Venturi Mufflers
 

Flyboy Dave didn't mention the muffler brand on another post but I suspect it was the Mac's. The Semco and Fox venturi mufflers are made as I had described, with a venturi tube though the center of the muffler and slits in the venturi tube. That works because the increased airflow drops the pressure of the air below the ambient barometric pressure. Dave showed another muffler with the exhaust exiting to the rear in a cocentric ring around the venturi tube. Not sure which would work better, but suspect the Semco and Fox method would be. But if the Macs balance the exhaust area so that the outside air would flow faster than the exhaust gas, then it may work better. Hard to do for all engines so in the real world I suspect the Macs worked better on some engines and Semco better on others. Not aware of any automobile mufflers that use outside airflow to reduce back pressure as Dave had described. I would like to know more about the auto muffler.