I have used a standard air box and K&N on my 2001 since new. It has never caused problems for the 87k km I have on it now. No increase in HP noticable, a little better throttle response at around 80km thru 140 fast acceleration and a little better sound.

I have lurked here for as long as it has been here on and off. THe standard airbox is extremely efficient and not half as restrictive as other models IMHO and others that regualrly post here.

GO for it!

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Niagara
2001 4.4i X5
Black Sapphire
Sport package
X5world DIYS Mods so far:K&N filter AC Schnitzer aluminum pedals, Piaa Super Plasma Fogs, M5 steering wheel, Chrome exhausts,Chrome gauge rings, Brembo rotors, powder coated calipers, replacement accessorize tray, replacement center console insert, IceLink Plus Ipod, X-pipe, Chrome staggered style 87 20' wheel's

Future MODS 10" Sub in rear, figure out how to replace stupid Business CD with monitor, lowering springs