[Steve C.]

"Like all other aspects of engine building, port size is relative to the required airflow of the engine. The smallest port that will deliver the airflow needed by the engine is desirable. That means we will have excellent velocity for port filling, but without any choke that may limit required peak airflow. The needed airflow depends on engine displacement, RPM, and VE at that RPM. And a port big enough to deliver 400 cfm may well be too large for any engine that can only use 300 cfm."

Jim Hand

https://www.youtube.com/watch?v=CmfxHNxk8ew


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[Jay S]

From what I recall Jim Hand was not of the opinion that Aluminum heads provide an octane advantage for increasing the compression on pump gas. That was why he did not increase the compression much with the KRE heads. I think he was both correct and incorrect on that idea. So long as the heads are identical, every test I have seen changing from Aluminum to Cast Iron has not shown any difference in octane tolerance or power. I think Jim was correct on that idea. But where I think he was incorrect is that a port volume increase does generally allow for more cylinder pressure on the same octane. I suspect increasing port volume moves air flow to where piston speed is faster, likely also leads to a cooler air charge, and also tends to move some engine scavenging around so the power band gets shifted a little higher of spread out. Depending on the car, some of those things will not necessarily improve the engines performance. If you could replicate an iron copy of a KRE or Eddy head, i think the Iron heads would run identical. The Iron to Aluminum contest to me more decide on how thick the castings are to manipulated them. The winner in that area is likely the Aluminum heads.

My 6X heads on my TA are ported to 185 cc. I tried 196cc on another set and lost 16 cfm at peak flow against the 185 cc head. I would expect the 196cc had more flow lift in it someplace, but I could not find it. Here is a couple photos of the 196cc 6X heads. The photo has a 1.77 valve that is sitting down on the valve guide stem.