[ItBites]

What Inner Diameter (ID) are your louvered Hedmans? If the ID is large enough, louvers vs perforations won't make a difference. Turn the louvers downstream, though. That reduces the thickness of the boundary layer and increases the effective ID where the bulk of the flow occurs, as compared to pointing the louvers into the flow. With a 331, if the ID is nominally 2.5inches or larger, you will be OK.