The most efficient filter is a flat ended cylinder type filter that has 1 inch of AC per 142 CFM of filtering and that cylinder matches the exhaust fans port size! Cylinder type filters like most people use in their grow rooms are very inefficient, only the first 3 inches after the opening maintains a greater than 50% efficiency, its called science, air will always flow at the greatest rate of speed where it encounters the least resistance, so if you don't have a flow rate tube that would equalize the flow rate along the entire length of the cylinder, then only the first 3 inches after the filters opening will ever maintain a greater than 50% efficiency. That's why most companies today allow for the user to flip the intake around because they know that only the first 3 inches of their filter is actually filtering most of the air being sucked in through the filter.