It's been over two decades since the EPA has done anything truly meaningful, and near three decades since the EPA outlived taxpayer cost effectiveness. It has become nothing more than a corrupt heavy-hand, wielded by whatever ideologue holds power at the time... but this is a topic for another forum so I'll end it there.
As far as the muffler mod thing...
How much you will gain, or more to the point how much you will notice, really depends on the engine itself... some are more responsive to a muffler modification than others. Near always a muffler modification will require carburetor changes, and without a fully adjustable carb (which is becoming rare) that ain't so easy. Whatever power enhancement you gain comes at a cost in higher noise levels and more fuel usage... which ain't linear, a 5% power increase means more than a 5% fuel usage increase. Typically a muffler mod begins with removing the muffler, measuring the exhaust port to determine total area, then modifying the muffler outlet(s) in some manor to 70-80% of that, and finally making carburetor adjustments... opening the muffler outlet(s) further than the "magic" percentage will actually hurt performance. Be warned, if done improperly it has the potential to damage, or even destroy the engine.
Before the EPA regulations on small engine emissions (noise and exhaust) manufactures built them how
they believed would provide the best balance of power, reliability, longevity, and whatnot... and most made some darn good power-to-weight. There was still room for performance improvements for the
true motor-heads to mess with, but again, those improvements came at a cost... typically in longevity. In order to meet the new regs, without spending billions on R&D, manufactures restricted exhaust systems... which in turn reduced power output and caused engines to run hotter... which in turn necessitated some changes to internal engine design and carburetors... which in turn reduced the power-to-weight ratios. That's not to say manufacturers haven't continued to make improvements... with some notable ones in just the last few years. But, under EPA regs, they will never be what they could be (heck, most aren't even what they were yet)... and likely the EPA will soon hit manufacturers with new, more restrictive regulations that will degrade performance even more (and likely make it more difficult for the end user to modify... which, by the way, is illegal in most areas of the country).
I've modified the exhaust on a couple of my 2-cycle engines with positive results (not my chainsaw though, it's pre-EPA), but I can't tell you what to expect by muffler modding your MS192 and MS290, I simply don't know... I don't even know if they have adjustable carbs. Your best bet would be to take this to the
Chainsaw Forum and ask the guys that hangout there... you'll probably find the best experiences with both saws, along with opinions, advice and direction (including some "
been-there-done-that").
