Here's my take on muffler mods.
Some mufflers, especially from earlier saws produced when EPA standards for power equipment weren't nearly as "tight" as they have been recently, may not need any help at all, and opening them up just increases the noise level instead of increasing power. Folks tend to confuse or associate increasing noise level with power increases, it's that way with many things related to mufflers and exhaust systems. By design a muffler may actually help scavenge the cylinder and reduce tendencies for "reversion", so just whacking a big hole or bunch of holes in one making the saw obnoxiously loud and no back pressure at all may do nothing or could be detrimental to engine performance.
Modifying mufflers can vary from a minutes work to very intrusive/labor intensive.
Far as Echo saws go, the CS-330/360T's are a good example of a very labor intensive muffler to modify, involving cutting them apart, removing the CAT, opening up some internal holes, then MIG welding it back together.
The CS-590/600/620's just a few minutes to open up the deflector slightly.
The CS-490 is described above and doesn't really need a lot of help.
CS-510's have an internal "pipe" to remove and open up exit hole slightly and the deflector.
CS-370/400's have a CAT to dig/saw out and open up the exit holes and deflector slightly.
I'm not a big fan of drilling extra holes and like to make internal mods to mufflers if/when needed, but still stock exit location and at most opening up the exit hole some.
It is best to sneak up on those things, making very slight modifications then do some testing, looking at increases in timed cut performance, etc. Try not to associate louder with more power, without some sort of controlled testing it may be difficult to really know if we're helping the saw out or not.........Cliff
