Headers are tuned to provide an extra power boost within particular target RPM range. This is due to the propogation of pressure/sound waves within the primary tubes and collector. The shorter the header the higher the RPM of the extra power boost. If a header is too short the power boost may come in at an RPM that is above the redline of the engine. At that point the only advantage of the header is its reduced back pressure compared to the stock manifolds. That's the extent of my knowlege