cubic inches vs. head flow vs. rpm

The way I see it is first you need to decide hp goal. With that in mind pick heads that can accomplish this. The rule of thumb is 2 hp per cfm but you rarely see output like that in non race engines with cams in the 265 to 285 range along with CR in the 9:1 to 10.5:1 range is gonna be more like 1.7 to 1.9 hp per cfm. Next you want to figure where your powerband will be idle to 5000 rpm or 2000 to 6000 rpm etc.. and thats where displacement comes in. Basically a 1 hp per cid engine is gonna peak around 5000 rpm and 1.25 peak around 6000 rpm. Obviously theres a lot of factors that come in to play that a very lose guesstimation.