Merge collectors have to be built to YOUR engine..
they are designed for a certain rpm and if you move
too far from that rpm they dont add anything.. so if
you feel you can buy a off the shelf merge and expect
to to add performance your kidding yourself... a straight
collector will do more if you make the collector 16"-18"
long... I made a few merge collectors at work to test on
the dyno... each one hurt power at most points(until it
crossed the RPM where it worked).. think a expansion
chamber on a 2 stroke.. they work on a certain engine for
a certain rpm range... I built a set of chambers for my
snowmobile.. it KILLED the top end but it turned it into
a tree puller on low end.. because I didnt have
the inlet cone and exit cone the right diameter
and length