When you "Open up" your muffler properly you should get a good power gain,but also you are accomplishing another really good thing-you are evacuating the heat out of the cylinder and engine much faster,upon a top end teardown I can see the piston has been running much cooler with less signs of overheating.In a motor with a really restrictive stock muffler you will often see signs of the piston overheating,PLUS a lot of carbon buildup on the piston crown and carbonizing around the rings and their grooves.
The best way that I modify any muffler,be it on a Stihl,Husky or any other is basically the same;
-I will order and use 2 of the "Deflectors" and screens from a 288 husky,they are worth the cost.
-Prepare to mount one on each side of the muffler at the front/top. Hold each deflector in place where you want to mount it,then using a small scribe,mark the inside of the deflector hole on the side of the muffler,because you are going to cut out the hole to full size before you attach the deflector and screen. I use a small tip on an acetelyne torch to cut out the hole,then finish trimming it nicely with a dremel. TIP- Don't use an actual acetelyne cutting torch,just use a small normal torch tip,turn it on to a normal flame,then start opening up the oxygen more until the flame is really "Hissing" and way oxygen rich,you have to turn the oxygen as high as possible to JUST BEFORE the flame "Pops" out. Now you have a really small,controllable cutting flame.You can easily cut the outlet holes out,but more importantly on the Stihl mufflers you can cut out any baffles and junk they have inside easily!
-Lastly,I lay my deflector(With the screen under it) in position on the muffler over the full-size hole I made and then I braze it down into place,both sides.
Then give the muffler a nice coat of high-heat(1500 degree) flat black paint.
I also use this method on the Husky 372 type muffler with the top outlet,so I end up with 3 outlets,works great.
I prefer to install the screens as I described for safety,they will never plug,and you can always wire brush them clean.
The 288 baffle just clears the chain brake handle on most models(Right side),and it directs the exaust away from melting anything.On the left side of the muffler you usually have to trim a small part of the top cover away to clear the baffle(Leave about 1/2" clearance),I mark it with a felt pen,cut it out with a hacksaw,and touch it up for a nice finish with a file.
This style muffler really roars and makes impressive power. If you if you follow this method you have a nice professional looking muffler with a somewhat factory appearance!:msp_thumbup: