Header primary tube sizes vary on many factors: engine combination, transmission type, gear ratio and intended use of the vehicle. For example a truck header will be designed using longer length and smaller diameter primary tubes to enhance low to mid-range performance, where a header for a Pro Stock race car will have larger primary tubes and large collectors for the highest flow available. Bigger generally is not better when it comes to street driven cars. when you put too large of a header or exhaust on a street driven vehicle, you usually notice a severe drop in low to mid-range performance due to a lack of backpressure. To combat this issue, Hooker sometimes builds 3 or 4 different headers for a particular application with varying tube and collector sizes.