Well it is kind of silly, its the throttle body. I mean really the limiting factor is the size of the throat feeding the intake manifold, be it a carb or whatever restriction there is.Ultimately of course it is simply the ability of the pump (engine) to inhale and exhale. The more air/fuel mixture you move through the pump, the more power you make. Anything that restricts throughput limits HP. That's the reason most 4 stroke engines don't show much gain when doing intake stuff, the ones that do have either very restrictive air cleaners or have had changes (cams, headers, etc) which require a better intake. Remember naturally aspirated means operating at 1 atmosphere so whatever gravity gives you is all you get. Thats why nitrous and turbo's and superchargers give such impressive gains because they are stuffing the pump!