[cujet]

A little bit off topic, but since this is an efficiency forum,,,,,

Air cooled engines sometimes have an advantage in efficiency. They reject less heat into the cooling system, generally by design. This is due to the surface area of the cooling fins and the fan's capacity. Sure, we could move massive quantities of air over fine pitched fins to try to mimic liquid cooling. But, generally, air cooled engines (at power) run with cylinder head temperatures in the 300 to 400 degree F range.

This results in a slight decrease in BSFC.

However, there is more to the story. Air cooled engines function well with 2 valves per cylinder. It's somewhat difficult to get 2 exhaust valves to cool properly (yes, it can be done). So, most are 2 valve engines. This can result in lower valve train frictional losses. But, the issue at hand is piston ring swept area. The lower output of air cooled 2 valve engines, generally means that more displacement per cylinder is needed for equivalent output. This results in larger pistons and lower RPM's. Piston ring swept area is greatly reduced in this situation. Therefore, BSFC is greatly reduced when some air cooled engines are compared to liquid cooled "modern" engines doing the same work.

Of course, this does not apply to all situations and all engine applications. But, it holds true for many modern air cooled engines. Consider two examples. The modern, well tuned V-twin, such as a Harley or clone. Compared with liquid cooled bikes of equal power, the Harley engine achieves a better BSFC number. Modern piston aircraft engines such as the Lycoming IO-540 can achieve BSFC numbers better than nearly all similarly powerful modern liquid cooled engines (generally 0.38-0.39 Lb/fuel/HP/Hr). In other words, your 350 small block Chevy won't make 300HP and do any better than 0.55.