The combustion chamber shape in the aluminum head is different from the WW2 head, but almost the same as the ribbed CJ-2a head I pulled off, just with a perfectly flat and shallower roof. The CJ-2a head had better power than the WW2 "F" head I had on before, especially noticable at the upper end. I assume the wider combustion chamber over the cylinder bore allows for better flow. The power increase with the Hickey or Chuchua aluminum head is probably mostly from the raised compression. A CJ-2 head would probably need about 1/8" milled off it to get the same compression ratio, as that is about the difference in the depth of the chambers. I'm not sure if the iron head could handle this much of a milling.
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