No CAI has ever been proven (dyno before and after) to have ANY HP gain over the stock set up.
Any CAI (like the Dinan) that goes all the way down in front of the right front wheel to pick up cool air dramatically increases the chance that your engine will be destroyed by water ingestion and hydro lock.
Any CAI that picks up its air under the hood, it is in reality a HAI (Hot Air Intake) and will actually cause a loss of power.
Any oiled filter media also puts you at risk for contaminating and ruining the MAF.
Any filter that has equal or less total surface area than the OEM and as these CAI's, claim better air flow , gains its air flow by allowing more and larger particles to enter your engine.
As the previous poster noted the OEM has a small opening that prevents (in most cases) water ingestion, and of course a much larger opening down lower to pick up the cool air and an effective free flowing filter element. The point is the stock set up is a more effective and much safer Cold Air Intake than any of the after market (and home brew) CAI.
Most who use them just like the sound (at the expense of performance and engine longevity). Its just a psychological performance boost.
chiefwej 2003 540i ///m-tech 6-speed
Black Sapphire Metallic w/gray