Compression Ignition Addict
Join Date: Nov 2011
Location: The great state of Warshington
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Won't notice any difference performance wise between a 4" and 5" pipe. The max DP size (without custom fab work) on an OBS is 3.5" with most people running a 3". That is going to be your restriction point. An addition inch after the downpipe won't effect the CFM that the exhaust can flow.
You will notice a difference in sound. 4" will let the turbo sing a little more, and the 5" will have more of a rumble to it.