Head differences -77 318 vs -89..?

Wyrmrider said

late model heads with the heart shaped chamber take less timing which indicates more efficiency.
There are 4 versions with various casting numbers, 273 #315 closed heart shape, 318 open #675, 318 closed '67 only, 318 closed #302.
use them if cc match what you want to do

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I fixed your post.
The 318 intake and exhaust ports all flow roughly the same , none flow better than the next, considering core shift as the variable.
The '67 closed heads have blind exhaust ports so that they flow even with eachother.