i recently built my own complete exaust system starting with 38mm from the heads going to 50mm y-piece but wasent to impressed with the Dyno run but after macking a few small mods I am well impressed with the performance and handling it goese licke my old ktm 660 sc I used to race pushing aut 75hp mods made maen jet drilled aut with 1,5mm drill bit lairfilter lid 1/3 opend up snorkels cut off and lid turn round so the front is clossed with a flat K&N filter and she goese licke shit of a shovel now 
