I measured it and I recall it being like 3 1/4 or so. The filter mouth on my K&N was probably a 1/4" bigger so I guess that makes since since I believe it's supposed to be a 3.5" mouth filter.

Don't quote me on the measurements.. I just know the filter mouth was a decent amount larger.

04 Subaru WRX "Eurosport bling bling fast and furious tokyo drift" "They have diarrhea of the mouth, and constipation of thought"