Magnaflow oval mufflers use a "straight through" design with a perforated pipe surrounded by mineral wool packing. Their round "bullet style" mufflers use the same design except there is less packing, thus they are advertised to be louder.
Flowmasters uses chambers with no packing in their mufflers.
Chambered mufflers drone like crazy on these cars. Magnaflow...not so much. Ask me how I know...
