We recently enlarged the air intake in the cowel of our legacy from 2.5" (factory standard) to 3.5" (the same diameter as the fuel servo.)  For you Continental folks, the Lycoming cowel has a third intake hole just for engine air below the prop.  We also moved the hole up closer to the prop to take advantage of ram air.  To our surprise, we got no boost in MAP and lost a few kts. airspeed.  Presumably the intake hole is now too big.  Does anyone know the formula for calculating the diameter of the air intake for a fuel injected engine (or know where we could find it?)