I know their is a pefect amount of tubing after the collector that wiil give you the most hp but alot of r&d goes into it with little results,The best thing we have found is 3 in tube 2 ft after collector with any blow thru muff, glass packs work if your into that but run them backwards so the air going thru isnt
disrupted.
Also have seen guys poiinting their turn downs at each other to cancel out each side for noise but flow the same.