I think the USA muffler has an 8mm side outlet the Aussie 660 has 12mm or bigger I don't remember exactly.
The slit is stock on both mufflers.
Yes the mag mufflers have a baffle it depends if you like really loud saw's or not most on here do lol
I didn't like the noise I cut the baffle out of a stock mag muffler with dual port front cover I also opened up the side outlet way bigger I could only put up with it for about a week and had to go back to a stock muffler with baffle and I always wear earmuffs using a saw.
I also felt it made the saw a little gutless with the baffle cut out and side outlet opened up it lost some torque thats for sure but that was just my experience in my part of the world with my saw on a given day in the timer I was cutting.
Yeah you could make the outlet bigger then the stock EPA pea hole and just get a dual port front cover.
We were lucky down here they didn't mess with our 660's they stayed the same right up to the end no chocked up EPA crap on paper Stihl say 7.1hp for the Aussie ms660 only .1 or .2hp less than a 661 but the 660 is a better saw for hardwoods in my opinion with it's heavier clutch.
it's to easy to get a 661 chain to stop dead and clutch slip! cutting hardwood here for me lol.
This pic is of one of the last Aussie ms660 mags before the 661 came in.
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