i dont know much so i cant give a good explanation but they have an adjustable header so you can tune it to what rpm range you want and different boat setups and different engine modifications require different lengths, and yes most don't have silencers because i suppose they would be cheaper without them and most people like their boats loud (i think) but alot of people also run stinger mufflers/silencers, and there quite a few pipes with built in mufflers such as some of the ones Mike listed, jeroentune, and there are a few others that i cant remember

heres one of the stinger mufflers if you don't know what i am talking about (down the bottom of the page)
http://www.redlinemotors.us/#/pipesa...tpa/4521241598