Drawing The Line - Six Cylinder Appeal

By James Parker, Photography by BMW

Twenty-two years ago, in 1988, Honda added two cylinders to the Gold Wing's four, resulting in the six-cylinder that's had the longest production run, by far, of any in motorcycling. Few were surprised by the change. Adding two cylinders to the Gold Wing's horizontally opposed engine seemed a logical, if not inevitable, way for Honda to increase power and torque as the market demanded more features, more comfort-more of everything.

I was surprised when I first saw BMW's new K1600 six. I hadn't imagined the K1300 four as a starting point for a six, but a closer look showed that its layout really lent itself to adding a cylinder to each side of the package.

The K1300 is unique among inline-fours. Its cylinder bank is canted far forward at 55 degrees, making it closer to horizontal than the basically vertical orientation of most other fours. This layout makes its cylinder head low enough that the frame beams run completely above it, not surrounding it as on most recent fours. The frame, in turn, is considerably narrower between the rider's knees, while simultaneously allowing for greater width at the cylinder and head.

The frame surrounds the throttle bodies, but the intakes are substantially narrower than the cylinder head itself. The cylinder angle also moves the intakes forward, farther from the ergonomically critical rear area of the fuel tank.

To help put the K1300's cylinder head low enough to fit under the frame beams, BMW's engineers chose a dry-sump oiling system, putting the engine oil in a tank above the transmission rather than in a sump below the crankshaft. This allowed them to lower the crank and thus the entire engine, the exhaust pipes now occupying the entire space beneath.

With its canted cylinder, forward intakes and low mounting, the K1300 moves the widest engine components away from the rider, leaving the seat/pegs/tank area relatively unaffected by engine width. At some point the engineers must have looked at these features and made an interesting observation: We can make this engine substantially wider without changing the basic architecture. But could they make it a six?

The K1300 four is rigidly mounted in the frame. A six would have perfect primary balance and could easily be rigidly mounted. Plus, BMW has a long tradition of building inline-sixes-about seven decades' experience in the automotive world.

Though the K1300 architecture could accommodate a wider engine, there would be limits. To make a six fit it would have to be narrow. Thus the engineers chose a relatively long stroke (67.5mm) and a small bore (72mm), with only 5mm between each cylinder-not even enough space for cooling passages. We're not told exactly how much width increased, but the weight is said to have gone up about 44 pounds.

To fit the six's intake components into a frame not much wider than the four's, BMW chose a novel approach. Rather than using one throttle body per cylinder as on most current bikes, six long intake runners are gathered into a central, single bore with a 52mm throttle plate. The long runners angling toward the center make the intake system narrower and provide perfect tuning for increased torque.

The K1300's dry-sump oiling system is retained, now using a tank cast into the cases at the rear of the transmission. With six exhaust pipes, the space under the engine is at even more of a premium.

Unlike the Gold Wing, where the four-cylinder was superseded by the six, the K1600 six will not replace the K1300 four, but will instead give BMW access to new markets and new buyers. Whatever the future holds for this newest six, visualizing adding two cylinders to the K1300's four was an impressive inspiration.

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