This flexpipe is to be used in place of the stock manifold pipe between the engine and the muffler. It's used to route the exhaust around and out of the cowl, instead of cutting a large hole in the cowl for an unsightly muffler. It has threads at both ends, one end to screw into the head, one end to thread into the muffler. I don't think these were meant to replace a muffler, although I'm not sure a four stroke engine really requires a muffler. But, as you said, if you ran this flexpipe exclusively, you'd have no way to pressurize your fuel tank. If you get one and do try running it without the muffler, please post your results here, as I have always been curious about running a 4 stroke unmuffled. Rich