Here's an interesting dyno shootout 160 cfm head on a 306, 333 & 393.

RAMM said

Haven't watched it but 2.06HP/CFM seems to be the sweet spot at least for me. Those HP levels you quoted are not impressive at all. While I agree its interesting it just isn't any kind of mystery in my eyes. J.Rob

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I don't think the hp/cfm level are impressive in themselves. Just how the 393 could squeeze another 81 hp out of a ridiculous small head over the 306, even though I imagine the 248 solid roller vs 212 hydraulic roller did play a big part but doubt it was responsible for the total gain. Just thought it was interesting.

But the 2.41 hp/cfm 393 engine did gain a 168 hp when he did put on proper heads on it, 387 vs 550 hp.