I was wondering if there are such things as inline air filters that would come in a 3 or 4 inch diameter pipe?

 

A simple wall of filter material in a pipe would have bad flow and get clogged quickly. However, if you can tolerate a hump in the pipe where the filter is, it seems like it should be possible to make something like that. I have looked before, but I don't think I found anything fantastic. I think some company that makes cold air intakes for Hondas make some. There are others that have a hose that goes to the filter, but the filter is also open on the sides (so some/most of the air doesn't actually come through the hose).

-Max