Like PE said, we have no real data about the stock muffler VS a Bisson or Slimline. All we have are opinions with nothing to back them up. The question about the OS 120AX and the muffler choices has been asked before, more than once and no one is able to provide any data like RPM numbers or DB numbers. Of course it would require someone to test the engine with the different mufflers and several props with the same fuel on the same day and be able to take reliable measurements. This would require someone to spend considerable money on all the items, (engine, mufflers, props). Most individuals are not going to do this just for grins. Perhaps a magazine engine guru would do some testing along these lines but I doubt it. So we are back to where we started with interpreting individual opinions we read on these forums. Oh well.