Installed my original 50s fentons (with straight down pipes) 18 months ago and added two aluminized 2" pipes with 30" steelpack mufflers. The place that installed it was the first one in So-CA back in the days who offered their own glass and steelpacks in various lengths.
Did some research before I made my decision back then:
Every brand creates their own sound by arranging different patterns inside the muffler where the exhaust air can enter the glass or steel fiber packed area.
I believe cherry bombs and the purple one have perforated pipes, which allow more flow.
The Smithy's have louvered perforations.
The various patterns of these cutouts determine the sound.
My steelpacks have a louvered spiral pattern for less backpressure.
Some like to turn them around against the flow for more 'bark' to the sound.
Basicallly the length determines how loud they get with increasing rpms.
I believe 26" is the most popular length.
My 30" have a nice deep dark bottom end and the steel vs. glass packing gives a more brittle metallic top end.
Glasspacks will break in earlier.
Steelpacks take longer to break in but are louder.
My friends tell me they used to fill the pipes with water to age them quicker and get them more nasty sounding.
Another old biker buddy adviced me against glass packs, as they allways blew out the packing in short order and the just ended up with straight through pipes.
