The advantage of a performance aftermarket valve like a Manley, for example (left), is not just in the stainless material. Most factory valves employ a sharp radius from the stem to the valve seat. Worse yet, many exhibit a ditch or undercut directly behind the seat that decreases flow. Depending on the application, new performance valves are only slightly more expensive than stock replacement pieces. Note the swirl polish and gentle radius from the stem to the seat in the aftermarket valve on the left.