"Newer" bike engine designs mainly use liquid cooling complete with radiators, water pumps, expansion tanks, cooling channels, etc. just like cars. It allows for a more uniform engine temperature both at different spots within the engine and over time/weather conditions. Because of this, the tolerances can be tighter and performance a little better with the same displacement (600 gsx-r vs. 600 Katana).
Air cooled engine designs rely on air flow and cooling fins to regulate temperature. In order for an air cooled engine not to fry itself sitting in traffic in Phoenix in the summer, the tolerances have to be looser to allow for variations in metal expansion with temperature.
Liquid cooled bikes probably do a better job of temperature manangement, but then you have to be aware of and fix any coolant leaks that may develop.
Last edited by RayO; 11-14-2002 at 08:24 AM.