The stock air box in the late model wrx's, lib's, foz's are very well designed and efficient and based on reading and my own experience. An aftermarket CAI depending on design may also create varying degrees of difficulty in tuning as well. My tuner confirmed this to me on two different occasions.

I do believe that CAI may in some cases be needed once you upgrade to a large turbo however. It also makes some great noise if nothing else.

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Funny looking German Subaru