I built this catch can when my R/T was having some angry blowby problems. It will work either way, but I had the side barbs used as an inlet for the crankcase gasses, one barb from the PCV system and one from the oil cap port. the outlet went to the small port on the airbox (the one that fills up with oil). worked out great. the added negative airspace ended up relieveing some of the pressure, so not much oil came through.