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Old 11-10-2011, 11:10 AM   #110 (permalink)
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Back to the issue of cooling which will need to be addressed before meltdown.

With experience gained from using grill blocks and the info posted by someone that with a fully ducted (in to the radiator and out) cooling system, the area that is needed is about 4 square inches per 25 horsepower. Now this is for a water cooled engine mind you, that is moving.

So, to extrapolate to fully duct an air cooled engine, the air intake needs to be in the high pressure zone in the front and the air exit needs to be in the low pressure zone in the rear. The air needs to be directed to blow directly on the engine, stay close to the engine and exit in an efficient manner. I would be hesitant to allow too much room for the air to swirl around the engine, the object is to heat the air and pass it out the back end as quickly as possible to keep the engine from overheating.

I didn't see Allans CRF230 up close, but as near as I could tell there was no ducting specific to sending air past the engine. Using the natural pressure relationships afforded by vehicle movement can produce pretty good cooling possibilities if done well. Brittens water cooled v-twin used a very small radiator behind the engine immediately in front of the low pressure area in front of the rear tire in the fender area. Filling in this low pressure area with hot air reduced drag, not to mention slightly reduced frontal area. The man was simply brilliant.

For the ~12hp needed, I'd at least double the 4sq in/25hp rule and use 4-6 sq inches and duct it closely around the engine. The increase in area isn't much to reduce temperatures.

The real advantages of water cooling are more even heat distribution within the engine, faster heat rejection from hot spots, allowing a higher state of tune (more output), being able to put the cooling radiator where you want it, the additional heat sink mass of the cooling system while stopped, and the ability to have a fan to cool the engine while stopped. There are many advantages to water cooling, but I don't like the toxicity of antifreeze and the additional failure modes of the cooling system. A larger air cooled motor working less hard can last a good long time if properly cooled.
Good design is simple. Getting there isn't.
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