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.

Describes the outside diameter of the header primary tubes. Headers are commonly available with multipe tubing diameter options in order to accommodate different levels of performance from a common engine type. For example, small block Chevy applications can benefit from using 1-1/2" to 2" primary tubes depending on the displacement, compression ratio and power band of the engine and the weight, gearing, tire size and usage scenario of the application vehicle. Hooker headers are produced in multiple tube diameter ranges known to be effective and popular for the intended engine/vehicle application. For stock to mild performance engines intstalled in a truck or heavy vehicle, choose a header tube diameter on the smaller end of the available offerings. For higher HP and max-effort engines in the same vehicle, choose a mid or large size tube diamater as available. Typically street driven V8 car applications from 250-400hp use a 1-5/8", 400-500hp use 1-3/4", 500-700hp use 1-7/8" and 700+hp use 2".