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

273 said
I thought it was pretty interesting myself.

I'd like to have seen them with the exact same combo on all 3.

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