I guess look for one with a removable baffle... and dont remove it?
Read reviews, but most aftermarket exhausts are made to make bikes sound a little more meaty :/

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Yamaha XT 660 Z T�n�r� 2009 black, Polisport Off-road front mudguard, Givi tall screen, Givi luggage rack, Yamaha top box, Touratech Hand Guards.