it was interesting they went old school on the R18, no water cooling. Seems the boxer model offers the best layout for head cooling due to exposure to air flow.
The Star Venture was Yahama’s attempt at such a thing. Uncertain the sales success of that model line in the cruiser/touring market though they too felt it had to be air cooled to target the Harley market. Seems though HD is moving more to water cooled bikes now days thus the Venture and R18 engines could be thought of as the last of their kind to be sold in big bikes.
Guess one will just have to wonder what could have been if BMW had tossed in some sort of hidden water engine head cooling on the R18 engine line.
Boxer motors are ideal air cooled engines because both cylinders are in the airflow. But there are some limitations to them that some may not like, such as limited foot placement, a perception that the bike is wider, and aesthetics.
V-twins like those in Harleys suffer from the rear cylinder being blocked to airflow. Here liquid cooling is an advantage both for comfort and emissions/power.
A V-twin turned sideways like in a MotoGuzzi has the same airflow advantages as a boxer engine, but also many of the limitations.
Liquid cooling a boxer, in my opinion makes little sense. May as well use a different engine config that has other packaging advantages. This is why I bought the last model year boxer GS before liquid cooling was added and why I bought an R18. I also have an older 1972 boxer motorcycle. I love the boxer in its raw air cooled form, but if a motor needs to be liquid cooled, I think other engine configs make more sense.
Another example of this issue, my GS is 40-50 lbs lighter than the liquid cooled version, even though I have a larger fuel tank. Triumph has the Rocket 3 with 160HP and 160 lb/ft of torque from its liquid cooled 2500cc triple and you can have forward controls if desired. The newer Bonneville line is also liquid cooled and puts out decent power in a nice compact design. My S1000R is only 1000cc and puts out 160 HP. Liquid cooling definitely adds more power, but combined with different configs makes more sense. I love my air cooled boxers, but if I have to get liquid cooling, I’d get other engine configs.