Making a muffler

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pdq67
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Re: Making a muffler

Post by pdq67 »

Me thinks that you are way over thinking this, but, JMHO...

Please read me again..

The best we can have are, "straight pipes", but John Law won't allow them. And the old WDT perforated smooth inner pipe REAL glass packs are as close as we can get.

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Re: Making a muffler

Post by Carnut1 »

1000hp with straight pipes would be a problem.
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Re: Making a muffler

Post by ptuomov »

Carnut1 wrote:1000hp with straight pipes would be a problem.
It only makes about 800hp at the flywheel with pump gas and 2x3" exhaust now, but with race gas it'll probably make that 1000hp once the exhaust is opened up.

This is what it looks like and sounds with 2x3" exhaust with small Borla XR-1 muffler in each pipe:
rear.jpg
children.jpg
The bad news is that it both has to become much quieter AND flow more exhaust gas. The good news is that the old system didn't have any cross-over (the new system will have one) and there's some unused space for a third muffler right at the rear bumper. That third muffler is the topic of conversation in this thread.
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Re: Making a muffler

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j-c-c wrote:I don't see here much about the actual acoustics of muffler design. That is the point, basically getting the generated exhaust waves to get 180Deg out of phase and then canceling, I thought. The issue becomes which frequencies to tune for and cancel, since wideband out of a single muffler per path has many drawbacks. And the issue is as with everything else we toy with, "everything effects everything else", point being I suspect very small changes in any one of thousands of parameters will change ones's outcome, RPM, tube lengths, number of chambers, chamber volumes, exhaust velocity, back pressure, temperature, orientation, material etc, etc. Is a very moving target, I can't see how what works for one set-up can with any certainty work well for another, and only in the best of situations only point in a favorable direction. Best aspect, when we are finsished with our solution, we all suffer from varying degrees of confirmational bias, and go about our merry ways. :D

I'd be very interested if there are some rules of thumb with some basis in theory that could be useful.

As a starting point, my guesstimate is that we're dealing with something like this as the starting point:
IMG_4114.JPG
IMG_4115.JPG
The old system is a true dual without a crossover. The new system is a dual 3.5 which is then merged to a something like 5" oval before it's divorced again to mufflers. I'm thinking that the new system is going to be more efficiently muffled because of this change in geometry.

After the rear axle mufflers, I am planning to merge the flow again at the rear bumper muffler. Recall that the old system had nothing but divorced pipes there. I'm thinking that I'll get some more high frequency cancellation in that rear muffler because one of the divorced paths will be about foot longer than the other. Hope's spring is eternal.

I use confirmation bias extensively in my life, but with loud fuel pumps and exhaust pipes I haven't quite been able to get there. It's a good idea, though. Maybe buying more expensive components would help strengthen the confirmation bias? Where are those inconel bellows again?
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Re: Making a muffler

Post by pdq67 »

I have two mufflers down each side now, an 18" and a 12" x 2.5" cheap JCW turbo's that exist stock on my '67 Camaro.

They were loud on my 406!!

And if I live long enough, I am going with as long a muffler string as I can get under my car of the WDT glass packs.

Say as long under my floor boards and then cross-wise from each side back by my gas tank like stock.

AND the SOB will be straight through!!!

I thank DV for his picture of just how real glass packs perform vs box mufflers!

As the old song, "I want to be Bobby's Girl", runs through my mind!! That young Lady could f** sing!!!

And don't forget, "Paul and Paula"!!! "Hey, Hey, Paul,---------------"

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Re: Making a muffler

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"Install WDT perforated inner pipe REAL glass packs! Too loud, then add another pair, end to end, to each side.

The suckers are true smooth straight through pipe mufflers here! NO back pressure at all.

BUT RUN THE TAIL PIPES OUT PAST THE REAR BUMPER!!

Have you ever thought about why the old WDT real glass packs aren't included in any of the muffler tests???

Same deal with why the old Holley 300-36 intake or Weiand 8016 Stealth aren't included in intake manifold tests????????

pdq67"

PS., as I like ta say, "You gotta Dime"....

READ ME AGAIN!!!
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Re: Making a muffler

Post by Carnut1 »

I am 99.99% sure those scoops inside the straight through mufflers are not helpful to max airflow. I ran glasspacks in the 80's and made more power with turbo style mufflers. Less issues with neighbors too.
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Re: Making a muffler

Post by ptuomov »

I attempted some math on the rear muffler.

The sound attenuation is mainly driven by the following formula: h=(m-1/m)^2, where m is the ratio of the pipe area to the muffler cross-sectional area. The frequencies that the muffler mostly impacts is driven by the length of the muffler.

Some observations: When the Borla XR-1 can is fed with a single 3.5" pipe, the ratio m is 0.32 and h is 7.85. When the same can is fed with two 3.5" pipes, the ratio is about 0.63 and h is 0.93. It unfortunately doesn't do much more that a simple pipe merge wouldn't do. For that tail can muffler to be effective, I need to about double the cross-sectional area for the can compared to the Borla can used as the center muffler per side. I basically need a canister that extends much further down, like a big oval muffler but sideways.

Then, there's the length. The can cancels none of the frequencies that are multiples of 0.5c/l where c is the speed of sound at exhaust temperature (about 500 m/s if the exhaust gas is under 400C by that point) and l is the muffler length (0.3m for that Borla can used at the axle). It cancels the frequencies 0.25c/l + 0.5nc/l the best where n is a positive integer. If the first muffler has 0.3m length, the second muffler should be either 1.5x the length of the first (0.45m) or 2/3x of the length of the first (0.20m). The same way, if you have a two-chamber muffler, you probably want to have one chamber be 2/3 the length of the other.

Agree, disagree?
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Re: Making a muffler

Post by j-c-c »

ptuomov wrote:I attempted some math on the rear muffler.

The sound attenuation is mainly driven by the following formula: h=(m-1/m)^2, where m is the ratio of the pipe area to the muffler cross-sectional area. The frequencies that the muffler mostly impacts is driven by the length of the muffler.

Some observations: When the Borla XR-1 can is fed with a single 3.5" pipe, the ratio m is 0.32 and h is 7.85. When the same can is fed with two 3.5" pipes, the ratio is about 0.63 and h is 0.93. It unfortunately doesn't do much more that a simple pipe merge wouldn't do. For that tail can muffler to be effective, I need to about double the cross-sectional area for the can compared to the Borla can used as the center muffler per side. I basically need a canister that extends much further down, like a big oval muffler but sideways.

Then, there's the length. The can cancels none of the frequencies that are multiples of 0.5c/l where c is the speed of sound at exhaust temperature (about 500 m/s if the exhaust gas is under 400C by that point) and l is the muffler length (0.3m for that Borla can used at the axle). It cancels the frequencies 0.25c/l + 0.5nc/l the best where n is a positive integer. If the first muffler has 0.3m length, the second muffler should be either 1.5x the length of the first (0.45m) or 2/3x of the length of the first (0.20m). The same way, if you have a two-chamber muffler, you probably want to have one chamber be 2/3 the length of the other.

Agree, disagree?
Agree, but the main takeaway for the masses I believe is your last comment in bold, meaning its best if sizes/volumes/dimensions/etc are never the same/repeating, I'm thinking doing so is one source of the often repeated "drone" complaints.
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Re: Making a muffler

Post by MadBill »

Wow. Moving back to the simple end of the discussion spectrum:
o Terminator boxes only work ahead of normal mufflers.
o For best effect the rear muffler should be tuned to deal with the frequencies the front ones don't, e.g. baffles vs. absorption.
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Re: Making a muffler

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MadBill wrote:Wow. Moving back to the simple end of the discussion spectrum:
o Terminator boxes only work ahead of normal mufflers.
o For best effect the rear muffler should be tuned to deal with the frequencies the front ones don't, e.g. baffles vs. absorption.
What do you mean by "terminator boxes" only working ahead of the normal mufflers? If you mean this in the sense of making the header think tail pipe ends there, I think you're right. If you mean as not muffling, I think they also work after regular mufflers. It's probably more efficient to place terminator boxes aka resonators ahead of the absorptive mufflers, but in my opinion they'll still cancel some frequencies even if they are the last thing in the pipe.

What this "dealing with frequencies" means in practice, I think, is that if I have 12" long muffler cases in the center section, either 8-9 inch or 16-20 inch long rear muffler would naturally cancel well the frequencies that are difficult for the 12" long center mufflers. The cross-sectional area would have to be large for them to be as effective as the center mufflers, like 8.5" diameter if round.

9"x9"x9" muffler? It's an 1980's car, so maybe I should paint it to look like the Rubik's cube! Joking, but only barely.
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Re: Making a muffler

Post by englertracing »

I would build a big aluminum oval, and put two perforated tubes in it, and stuff it full of fiberglass.
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Re: Making a muffler

Post by j-c-c »

Well the purpose of a muffler is reduce acoustic energy output, and those methods have been touched on here in numerous responses, but that acoustic energy is normally converted into heat, and if your alum muffler is effective, it won't be for long, as it starts to drip alum. :D
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Re: Making a muffler

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englertracing wrote:I would build a big aluminum oval, and put two perforated tubes in it, and stuff it full of fiberglass.
This will be stainless steel. Likely internals will be perforated stainless steel tube with 20-25% porosity, stainless steel wool sheet wrapped around it, then the rest of the can filled with some acoustic packing material like fiberglass.

Do people every worry about water condensing inside mufflers?
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Re: Making a muffler

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j-c-c wrote:Well the purpose of a muffler is reduce acoustic energy output, and those methods have been touched on here in numerous responses, but that acoustic energy is normally converted into heat, and if your alum muffler is effective, it won't be for long, as it starts to drip alum. :D
Here's an empirical question that is relevant for the dimension calculations. Does exhaust gas heat or cool as it goes thru a muffler? I always assumed that outlet was cooler than inlet, but "assume makes an a$$ out of u and me" as they say.
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