H-Ds have whats called 'knife-fork' rods , one rod sits inside the other. You have 2 cyl. so , 360 degree times 2 = 720 . One cyl. fires at 315 degree and the other fires at 405 degrees ,there is your sound ! Put your hand {with a glove} on the end of the front exhaust pipe ,and notice how many times it fires ,now do it on the back pipe , ALOT LESS , YES ? Each motor ,has its own sound {knucks ,pans,shovel,EVO ,TC } do to there heads and cams ,pipes ,etc. In my opion , the shovels had the sweetest sound ,just don't be in a closed garage with one running ,because you will die from the gas fumes ! Only 1/2 of the gas in the combustion chamber burns ,unless you 'dual plug' them !