this is purely a harmonic thing and would be diffrent for every engine its state of tune its also dependent on wht rpms you want your power to be
engins design speed, exhaust valve timing and its duration is what will denote how header length will work to benifit.
a good length will make a pressure wave to create a low pressure at the exhaust valve pulling air/fuel in to the cyl at valve overlap (good scavanging) and a bad length will have the pressure wave reflected back making a high pressure at the exhaust port (poor scavanging) pushing exhaust back in
there is no magic header length or style that suits all engines its more of a case of finding what works best for what you have
equal length for equal cyl power ballance.
longer headers genraly work better at higher rpms.
spent months at uni on this stuff
