Iron vs. aluminum head compression

[wamnram]

The aluminum heads have a higher compression ratio than the cast iron heads. This is true for both the 48 series and 68 series heads usually found on the 36 model engines. Both heads list CR of 7.52 for aluminum and 6.06 for cast iron with stock bores. 40 series heads are 6.5 aluminum and 5.42 for Cast Iron.