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the blue is the better of the two because more air is alowed to pass through, and you can find it in more places. also from what i have seen the blue one has an opening on the front of the filter to alow more are in (not sure if it helps gain HP) but it make the car a little bit louder. the grey one does not have that. you will get a some drone but its sounds good in my opinion "its the way it should shound and preform". just look into the see if its worth it because im trying to research if it will void the bmw warranty. good luck
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