Bro.. which 4-2-1 header u using ?? 1 muffler for twin pipe? u mean da muffler that 1 inlet and 2 outlet muffler? can share abit? haha.. my friend suffering from a prob..that he now using superstreet 4-2-1 extrator. from piping centre bullet to muffler all standard.. jus with a assist of drop in...