The problem with traditional 'glass packs' is that there is only so much glass fiber you can pack in that skinny tube. Then the heat burns it off and they get insanely loud. Using a welded case muffler like the Pypes Race Pro, Magnaflow and DynoMax Ultra, there's a lot more space for packing. In the case of the Pypes and Magnaflow units, they are packed with Basalt material. It's like the upgraded carpet underlayment. It's a thick pad that gets ribbon layered in the muffler. That material does not burn up. So the muffler keeps it's sound qualities pretty much permanently. Flow wise, they are straight through design with perforated holes in the inner tube. Perfect for reducing sound in a 'dump' system and perfect for flow.
