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Part Six: The Exhaust System

Cerberus

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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 :p



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)
 

Cerberus

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Couple quick Q's:

- Generally speaking, would it be preferable to have more total exits on a muffler (if the total exit area were a fixed-area?)

- I'm VERY interested in the effects of volume of the muffler, for instance if your blowdown required 24.5deg for near-atmosphere in the cyilinder, surely it'd require LESS time (IE more time for transfers!) if a given exhaust-port-area somehow "moved quicker" and of course a bigger muffler-body means less back-pressure to the expanding charge means less resistance means quicker blowdown (in same degrees of crankshaft rotation)

Thanks a lot for insight on either am having trouble finding info on these 2!
 

Cerberus

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OH Dyno Joe!! Thanks am glad to re-watch that one, LOVE his content, at any rate I thought that one was very interesting because if you look at the middle evaluation (4:45) you'll see that, of the 3 real 'short-pipe' mufflers, the 1 OEM box muff, and the megaphone, things were as-expected for all-but-one:
- short-pipes were best 3, then well-below them was:
- OEM box muffler, but slightly-below that was:
- megaphone style,
so I think that right there negates all of the "no muffler could be made an optimum" thinking (right? Since the megaphone would be the ultimate of that, I guess you could argue it causes too-much suck, therefore evacuates and subsequently pulls-out too much but that's a hard thing to think IMO, but very eager to hear you guys' thoughts on these things!!
 

Dieselshawn

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Cerberus: The baffle is Stihl’s attempt to meet epa emissions. It slows the exhaust pulse coming out of the cylinder to keep the fresh fuel/air mixture in the cylinder longer until the piston closes the exhaust port.

By doing this also, it forces the piston to do the actual pushing to get the mixture up the transfer ports.

The saw will run fine this way but it’ll be anemic power.

The baffle also quiets the saw down more.

Think of the big v8’s of the 70’s when the government forced the manufacturers to be epa compliant.

Big turd engines. Only enough power from the big displacement torque to move the heavy vehicles.
 

Ketchup

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Good stuff. Just thinking here…

The exhaust has two distinct forces moving outward on the down stroke. One: the sonic wave, Two: the expanding gas wave.

The sonic wave moves very fast. It probably exits the port, hits something in the can and then bounces back into the exhaust port before the transfers even open. Maybe slightly later. That should be easy to figure out. I could see how a concentrated sonic wave coming back up the exhaust port could slow the gas pulse. It’s feasible that diffusing that wave could allow gasses to escape more quickly. This may help to explain why a straight pipe improves performance: the sound wave almost entirely escapes. I may be grossly exaggerating the sound wave effects though.

The gas wave is moving out slower. It needs room to expand and is highly affected by friction and turbulence. I think most stock mufflers have too much in the way for the gas wave to fully exit the chamber in the time allowed. Both too much turbulence and not enough space. The outlet size of the muffler dictates how much exhaust gas is trapped in the can per cycle. Enlarging the muffler outlet allows more gas to escape per cycle.

Unless the interior of the can has too much turbulence. Baffles, scrubbers and sharp changes in trajectory hold the exhaust inside the can long enough that the next cycle is already pushing more exhaust in before the previous cycle has cleared.

If the exhaust gases cannot escape rapidly enough there will be a point on the upstroke where the expansion of those gasses overcomes the transfer pressure and they back flow into the chamber (or never fully exited).

In the reverse of the blowdown phase, exhaust pressure in the can could also be greater than the upstroke and back flow could occur.

I guess I see some benefit to back pressure in the can because it will aid the build of chamber pressure on the upstroke prior to the exhaust closing. It does seem like there is a sweet spot where the transfers flow optimally, close and then chamber pressure begins to build during the reverse of the blowdown phase.

I think pressure in the can could still be pretty low at that point and the muffler outlet size can control that without the baffle. I guess the sound wave might cause less problem with a baffle, but it seems like that can be addressed without a baffle that inhibits gas flow.
 

Ketchup

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Couple quick Q's:

- Generally speaking, would it be preferable to have more total exits on a muffler (if the total exit area were a fixed-area?)

- I'm VERY interested in the effects of volume of the muffler, for instance if your blowdown required 24.5deg for near-atmosphere in the cyilinder, surely it'd require LESS time (IE more time for transfers!) if a given exhaust-port-area somehow "moved quicker" and of course a bigger muffler-body means less back-pressure to the expanding charge means less resistance means quicker blowdown (in same degrees of crankshaft rotation)

Thanks a lot for insight on either am having trouble finding info on these 2!

I think a single circular outlet will always flow the most at a given area. It has the least turbulence.

As I see it, engine displacement, port width, volume of the can and the size of the muffler outlet all work together to determine how much blowdown is needed. Open the outlet and widen the exhaust and you should need less blowdown. A high RPM saw probably wants more blowdown than a torque monster.
 

Ketchup

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In regards to the speed of the sound wave, I did some crude maths.

Speed of sound= 343 m/s (in dry air at 20C)
= 34300 cm/s

Distance from Port to Muffler wall = 10cm (just an easy number that's close-ish)

10000 RPM = 166.66 RPSecond (that's fast!)

If it's 10cm across the muffler, and 10cm back, the sound wave will travel 20cm before interfering with discharge.

20cm/34300 cm/s = 0.00058309…seconds to travel 20cm.
(Probably got that wrong)

1 revolution is 0.16666 seconds/360degrees = 0.00046294 seconds per degree.

If my crude math above is correct, it takes 0.00058…seconds for the sound wave to get back to the port opening. The port is opening 1 degree every 0.00046 seconds.

0.00058309/0.00046294= 1.2…degrees.

So the strongest sound wave mostly hits the piston wall. The arc of the port roof probably isn’t even at full width yet. I guess there are many reflective waves after this, but they probably diffuse pretty fast in effect.

And this is a hotter and denser atmosphere, so the sound wave is moving faster.

My figuring makes me think the sound wave has a minimal effect when dealing with a can muffler. The geometry of the can isn’t a perfect mirror, and the sound wave moves too fast to effect the main volume of flow.

Rip all the baffles out and put a big hole in it.
 

huskihl

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In regards to the speed of the sound wave, I did some crude maths.

Speed of sound= 343 m/s (in dry air at 20C)
= 34300 cm/s

Distance from Port to Muffler wall = 10cm (just an easy number that's close-ish)

10000 RPM = 166.66 RPSecond (that's fast!)

If it's 10cm across the muffler, and 10cm back, the sound wave will travel 20cm before interfering with discharge.

20cm/34300 cm/s = 0.00058309…seconds to travel 20cm.
(Probably got that wrong)

1 revolution is 0.16666 seconds/360degrees = 0.00046294 seconds per degree.

If my crude math above is correct, it takes 0.00058…seconds for the sound wave to get back to the port opening. The port is opening 1 degree every 0.00046 seconds.

0.00058309/0.00046294= 1.2…degrees.

So the strongest sound wave mostly hits the piston wall. The arc of the port roof probably isn’t even at full width yet. I guess there are many reflective waves after this, but they probably diffuse pretty fast in effect.

And this is a hotter and denser atmosphere, so the sound wave is moving faster.

My figuring makes me think the sound wave has a minimal effect when dealing with a can muffler. The geometry of the can isn’t a perfect mirror, and the sound wave moves too fast to effect the main volume of flow.

Rip all the baffles out and put a big hole in it.
You’ve mathed why pipes are around 2’ to the reflecting cone.
A can-type muffler only has the ability to build back-pressure, not reflect any sort of sound wave (in a beneficial way, at least). Back-pressure does give a sense of added torque or forgiveness, though. On the same saw, it takes more levering to bog with a restrictive muffler than what one with a wide open muffler can take
 

CR888

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Tuned pipes are so long and bulky for scientific reason..cans are way small to utilise sonic resonance.
 

Terry Syd

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I'm wondering if it is the 45 degree angle that the exhaust wave hits that prevents a return wave from pressurizing the exhaust port. This is an interesting concept that needs some dyno time.
 

davidwyby

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I figured all those 90° bends were bouncing fuel back into the port like a pipe. Hadn’t thought about the 45 much. I know Ben is super busy right now but maybe he will comment. I intend to try one myself.

@Red97 is there a saw in my sig that you also have or have access to? I mainly want to try it on my 7900.
 
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