Describes the outside diameter of the collector outlet. The larger a collector diameter is, the higher its mass flow potential will be with an inverse effect on velocity. Maintaining velocity is critical to supporting low-mid RPM performance and should be an important point to consider when choosing header collector size. For most V8 equipped vehicles with an exhaust system after the headers, 3" collectors are the limit for practical collector diameters. Dedicated drag race vehicles equipped with open headers can benefit from running larger (up to 4.5") and longer (between 16"-20") collectors than typically used on headers developed to be connected to an exhaust system.