!”

This engine is different. Because such horizontal-opposed twins have inherent primary balance (the two pistons move in opposite directions), traditional cruiser riders might pine for the vibration that says the engine is “alive.” Therefore the engine is given what the engineers called “pulse” by omitting that balance shaft, a perfectly okay solution because peak power comes at that low 4,750 rpm and redline is 5,500, keeping those forces moderate; inertia forces increase as the square of rpm.

As heavy cruisers and touring bikes have grown and riders seek performance appropriate to today’s speeds, engines have become larger and so has the heat they generate. On a traditional V-twin, you sit on top of that heat, but Max Friz’s 1923 common sense located the boxer’s cylinders to right and left, where their heat is carried away by free-stream air when moving and by vertical convection in stop and go.

six-speed gearbox
Ghosted view of the six-speed gearbox. The large blue spring loads the cam-and-saddle torsional absorber. The output shaft is at the lower right of the gearbox.BMW

The black steel-tube chassis describes a straight slope from steering head to rear axle, with the long 23.3-inch swingarm and its direct-acting single shock as a part of that line in faux hardtail style—no Paralever here. Although we were not shown the seating, there is nothing preventing it from being fashionably low. When more than one journalist objected that the cylinders are in the way of the traditional cruiser’s “pelvic pegs,” they were told, “A boxer is a boxer!”

When I asked an engineer about overall weight in relation to current cruisers, I was told, “It is in that class.” Not shown were the steel flywheel and single-plate back-torque-limiting clutch. We also did not see a final version of the intake system.

The six-speed gearbox is driven through a cam-and-saddle torsional absorber. All gears are helical—both for noise reduction and to transmit loads through more than one pair of teeth at a time—and are shifted by dog rings, not by axial movement of the gears themselves. Shift forks and shift drum are standard modern BMW practice in being made of lightweight aluminum rather than the traditional steel.

On the bike’s right side can be seen the driveshaft and its two universal joints. The warmer color of the plating on this assembly is electroless nickel; early BMWs had nickel brightwork rather than chrome. The shaft and joints are completely assembled before plating.

An external-rotor 21-pole alternator mounts on the tapered front end of the crank. When asked about electronic rider aids, an engineer replied, “It’s the state of the art in electronics.” Electronics are the new norm, no longer news. Get used to it; safety authorities may one day make them mandatory.

A real surprise in BMW’s presentation was discussion of how the team had telescoped new production development and testing through high-speed data acquisition, processing, and storage. We were shown two machines, an R 1250 GS and a large scooter, equipped with Swiss-made “measuring wheels” equipped with accelerometers and also covered with strain gauges and other sensors. Up to six video cameras capable of 140 frames per second can also be applied. The instrumentation on these two machines represents 300,000 euro.

I was told BMW’s US sales this past year rose 9 percent, and that this growth is driven by a steady flow of attractive new products. That used to mean long-lead-time testing, analysis, and validation, both costly and slow to respond to market change. Thanks to new high-capability data handling, the process of assuring the performance and durability of new models has been greatly accelerated.

This parallels methods that have long been normal in aviation, where vast amounts of data must be gathered from a multitude of sensors and quickly put into forms that help engineers optimize the flight-test process. As with aircraft, motorcycle performance cannot be accurately predicted just by computer; physical testing remains essential. Now higher-density data gathering extracts more usable information from each test mile.

A better understanding of how BMW motorcycles are used worldwide comes from anonymized ECU user data gathered with owners’ permission. As an example, it was stated that South African riders display by far the most aggressive throttle use.

We were also given some insight into industry planning for the expected future of more connected vehicles, increasingly robotic cars, and less-attentive drivers. A beginning is adaptive cruise control, which maintains safe distance from vehicles in the same lane. We were shown the recent progress in automotive radars and cameras, which are now small enough for motorcycle application. Just two weeks prior to this presentation, there was a meeting of the Connected Motorcycle Consortium, whose core members are BMW, Honda, and Yamaha, plus development members. They seek to define the place of motorcycling on future streets and highways.

Also shown were a lighted vest capable of flashing warnings (“This is not the horrible yellow safety vest that we all hate,” the presenter said), and a head-up display (HUD) built into spectacles of near-normal size.

In all, 13 BMW motorcycle engineers and project leaders spoke on a range of topics and were available for questions. This was an information-intensive day.

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