Thanks for the link Ketchup, am blown-away at how much great stuff I'm seeing here, I have to leave a couple replies for later to keep this from being too long but there's too much that's "almost on-point" here!!
I don’t totally understand back pressure in a can muffler. It doesn’t seem like a can is able to back-stuff the cylinder like a pipe.
It actually
is able to, it cannot help it, when the exhaust pressurizes the muff it can hit like 7PSI in there (and, on a tuned-pipe, the divergence-cone's wave - the 1st of 2 waves from an expn.chamber pipe - the div.cone wave causes suction/pull on the exhaust port, not only pulling exhaust but actually aiding movement of charge up through transfers as it creates vacuum in the jug)
BUT we do not have tuned pipes....and boxes would be near-impossible to tune for sonic/acoustic reliability,
this is why I suspect we're seeing so many more 2-stage mufflers w/ heavily perforated baffle-walls, to "break-up" the exhaust pulse, because even with some obnoxiously over-sized muff outlet you still have a constriction on the exhaust-port meaning the charge will pressurize the can
and bounce back at the flange, this is desired/tuned-for on pipe mufflers (expansion chamber type) but we cannot do that w/ boxes, so my suspicion/my bet is that the OEM's are simply aiming to diffuse the bounce-back (since it cannot be controlled/used, nor eliminated) actually to that end I just made a baffle-wall plate for sandwiching between the front&back portions of a 660 muff,
tons of perforated holes, should break-up the exhaust pulse nicely --- w/o exits near the force of that initial pulse pressure, that's how you get chaotic turbulent exchange at exit(s) port(s) on the muff and introduce atmosphere that may end up mixed back in.
My crude thinking is that a saw that blasts all the exhaust out should run the same, no matter how much raw charge makes it out after the exhaust. Have you guys tried tuning a saw with no can? I’m tempted to think it will make the same power if properly tuned.
IF we presume the carb & fuel and all that were setup to deal w/ higher throughput (which'd absolutely be the case here), and setting the port time-areas to properly feed it, if you got this setup/optimized/tuned I still do not suspect it could beat a properly done can-muffler, because:
#1 - it seems can mufflers
can be setup to give some "static"(well, spread/weakened) return-pulse from the exhaust, giving a low intensity / extended duration "plugging" effect (nothing like a pipe's baffle-cone but better than nothing) On a pipe-setup you want your 'pressure bleed resistance' (inlet//outlet ratio) to be 100inlet//58-62%exhaust the exhaust (exit or outlet pipe on a real exhaust) is of course going to want more "resistance" than us (because we're not utilizing enhanced push/pull like a real pipe) but we are using
some, which leads me to:
#2 - By proper design of the exhaust tract we can most definitely "create suck" which means that, once blowdown/pressure-equailzation is achieved, you're going to have atmosphere getting muddled-in as piston's around BDC (when it'd otherwise be getting charge//exhaust mixture)
I wonder if any of the saw competitions have a "no pipes" class? If so, whether they use boxes or not would be the answer right there
I guess too little back pressure could inhibit scavenging. Especially if the transfers weren’t aimed well.
I suspect that, while we get little help from post-port exhaust components here, I'd expect we get far more pull (from divergent exhaust-tracts) than we get plugging/pressure to prevent charge-spill (and if I understand it correctly we basically want minimal plugging anyways, not zero but not a lot, because our scavenge-loop setups are kinda predicated around our overly-square designs (wherein loop-efficiency isn't as important, short-circuiting not as problematic, etc compared to longer stroke/tighter bore)
Re transfers being aimed poorly, you can aim them so bad you entirely short-circuit the new charge w/o even disturbing the exhaust left in the upper portions of the jug (even if blowdown is achieved / pressure is 0psi, there's still volume at top portion of jug that'll still be filled w/ exhaust that needs to be 'swept out', am surprised upwards-angles aren't more aggressive- even more common, many saws just have directly horizontal uppers...)
@Dieselshawn , have you put the 661 on the dyno after a carb reset with no muffler?
This of course assumes the rest of the saw can keep-up (ie even if it didn't crush performance, it'd be fair to expect a very marked increase in fuel consumption)
A 2 stroke needs some back pressure to prevent some of the air/ fuel charge from being sucked out of the cylinder by the high speed exhaust pulse.
What do you think of the idea that more modern OEM muff designs are moving to these 2-portion, perforated-wall-divided mufflers? I've been theorizing it's an attempt to "diffuse, spread&weaken" the exhaust pulse, since we cannot time/control it (or get-rid of it) so, instead, the goal would simply be to aim to make it so the backpressure @flange is as consistent as possible, whereas the polar opposite would be to make a nice flat mirror-finish wall to aim dead-center back at the flange-- this would mean you'd get a short, pronounced plugging effect (but it'd hit the piston at different points in its rotation base on what RPM you were at, & could not be tuned like the baffle-cone's wave on a real pipe, so modern can-muffs are aiming to spread/make-consistent the backpressure, turn a strong/brief pulse into a consistent / smooth / weaker back-pressure!)
I’m not convinced the saw loses more fuel without a muffler than it replaces. If the transfers are clearing the whole chamber of spent gasses, then the chamber should be full of fresh charge. It seems more likely the scavenge loop is short circuiting and spent gasses are staying in the chamber while fresh charge gets ejected.
But that's normal, once on compression/upstroke the mixture - at *best* (based on relative crankcase//cyilnder volumes) would be like 80%, presuming no chatoci intermingling of exhaust&charge (which is not how it actually works, but thankfully the more over-square it is, the less sensitive to short-circuitign or asymmetrical transfer loops and the like, "bulk flow" seems to become the prime factor)
)
