The following are examples of tampering to original equipment EPA approved exhaust systems. Most owners who tamper with these mufflers are not content with the resultant increase of noise, unless there is a straight through passage from the muffler inlet to the outlet, ie., "hollow core muffler."
The left photo is what the unmodified original equipment muffler for all 1983 to present Harley Davidson FL touring models looks like. On the right is the tampered with version. Upon a closer inspection you would be able to see straight through from the exhaust outlet to the inlet of this muffler. Note the jagged edges.
Below is an example of tampering to Harley Davidson's own, EPA approved "Screamin' Eagle" line of aftermarket muffler for FL touring modelsii-1. On the left is what the unmodified exhaust outlet end of the muffler looks like. On the right is the result of the owner grinding off the two small opposing beads of welds (left photo), then removing the core of the muffler, sawing off protruding tubes at both ends of the core, and then reinstalling the heavily modified core. Upon a closer inspection, you would be able to see straight through from the exhaust outlet to the inlet of this muffler.
From a 2003 Yamaha YZF600. On the left is what the unmodified exhaust outlet end of the muffler looks like. On the right is the result of the owner grinding off the hardened rivets that secured the end cap, and sawed off the last inch of the muffler. The end cap was reinstalled with common sheet metal screws. Note the jagged edges.
From a 2001 Harley Davidson FXST. The owner turned this original equipped muffler into a hollow core muffler by cutting out the center, and left a jagged and out of round edge.