Just to clarify, your current compression is actually about 8.8:1, when you allow for the space above the rings, and the compressed head gasket. If you opt for the 64cc heads, this will remain unchanged. If you install 58cc heads, your actual compression will go up to 9.4:1, which, by itself, would increase your horsepower by about 18 hp. Switching to aluminum heads would also shave 60 pounds off the engine, and permit higher compression than iron heads would.
Take care,
-Chris
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