PHYSICS to correct a poor previous (no gain) Muffler Mod in a OEM muffler.

[one.man.band]

resonant cavity: Cavity Resonance

Polarity of waves: Issue of assigning polarity of the system is where the puzzling point is for me. In agreement with the polarity concept as you see edisto. My picture of the what the entire system resembles: during the piston cycle, the exhaust port alternates between a 'two end open' pipe, to a 'one end closed' pipe. The polarity is....? The second half of the system being a resonant cavity during the times where the exhaust port is blocked by the piston, and a 'two end open' pipe when it is not. The polarity is.....? Complication to this system is also due to the reflected waves (90 degree) from the front face of the muffler. Would these be + or (-). Not sure where any of these waves change or retain polarity.

As woodheatwarrior mentions, the amount of reflected waves out of the exhaust port hitting a 90 degree wall and re-entering at the right phase is a long shot due to being outnumbered by the amount of other frequencies being produced. During the piston cycle, the muffler could behave as a resonator for a very short time (2 times per single piston stroke). Because at say 9500 RPM, the piston is moving at 158.33 Hz (158.33 times a second). Trying to exploit the fact that once the resonant frequency is reached in the muffler, it will continue to oscillate, through the times where, the muffler is not a true resonant cavity. The muffler is (and most all are) greater in volume than a single piston stroke of exhaust gases. The damping of the previous exhaust gases (which outnumber the single piston stoke volume), in the muffler should help keep it in resonance. (Counting on that anyway). As you said, due to RPM changes, this effect would be diminished most likely.

Hope you are wrong on your last statement edisto. (Crossing fingers). Hahahaha. Don't enjoy the pipe flow calculations at all. Did enough of those 25 years ago.

There is a great research paper linked that I just found linked below. Crankcase volume/intake/exhaust tuning. Maybe you guys have it already. Given me a new area of frequency, to investigate: Pulse timing. Worked out some more calculations which will be posted. Check out Figure (12). The correct amount of phased pulses has a big effect of delivery ratio and scavenging as well.

 

[sachsmo]

That's actually physics...


...just sayin'

Hahahahhahahaha,


Ok,

No fancy math.

 

[one.man.band]

One limiting factor in tuning the muffler is obviously the point that it is shape a box, and not like a pipe. The other is lack of sufficient length to tune it using regular pipe formulas.

no fancy math. as per sachmo.

(from my calculated values).

pulses range between 2.88 to 12.33 from 9000 RPM to 16500 RPM. optimum being 3. <(per research paper). lower is better.

tuning lengths between: 3" to 7" were used to get those numbers. the better numbers of pulses relate to the longer pipes. the shorter the pipe relates to the higher # of pulses. opposite is also true.

tuning frequency will need to be computed and adjusted to reach the optimum 3 value in all cases.

thoughts?

 

[Terry Syd]

The main thing I got from the Nagao & Shimamoto paper was graph #14. That graph shows the effects of blowdown on the delivery ratio of the engine. You can see the one curve where the engine is not firing, only motoring along. The other graph shows what happens when you ignite that mixture and create extra volume with the exhaust gases.

The key point to realise is that graph was done with a straight pipe, not a muffler exhaust. If there had been a restrictive muffler the delivery ratio would have decreased even more. Thus, I believe that a chainsaw (especially one with low blowdown figures) benefits greatly by having a less restrictive muffler.

The paper does show the tremendous advantages of tuning the intake. I found that out when I was tuning intakes, the delivery ratio jumps right up and the torque increase is quite pronounced. I have always thought that for saws, there was something left on the table regarding a tuned intake.

EDIT: One of the things that mitigates against having a tuned wave from a muffler is the speed of the gases coming out of the cylinder. The exhaust gases are traveling at about the speed of sound. A reflected wave would thus have to travel back through a medium that is moving at the same speed, in other words, it is impossible for the wave to return while the engine is still in the blowdown phase. Since we can't get that negative wave to return during the blowdown phase, the best we can do is to make sure there is as little backpressure as possible so that the exhaust gases can exit.

 

[sachsmo]

Ma MOPAR did a bunch of work with intakes back in the late '50s early '60s.

Them ultra long cross rams and the shorter ones on the SS Hemis show there are gains to be had on the intake side.

They called them long ones sumthin' like Sonoramic.

 

[Terry Syd]

Yes, there are definite gains to be had on the intake side. Many of the newer cars and bikes have tuned intakes and air boxes. There have also been some intakes that were variable, that is, there was a valve that was rpm controlled that opened up more volume when the rpm dropped to a certain level.

I used a long hose on the intake manifold to increase the volume. Depending upon the volume, I could place that boost of power where ever I wanted it. I used a long hose that gave a boost very low in the rpm range to help the enduro bike lift over logs and rock faces. I even put a ball valve in the hose so that I could bring the boost in higher for motorcross work.

On a saw the same thing could be applied. A long length of hose to bring in the boost around max torque in order help with bucking bigger logs. A valve in the hose could decrease the volume to bring in the boost higher for limbing.

On a smaller saw, the hose could be fitted up inside the handle so that the mod would be out of the way.

 

[one.man.band]

cut from terry above.

EDIT: One of the things that mitigates against having a tuned wave from a muffler is the speed of the gases coming out of the cylinder. The exhaust gases are traveling at about the speed of sound. A reflected wave would thus have to travel back through a medium that is moving at the same speed, in other words, it is impossible for the wave to return while the engine is still in the blowdown phase. Since we can't get that negative wave to return during the blowdown phase, the best we can do is to make sure there is as little backpressure as possible so that the exhaust gases can exit.

Optimum of 3 pulses: The paper did not assign pulse directions. Just stated fluctuations. If the fluctuations were: out-in-out

trying to draw this here.

Exhaust port end (piston side). Exhaust port end (muffler side).

.................> 1 out (piston downstroke)
<............... 2 in (bounce reflection supercharge intake effect)
.................>3 out (also phased with piston down stroke of next cycle)
and repeat

what i read: to phase the exhaust blowdown pulse, with exhaust period of same cycle, and at the same time, so phase remaining fluctuation after exhaust port closing with blowdown of next cycle.

for a motor turning 9500 RPM, piston 158.33 Hz.

if muffler could be tuned an to get exactly 3 pulses per stroke. it would most likely be tuned to the 3rd harmonic of 158.33Hz. 3 x 158.33Hz = 474.99Hz.

due to damping, x-sect area change, and other factors it would be difficult to do. only if the muffler could behave as a tuned resonator.

 

[edisto]

The main thing I got from the Nagao & Shimamoto paper was graph #14. That graph shows the effects of blowdown on the delivery ratio of the engine. You can see the one curve where the engine is not firing, only motoring along. The other graph shows what happens when you ignite that mixture and create extra volume with the exhaust gases.

The key point to realise is that graph was done with a straight pipe, not a muffler exhaust. If there had been a restrictive muffler the delivery ratio would have decreased even more. Thus, I believe that a chainsaw (especially one with low blowdown figures) benefits greatly by having a less restrictive muffler.

The paper does show the tremendous advantages of tuning the intake. I found that out when I was tuning intakes, the delivery ratio jumps right up and the torque increase is quite pronounced. I have always thought that for saws, there was something left on the table regarding a tuned intake.

I read this paper awhile back, and what stood out for me was the finding that crankcase volume did not affect peak delivery ration...it only influenced the engine speed at which it occurred.

As Terry points out, the other eye-opener is that there is much more to be had by tuning the intake than there is by tuning the exhaust.

 

[J.W Younger]

I may be missing something here but it seems to me to put this much and effort/ thought into maxing performance using any type of can in the ex system is going to be marginal at best.Also when you move on to other areas such as crancase volume, port timing, intake, etc. it will change what worked before.

 

[weimedog]

opcorn: always wonder when all the theory is done and it gets to action..what happens..wonder how many geometric special conditions basic formula can't emulate...so the theory might point in a general direction but trial and error & empirical data ultimately wins doesn't it? opcorn:

 

[openloop]

The events happening inside a box shaped muffler are tantamount to chaos, I.E. immeasurable. Any discussion trying to decipher the reverberations contained within is a futile attempt. The shear number of harmonics present inside that muffler ensures ANY gains from wave tuning would be minimal or within a very narrow rpm band....so narrow it would lose all practicality. The ONLY way I could see wave tuning inside a box muffler happen is if a coiled up length of tubing was used to adjust the port length within the muffler. But as others have said more gains could be had by running a proper tuned pipe and intake length.

 

[one.man.band]

opcorn: always wonder when all the theory is done and it gets to action..what happens..wonder how many geometric special conditions basic formula can't emulate...so the theory might point in a general direction but trial and error & empirical data ultimately wins doesn't it? opcorn:

fixing to do just that. idea floating in the brain to weld up a test stand. do not have the funds to divert to a complex system with data logging. simple peak torque only. my idea is based somewhat on a simple prony brake system. using the saw power-head driven into a very small diameter disk or mechanical drum brake. get the saw spinning and turn a bolt (or air compressor pressure regular) up until motor slows to a chosen rpm. the # of turns in of the bolt could be seen, or the pressure regulator setting. finding the point of max torque by way of brake pressure on the drum or disk. can not get an actual #, using this method, but with a tach and laser thermometer, could get an idea of improvement or worsening. the more brake it can handle, the stronger it is at that rpm (if i can actually hold it to an fixed rpm).

the stand and especially guards on everything will be overbuilt, because i do not want the drum or disk, bearings, shafts blowing apart at those rpm's (which will most likely happen). idea is to enclose them within a welded box. could use some ideas for the drum or disk. smaller diameter the better. any ideas appreciated.

will buy 2 or 3 stock muffler mufflers to tweak. measure stock torque. frequency tuned. and pulse tuned. have the rest of the winter to monkey with this. (unless my big willow tree decides to suicide).
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like the idea of intake tuning as well. correct tuning length to get a 3/4 wave will mostly require more length than is available. the needed port time area was enlightening. hope someone will pursue this further. great research paper. there are many more since then, but pay per play.

 

[one.man.band]

need some help on the drive system. do the small dynos run a damping coupler (do not know the correct term) like a large electric motor? would the saw having anti-vibe handles be enough? first thought was to run a chain. tensile strength of chain might be an issue. keeping the chain aligned true to the gear would be another if using stock clutch. would like to be able to run the stock clutch if possible.