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a. you lost psi.
b. the exhaust its too big for your application and you lost back pressure.
c. you have bought a bunch of "cool mods" and no fuel mods, chances are you are running lean and possibly "♪♫knock knock knocking on heavens dooooor...♫1♪.
d. you dont do your homework before picking your mods.
e. you are a noob without a tune.
f. all of the above.