i"don know about this, but for my observation & story from some trusted people, why the big size pipe are used by a turbo engine, because they want to let lose the presure faster so they unharm the turbo when the engine at max rpm. But for the na engine there are many reson to used short pipe, or big mufler. For xample drag racer used a short pipe to get max rpm at a short time. But for daly used i sugest u not to used too big pipe for the na engine. using much smaller pipe might couse little damage to engine because of the back presure is to high. Na enrine need back presure but not to high back presure. Anyway this is just a tougt. So correct me if i'am worng.