All I can add is add is what I found. On my best set of Edelbrock heads I porteded I tubed the pushrod holes and took every bit of available area and they flow slightly over 315cfm. The air speed at the pinch was in the range I like to see. I matched a port on a new set of heads and then I put aluminum round rod in the hole and had Harlan Sharp make me a set of rockers .350 offset so I can open the pinch even more. Now this port was already flowing 315 cfm and when I opened up the pinch .200 more I gained nothing. There are other things I want to try in the port but I kinda lost interest in that project. Hopefully this winter I will drag it out again