This is for nofear69, There would be virtually no difference to the engine reguarding primary diameter with zoomies on a naturally aspirated engine. simply put zoomies are zoomies, diameter be damned. As for the tiny 2.125 primarys on a 582 racing engine with a step to any diameter would work ok on a 500 to maybe 600 horsepower hyd cam mild engine in a heavy street car. That small of pipe may not even clear the entire exhaust port on most racing heads now a days for a engine this large. The 582 will require a 2.25" primary in most any power level baring the previous example. The 4 in to one collector helps scavenge the spent gasses out to help aid in initiating the incoming air charge. zoomies do nothing for a naturally aspirated engine but make a cool sound according to some, or so I am learning. I hope this additional explanation helps my earlier entry make sense.