Most all over the counter headers are designed for
a street version engines, the primary tubes are long
and the collectors are way too short and usually too
small in diameter. A set of headers needs to be designed
for the engine... primary tube diameter, length and
collector diameter and length. A merge collector isnt
a one size fits all, cone shape and length mean ALOT.
Also if you run a exhaust, the extension pipe going
to the muffler is part of the collector(any length
before a expansion area)... collectors go in length
of multiples for tuning... if 18" is the best, then
36" is the next tuning length(sound wave)