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I am having trouble getting a precise answer- is back-pressure necessary on a little single-cylinder 40cc 2-stroke like a chainsaw or a blower? I ask because people frequently modify mufflers to increase flow (which does work, basically just 'defeating EPA restrictors, sometimes its literal plastic tabs blocking fuel-jets :P ) but there's always someone who'll bring-up back-pressure and how some unspecified amount is of benefit to this type of engine. I'd think not (since the impulse-line is the 'cyclic back-pressure' of the system) but wanted to know for sure before modifying a muffler on a brand new / high-end saw :)

PS - on these types of mufflers, in theory, can anything travel inward to the cylinder via the exhaust port? For instance I noticed I'd left my saw on a dusty pile and its exhaust was creating such a dust-cloud around it that, if in-between that rapid up&down of the piston, if air(and here, dust!) can travel inward through the cylinder's exhaust-outlet port, I can picture both Yes & No because there's gotta be suction on 1 stroke but hard to picture that pressure is equalized via sucking-in-air through the muffler!
If in normal operation, a chainsaw/blower takes in nothing (via exhaust-port) while operating, and backpressure is irrelevant, that'd mean 'optimal' was no muffler & that mufflers for optimization would be built so they could muffle as much as possible w/o restricting flow -- a tall order for non-engineers but some muffler is quieter than no muffler and these things scream regardless.. Thanks a lot, have never done a muffler on a saw as-important as this guy so really wanna ensure I've got the principles down right, going to braze a tubular pipe onto the rectangular muffler and would certainly braze a mesh-screen into it if I knew solids could potentially find their way into the muffler in some odd circumstance / weird cutting position!

[PPS - I know "It's naturally aspirated ergo compression is king" but the type of block in this saw doesn't allow changing floor height so porting the block itself is of little use, only advancing the flywheel's ignition-timing and increasing muffler/air-intake throughput can give boosts so want to ensure I don't do something that's causing as much harm as good or doing something unknown that I'm missing!]

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For a 2-stroke engine, the least amount of back pressure inside the exhaust the better. If you look at performance chainsaws (yah, it's a real thing), you'll see an exhaust system which has a broad section in the middle of the pipe, then goes back to a smaller size. This broad area allows for the gasses to speed up as its cooling down, thus creating a draw or scavenging effect within the exhaust system. The draw is a lower pressure area which pulls trailing exhaust pulses from in front increasing the performance of the saw (this applies for any 2-stroke performance build). If pressure were required, it'd be a short length of tube without any thought of scavenging.

The "myth" you are getting told about a vehicle needing back pressure is old school thinking. The idea was if you allowed the exhaust to escape too quickly, you'd burn the exhaust valve. Obviously, your 2-stroke engine doesn't have an exhaust valve so there's no worries there. In today's engines, the main purpose of an exhaust system is to direct the exhaust away from the engine in as compact a space as possible without killing performance. This provides safety for those in the vehicle (exhaust gasses can kill!), as well as helping to keep underhood temps down. Really, the idea is just to direct the exhaust flow. With performance applications, the main idea is to increase the scavenging affect through long tube headers. If you can pull the exhaust along and get it out of the way, the better for performance.

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The exhaust back pressure will be affected by the inlet to exhaust can diameter, can volume and exhaust can exit diameter as well as the flow rate.

I made an exhaust to silence - well reduce the noise from a small engine 1” dia pipe 3” long with a washer and a 3/8” hole for the exit - made a huge difference.

One source you may consider is Tuning Mini Engines by David Vizzard - he goes into tuning exhaust ports and reflected waves.

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  • I feel so moronic because I can't tell if your answer was more of a "Yes" or "No"... if I've got a tachometer on my saw, and I'm taping (and later, timing&comparing) saw-cuts ie 3-cut-speed-tests through a given log for comparing/testing.......I KNOW that every saw I've ever seen produced in the past ~30yrs+ has had, at minimum, muffler-restrictors/baffles/etc to meet EPA guidelines (and carburetors often tuned very lean sometimes dangerously so) and that it's very common-fare for saw-enthusiasts to, at minimum, remove impediments & grind/cave holes in the muff-- so wouldn't no muff be best? – 2-stroke-q's May 19 '20 at 19:58
  • Hope I didn't come across 'dismiss-y', I am interested in 'theory' and more understanding however I've got a new saw not a week old, on-the-bench and not going to bother painting muff til it's properly finished/tooled, but this guy came w/ an "open-box" muffler like literally just an empty box, the exhaust-gas is only slowed by the fact the muffler's output-slot is a [MUCH!]smaller circumference than the block's exhaust-outlet-port, so 'correcting' this (and the needed richening carb-jets) allows "up-to" full-flow from the block, up-to-user to decide what level of "blockage" to leave.. – 2-stroke-q's May 19 '20 at 20:02

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