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muffler performance

  1. The ZX6R
    so ive been searching around for a muffler for my 2015 ninja 636, Ive been looking at the two bros $400ish or the m4 $307 the two bros said it can add 2hp more then the M4 so idk what to get. I would like to get the M4 because it has a better look Any one have any input I would like to know the...