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Part Six: The Exhaust System

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.
 

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.
 

davidwyby

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My EC saws finger ported 3120 muffler has a bunch of small ports. I want to change it to one pipe out the side. I know having the pipe further into the can is quieter, but what effect does it have on performance?

Thanks
 

Ketchup

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My EC saws finger ported 3120 muffler has a bunch of small ports. I want to change it to one pipe out the side. I know having the pipe further into the can is quieter, but what effect does it have on performance?

Thanks

My logic says a longer pipe will create back pressure. Especially if it sticks into the can. That isn’t really something I would want.

But my experience is that a pipe that sticks into the can an inch or so and out about the same pulls exhaust out faster and actually encourages the scavenge loop.

Take both with a grain of salt.
 

Ketchup

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It might be interesting to 45 the inner end of the pipe and face it’s opening towards or away from the exhaust port.

I would think that the straightest line from port to outlet would move the most gas. There could be some benefit to a separation from port to pipe. Maybe the can becomes an overflow.

I wonder at what length of tube exhaust flow starts to slow down.
 

davidwyby

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I would think that the straightest line from port to outlet would move the most gas. There could be some benefit to a separation from port to pipe. Maybe the can becomes an overflow.

I wonder at what length of tube exhaust flow starts to slow down.
I agree, turning it away would probably quiet it down.
 

Woodwackr

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Here’s a question.
What would you suggest for making the loudest, most obnoxious sound as possible?
Why?
Spook house prop saw, Stihl 170s, used outdoors
No chains are used, at least the spinny kind :p
 
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