muffler

By removing the baffle you are creating less back pressure. The less back pressure the more power. You will get a performance gain but it will be so small that you wouldn't notice without a dyno machine.

If you want to make the most out of your stock exhaust, get one of these...

Billet Power Tip for Honda 400EX / 400X ATV Exhaust: eBay Motors (item 350374981447 end time Jul-29-10 11:02:35 PDT)

It will get a few db louder. Mainly just a deep growl sound. Not the high-powered sewing machine sound