Could someone help me understand the need for I/D exhaust size to be larger than what comes out of the motor itself? doesn't the exhaust gas get cooler the further it gets out, lessening the need for larger pipe? if you goto smaller pipe after pre-cats or y-pipe, aren't you creating more force back towards the motor? (maybe this is necessary for the "dynamic supercharging")

 

as exhaust gasses cool. they slow down .. creating a wall of slower air that is constantly getting interuppted by fresh burn. creating a back pressure...

path of least resistance up to a point of muffling is the ideal situation for performance and some 'back pressure'. constricting too soon can lead to poor exhaust velocity and slow down the effiency of the process. tuned exhaust give more power