On a motor with a well designed head, it is possible to reach 2.45 hp per cfm in a high hp buildup. 2.3 hp per cfm is more likely though, and pretty stout performance on a bigblock type build. My 528 makes about 880 hp, and has a headflow of 368 cfm at .700, the highest even number where the cam actually lifts the valve. There are so many variables involved that this number can't be taken too seriously, it is just a rough benchmark.