Making a muffler

[pdq67]

This is posted for Big Joe !

Joe,

I really love the cackle of the old Walker/Dynomax/Thrush smooth perforated inner pipe REAL Old-Fashioned glass packs.

Say like their 2.5" in and out that are long like 20" or maybe longer down each side.

How bad will these choke a 550hp, 496 BBC down to?

If they are TOO loud, then I will add a 12" down each side end to end to act like a resonator.

I really can't see them killing all that much power because of being smooth perforated, but I will concur to you.

pdq67

[ptuomov]

Exhaust video update by John: https://www.youtube.com/watch?v=zq6VwW_p-zc

The rear muffler can project is shown at the 5-minute mark:

https://youtu.be/zq6VwW_p-zc?t=5m

The stubs protruding into the can will be mostly cut off. He'll wrap a perforated steel sheet in an oval "funnel" cone around the three pipes. Then, there will be two thick layers of stainless steel mat, and kilo or so of the most durable acoustic packing. A small condensation and acid drain hole in the bottom of the end plate.

[pdq67]

Are you Big Joe 1?

pdq67

[pdq67]

This is posted for Big Joe !

Joe,

I really love the cackle of the old Walker/Dynomax/Thrush smooth perforated inner pipe REAL Old-Fashioned glass packs.

Say like their 2.5" in and out that are long like 20" or maybe longer down each side.

How bad will these choke a 550hp, 496 BBC down to?

If they are TOO loud, then I will add a 12" down each side end to end to act like a resonator.

I really can't see them killing all that much power because of being smooth perforated, but I will concur to you.

pdq67

OR D/V here!!!

PLEASE POST BACK BOTH OF YOU.

pdq67

[ptuomov]

Advance apologies to pdq67 about me polluting his thread with these muffler design links. Here's a link I'd like to share:

http://file.scirp.org/pdf/JFCMV_2016071214300784.pdf

I think this is relevant for muffler design for a high-compression engine with early EVO, and possibly more generally. The study looks at how shock waves, in particular exhaust blast waves, propagate in a muffler. For attenuating shock waves that are generated at the exhaust valve open and shortly after that, a large muffler can with a straight-thru perforated tube and acoustic packing material looks like the best solution, better than more complicated multi-chamber models. Is this why say Borla just makes that one kind of muffler?

[j-c-c]

That link is in the deep end for me, but after a quick review, it seems to center primarily on the issue in car exhaust concerns in the authors words: "Such noise is also referred to as high frequency exhaust noise," and also the "slope front entrance" being a positive feature, which I always felt that a "decoupled" entrance was more preferred. I think the difference here is the author is focused on high frequency generating blast waves, and is not addressing the low frequency stuff, intentionally.

[ptuomov]

That link is in the deep end for me, but after a quick review, it seems to center primarily on the issue in car exhaust concerns in the authors words: "Such noise is also referred to as high frequency exhaust noise," and also the "slope front entrance" being a positive feature, which I always felt that a "decoupled" entrance was more preferred. I think the difference here is the author is focused on high frequency generating blast waves, and is not addressing the low frequency stuff, intentionally.

I'm not sure about what kind of noise a supersonic blast wave generates. Does anyone know?

I think (but don't know) that it can generate broadband noise. An example is lightning and the resulting thunder, that's a blast wave like exhaust blowdown blast wave. Thunder gives a broadband noise and things like the length of the lightning arc determine the peak frequency. Here's a wikipedia example graph of a thunder spectrum:

https://upload.wikimedia.org/wikipedia/ ... ectrum.png



Reading how blast waves kill people in tunnel systems and what kind of geometric obstacles can be used to mitigate the shock waves, I wouldn't be shocked (pun intended) to find out that the exhaust geometry is important in determining how the exhaust shock wave produces audible sound and at what frequencies.

An interesting reference in that paper, by the way. One of the papers verifies that car exhaust port generates a supersonic blast wave. I think they quote Mach 1.1 wave generated from the expansion in the exhaust port during the exhaust blowdown phase. Interesting.

[j-c-c]

"I'm not sure about what kind of noise a supersonic blast wave generates. Does anyone know? "

Thinking out loud, if a blast was say a single square wave event, if would be by definition, be every frequency, and the tone of what we hear, is result what individual frequencies that have not been canceled or absorbed, correct?

[Kevin Johnson]

Friedlander wave:

https://www.google.com/search?q=friedla ... 8690658029

ADA109896.pdf

Many more non-classified reports available to help cure insomnia.

[ptuomov]

"I'm not sure about what kind of noise a supersonic blast wave generates. Does anyone know? " Thinking out loud, if a blast was say a single square wave event, if would be by definition, be every frequency, and the tone of what we hear, is result what individual frequencies that have not been canceled or absorbed, correct?

I don't know. All the research that I've been able to find says or hints that the early part of the exhaust wave is a supersonic blast wave and that blast waves generate broadband noise. However, thinking about it out loud, it must be the case that it really matters what that wave hits, what is the resonating frequency of the object that the wave hits, and what frequencies get cancelled out by (re)reflections. But the short answer is I don't know.

Friedlander wave:
https://www.google.com/search?q=friedla ... 8690658029

ADA109896.pdf Many more non-classified reports available to help cure insomnia.

My reading of that research is that the blast wave generates broadband noise spectrum in open air. What it does in ducts or caves or whatnot is not explained by the published research that I've been able to find and understand.

What this research (including the muffler paper I linked to a couple posts ago) tells me is that if one is trying to design an exhaust termination box, one should maximize the cross-sectional area of the termination box and fill that exhaust termination box with acoustic dampening material. The empty box reflects back much more of the blast wave than say fiberglass packed box. Thus, if I'd be making an exhaust termination box, I would make it as large as possible (duh), have a perforated pipe go thru it that has very high porosity, wrap that in stainless steel mat or mesh, and then fill the rest of the box with rock wool or something like that. Table 1 of (http://file.scirp.org/pdf/JFCMV_2016071214300784.pdf) says that a simple expansion chamber with a perforated tube and acoustic packing reflects the least amount of blast wave back to the inlet pipe (83%). Every other configuration reflects back more based on that study. Now, for muffling purposes, those reflections may be useful. However, if the objective is to mimic termination into open air, then I'm thinking they would be counterproductive?

[pcnsd]

"I'm not sure about what kind of noise a supersonic blast wave generates. Does anyone know? " Thinking out loud, if a blast was say a single square wave event, if would be by definition, be every frequency, and the tone of what we hear, is result what individual frequencies that have not been canceled or absorbed, correct?

I don't know. All the research that I've been able to find says or hints that the early part of the exhaust wave is a supersonic blast wave and that blast waves generate broadband noise. However, thinking about it out loud, it must be the case that it really matters what that wave hits, what is the resonating frequency of the object that the wave hits, and what frequencies get cancelled out by (re)reflections. But the short answer is I don't know.

Friedlander wave:
https://www.google.com/search?q=friedla ... 8690658029

ADA109896.pdf Many more non-classified reports available to help cure insomnia.

My reading of that research is that the blast wave generates broadband noise spectrum in open air. What it does in ducts or caves or whatnot is not explained by the published research that I've been able to find and understand.

What this research (including the muffler paper I linked to a couple posts ago) tells me is that if one is trying to design an exhaust termination box, one should maximize the cross-sectional area of the termination box and fill that exhaust termination box with acoustic dampening material. The empty box reflects back much more of the blast wave than say fiberglass packed box. Thus, if I'd be making an exhaust termination box, I would make it as large as possible (duh), have a perforated pipe go thru it that has very high porosity, wrap that in stainless steel mat or mesh, and then fill the rest of the box with rock wool or something like that. Table 1 of (http://file.scirp.org/pdf/JFCMV_2016071214300784.pdf) says that a simple expansion chamber with a perforated tube and acoustic packing reflects the least amount of blast wave back to the inlet pipe (83%). Every other configuration reflects back more based on that study. Now, for muffling purposes, those reflections may be useful. However, if the objective is to mimic termination into open air, then I'm thinking they would be counterproductive?

What about a muffler design that uses two or three combined elements that would provide a freq tuned anti reversion shape to the chamber entry while the chamber itself was also wrapped with acoustic damping material? The chamber would still provide Blair's optimum expansion area.

[MadBill]

...I don't know. All the research that I've been able to find says or hints that the early part of the exhaust wave is a supersonic blast wave and that blast waves generate broadband noise...

Here's an only vaguely related thought: If, as widely stated, the cylinder pressure of an NA engine at WOT/EVO is typically in the range of 50-100 psi, then if you pressurized the cylinder with a leakdown tester to 100+ psi and momentarily popped open the exhaust valve by striking the tip with a dead-blow hammer, a sonic pulse should result. I can say it is loud...

[ptuomov]

What about a muffler design that uses two or three combined elements that would provide a freq tuned anti reversion shape to the chamber entry while the chamber itself was also wrapped with acoustic damping material? The chamber would still provide Blair's optimum expansion area.

Here are the chambers/mufflers that they tested with exhaust blast waves. The simple packed chamber d passed on the smallest wave to the outlet AND reflected the smallest wave back to the inlet. What you describe sounds a lot like either (e) or (f), but they by my reading under performed (d) as a termination box for the exhaust blast wave.

MufflerTypes.jpg

[n2xlr8n]

My reading of that research is that the blast wave generates broadband noise spectrum in open air. What it does in ducts or caves or whatnot is not explained by the published research that I've been able to find and understand.

Waveguide tuning explains it very well.

What this research (including the muffler paper I linked to a couple posts ago) tells me is that if one is trying to design an exhaust termination box, one should maximize the cross-sectional area of the termination box and fill that exhaust termination box with acoustic dampening material. The empty box reflects back much more of the blast wave than say fiberglass packed box. Thus, if I'd be making an exhaust termination box, I would make it as large as possible (duh), have a perforated pipe go thru it that has very high porosity, wrap that in stainless steel mat or mesh, and then fill the rest of the box with rock wool or something like that. Table 1 of (http://file.scirp.org/pdf/JFCMV_2016071214300784.pdf) says that a simple expansion chamber with a perforated tube and acoustic packing reflects the least amount of blast wave back to the inlet pipe (83%). Every other configuration reflects back more based on that study. Now, for muffling purposes, those reflections may be useful. However, if the objective is to mimic termination into open air, then I'm thinking they would be counterproductive?

I'm accustomed to waveguide tuning for specific frequencies, but imo to attenuate specific frequencies, one would have to tune the cavity such that the signals algebraically cancel, which would require some reflection. What I think they are doing is sizing the muffler "cavity" based on the fundamental frequencies of an engine, and using the dampening material to attenuate / decouple a small range of frequencies.

The math for frequency attenuation is in the Sound Reference book- it's pretty simple stuff.

[ptuomov]

What this research (including the muffler paper I linked to a couple posts ago) tells me is that if one is trying to design an exhaust termination box, one should maximize the cross-sectional area of the termination box and fill that exhaust termination box with acoustic dampening material. The empty box reflects back much more of the blast wave than say fiberglass packed box. Thus, if I'd be making an exhaust termination box, I would make it as large as possible (duh), have a perforated pipe go thru it that has very high porosity, wrap that in stainless steel mat or mesh, and then fill the rest of the box with rock wool or something like that. Table 1 of (http://file.scirp.org/pdf/JFCMV_2016071214300784.pdf) says that a simple expansion chamber with a perforated tube and acoustic packing reflects the least amount of blast wave back to the inlet pipe (83%). Every other configuration reflects back more based on that study. Now, for muffling purposes, those reflections may be useful. However, if the objective is to mimic termination into open air, then I'm thinking they would be counterproductive?

I'm accustomed to waveguide tuning for specific frequencies, but imo to attenuate specific frequencies, one would have to tune the cavity such that the signals algebraically cancel, which would require some reflection. What I think they are doing is sizing the muffler "cavity" based on the fundamental frequencies of an engine, and using the dampening material to attenuate / decouple a small range of frequencies.

The math for frequency attenuation is in the Sound Reference book- it's pretty simple stuff.

Even for supersonic blast waves? Those are quite far from the acoustic waves which are assumed to move at the speed of sound and be small. These are big and fast. It's my reading that the acoustic theory is not very predictive when waves get bigger aka finite waves, and it's pretty far off for shock waves in general and supersonic blast waves in particular.

My thinking is that the first section of the exhaust needs to deal with the supersonic blast waves one way or another. Methods that I'm thinking about is a single large pipe that combines all eight pulses per revolution. I don't have room for acoustic packing there, so I just have to try to use geometry there. I need to reduce the amplitude of the shock wave enough, as the amplitude determines the wave speed. If I can get it down to sonic velocity, then the shock wave will die and turn into a normal finite wave. I think.

Then the middle section can be tuned and predicted more with the 1D finite wave sort of methods. Finally, if the middle section has enough volume and dampening, then the rear muffler can is probably adequately modeled with the acoustic tools you reference.

This is all wild speculation on my part, so don't take any of the above as facts, just food for thought.