I think "flowmaster" is being used as a general term to describe people with 2 chamber or american thunder set-ups. When making a comparison to flowmaster, it makes a world of differance if your talking about a 2 chamber american thunder muffler (p/n 942543) and a original 40 series 2-chamber (p/n 42543). The american thunder kit comes with 942543's, there is no kit that comes with 42543's, flowmaster's loudest muffler. A friend of mine got an american thunder kit but welded 42543's into it and it is crazy loud. So when your talking american thunder, remember they are quite compared to 40 series 42543's. just my .02