My arc dump valve has an adjustable bit on top , undo the lock nut and adjust the threaded part but what for ? is it just for loud or quiet dump sound ? if you adjust it so its not so loud then is there any purpose in having it , or does adjusting it do something else??

 

Its normally to adjust the tension of the spring. Making it harder or softer will affect the level of boost needed to make it vent. It is also used for cars that don't idle great with DV's.