We have built set on kill engines of all sizes and have found air flow is king, we make the same hp with a 565 as we did with a 632. The thing with the 565 was it was much easier to race. e were definitely faster with the 565. these engines were raced in nmca 10.5 . we were normally in the top 1/3rd of the field qualifying . I would like to see where we out ran the head flow. It would be my guess around 520 inches. So with all that air flow is the name of the game in my opinion.