I'm using a Baldwin that is the same as the 6637. Last night I ordered a tapered open cone type filter made by Donaldson. My theory is that the if filter is crammed in between the battery and the fuse box, the fender and the hood insulation, it has to be restricting the air flow. I am going to try a smaller diameter cone shaped filter with a hollow end for more surface area. This and the Zoodad ought to be an improvement. Hope so anyway. I installed a chip in the truck this past weekend and it is screaming for more air!