I was looking to replace the stock mufflers on my P71 with a pair of glass bags and after some research I settled on Magnaflow for their heavy duty flow through design. Measure seven times, cut once (or order ;) once, as they always say, but it turned out these mufflers were missing about an inch of connection with the standard exhaust pipes. Plan B: Downspouts. Quiet at idle and minimal hum when accelerating at cruise, they made my 4.6 sound average and I saw an immediate increase in fuel efficiency after recycling nearly 50 pounds of scrap metal. My mistake turned out to be the best and I highly recommend Magnaflow IGUs.