[Alan Roehrich]

If the primary length varies enough that some primaries are off enough to lower the amount of HP produced by those cylinders, they'll just lower the overall average HP, in the vast majority of cases.

The reason is that you have an approximately 3" "window" for tuned length. After that, the length of the primary is far enough from the correct harmonic length that it hurts HP over the entire range. That happens because you lose way more off the peak than you ever gain everywhere else.

The "tuned" in the term tuned equal length header refers to the length of the primaries and the collector being tuned to one of the natural harmonics created by the rest of the combination, cam timing, port size, bore, stroke, rod length, etc. If the primary and the collector do not match one of those harmonic lengths, you are not operating in a "tuned" state, and therefor the header is not working with the engine, it is working against it, by disrupting those harmonic tuning lengths. That's a crude layman's way of explaining it, used here for the sake of brevity and clarity.

Obviously, given the constraints imposed by the stock engine location, mostly stock chassis, and ground clearance on a Stock or Super Stock car, it is next to impossible to build absolutely perfectly tuned headers. In some cases, it can be impossible to even get within the 3" "window", at least on the primary tubes, no matter what you do. Also, in some cases, even if you add extra bends to move a tube to get close to the window, between the extra bends and the inch or two that end up from perfect, you're better off to just use fewer and/or less severe bends and live with the incorrect length.