I had opened up the louvers on the MS180 muffler a while back, didn't see much gain though. Finally pulled the muffler and drilled 2 1/2" holes in the section behind the louvers. It really cuts much better now. Seems like it's burning more gas too. I'm still running the stock bar and chain, semi-chisel, .46 gauge. Might try 3/8 lo pro. Kinda proud that it worked.