I really wonder what Honda used in their "experimental" aircraft
   engines.
   Anybody knows this?

   Rolf Pfeiffer

   The big problem with liquid cooling is instability of the cooling
   system. When it reaches the boiling point of the coolant it goes
   unstable. While an air cooled engine continues to increase it rate
   of heat transfer as the engine warms up. Delta T in a liquid cooled
   engine is limited to about 110 F. Delta T in your typical air cooled
   aircraft engine can be up to 400 F.

   Read my "How to Cool your Wankel" article  on the subject on the ACRE
   NL web site.
   http://www.rotaryeng.net

   Paul Lamar

The Honda aircraft engines have been both air and water cooled.
Bill Jepson