Well I asked basically the same question a while back and the answer that I received was that the preference for air cooled comes down to the sound of the engine
http://www.motorworld.net/forum/show...=120611#120611 . An air cooled engine has a more mechanical sound due to the whirl of the fan and the lack of a noise insulation that a water jacket surrounding the internal engine parts creates.
IMO a water cooled engine is able to more efficiently cool the engine which should increase reliability and thermodynamic efficiencies. The only down side other than the sound is added weight and potentially added mechanical problems with a more complex system. I’m sure that a water cooled medium with water, radiator and pumping that runs to the front radiator has to weigh more and certainly there are more parts that could fail. The other aspect of air cooling is the nostalgic association to a relatively unique Porsche characteristic. Porsche fans love nostalgia
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