Maybe Sin can EXACTLY explain it to you...but the stock intake box is EXTREMELY restrictive, the whole process of increasing horsepower in your engine is an easy principle...."breathe in and breathe out" the better you make both of those possible the more power you will have...The FIPK is designed to create a more fluid flow of air into the the engine, it is extremely less restrictive then the stocker, so you should be getting better flow...i was always under the impression that the FIPK gave you better accelration rather then top end, i could be completely mistake though..
-Alex