Just put a 18'' magna on my 07 today,2.25 dual in 2.5 single out. When i went to the speedshop thurs i was going to buy the 14'' but i let the guy at the speed shop and my buddy talk me out of it. They both were convinced it would be to loud.I should have done what i wanted because the 18'' is not loud enough. It is only slightly louder than stock. I would suggest the 14'' for a good sound. The three chamber mufflers are even louder than the straight through because there is no sound deadening material in them. It is just a hollow can.