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Originally Posted by greeny

Being aircooled is not in itself a barrier to revving hard or making power. For example aircooled formula A kart engines (100cc A/C) rev to 20,000+ RPM and produce 40hp. However their engine life is less than an hour. These guys rebuild engines between heats, and have specialist qualifying engines.

The moral of the story is that yes you can get big horsepower from a/c but the reliability and durability of the engine will be severely compromised. Dazza and Danny are right. The more power the more heat you produce. If you fill a balloon with water (try this at home), and hold a cigarette lighter to the balloon it will not pop. Do the same with an airfilled balloon and it will pop almost instantly. This is because water is far more dense than air, and theirfore disperses heat far more efficiently throughout the water in the balloon. An aircooled motor cannot dump heat as well and this has several effects:

1. When an engine is hot, the air going inside is also hot, making it less dense. If the air is less dense, effectively there is less of it and as i have said before, More air = more power. This is the sole role an intercooler plays in a turbo setup, to cool the inlet charge. Similarly, engines will make more power in cold weather at sea level as the air is more dense.

2. Due to the inability to drop heat, A/C barrels distort unevenly, especially around the exhaust port where temperatures are very high. This uneven distortion serves to change the shap of the bore, forming an oval. This opens tolerances, reducing power and secondly increases wear as the piston no longer effectively fits. To take the formula A example again. At the end of a straight you will see the driver choke the engine by putting their hand over the airbox. They do this under brakes before they get off the throttle. The reason they do this is that at the end of a straight the piston is so hot that it actually does not fit in the bore, it is an interference fit. If they came all the way off the throttle, the engine would have no fuel (oil) and would intantly seize. By putting their hand over the inlet, they dump a whole bunch of nice cool fuel onto the piston to cool it, shrinking it just enough to keep going when they get off the throttle. These are the types of lengths you have to go to to get equivalent power from an A/c engine.

Hope that clears up the cooling issue, cheers

nice said

good work