I have seen hp gains by pulling air intakes from a large airbox, or chamber. for instance on several FI dirtbikes on the market, the Cannondale would make more hp using the stock airbox, then vs the guy who ran his bike wide open with no airfilter, kinda strange but I think it has something to deal with air velocity pulses of a single cylinder engine. so by enclosing the air intake the air pulse being created by the pistong acts like a little supercharger when the airbox is closed and sealed vs fully opened free flow throttle body.