I had Lone Star sidepipes with 2 1/2" core and 24" long mufflers. The louvers were facing so they did not catch air. This would give max air exiting the muffler. I replaced them with Cobra Pack 3" x 28" mufflers. I installed them so the louvers would scoop the air into the muffler. Four inches longer gave more volume.
Before you could heard me from 1 1/2 miles away. I've got witnesses. Lots of them including one police office, pictures in my gallery.
No ticket.
After I changed the mufflers, it's a "lot" quieter and I "think" it makes more HP. I made other changes between dyno runs so I don't know what the muffler change gained me. But I feel very confident the new sidepipes help.
I now have a quieter exhaust system with a deep health rumble. I like it.
It's that air in - air out thing. I have larger mufflers to handle the same amount of air, so it should be quieter.
A muffler is a place to store air till it can sloooowly exit the muffler. The reduced speed of the air exiting the chamber reduces the noise.
IMHO
Dwight