The 2016 YZ450F was Dirt Rider's open class motocross shootout winner, but that doesn't mean it can't benefit from a little engine massaging. Kris Keefer decided to to try an FMF SS Aluminum 4.1 muffler system to see if it improved the already potent engine package the Yamaha has to offer. FMF's aluminum 4.1 version still incorporates a carbon fiber end cap and comes with a spark arrestor screen for all you off-roaders out there. The complete system system will cost you $799.