Originally Posted by RDN
haha no it isn't dude 3.5mm or .14in is a very minimal difference in exhaust sizing.
Min = Mout from fluid dynamics, so a 3.5mm difference in radius would mean that a 12% difference in sectional area between the 60mm pipe and 63.5 mm pipe.
how that effects flow and the velocity of the fluid is pretty complicated, but it's not a small effect.