If noise isn't a concern, you don't really need a muffler at all. A piece of pipe or even just the stub will work fine, but will be quite loud. I'm running both my YS.91's with no mufflers at the moment.....just an exhaust deflector zip tied onto the muffler stub. I don't know how long the deflectors will last though....one of mine has just disintegrated. Of course, if you need muffler pressure though, you're stuck with one.