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DIY: Flow modifications to the Internal Construction of the OEM 2.0 Muffler

Background information: This thread is a continuation of an earlier thread which I started here: Modifying Stock 2.0 Muffler For Beter Sound And Flow

Here in this new thread I intend to document my upcoming modifications to the OEM system. My goal, as presented in the earlier thread linked above, is to try to come up with a less restrictive muffler, sort of halfway between the loudness level of the stock muffler and the roar of a straight-thru system. So I intend to remove some restriction, while remaining 'just enough' to suit the above goal.

Yesterday I met with fellow Forum member Chaadster, and obtained his old used OEM system from his former 2.0 Ti. Upon getting it home and doing a bit of cleaning, my first observation was the discoloration of the outer casing at the center area of the muffler. See first picture, where I highlighted with white squiggly lines the area of the discoloration. This indicates that this area of the muffler runs the hottest, and therefore would be the general location of the most flow restriction.

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The next step was to open up the topside of the muffler using a grinder. And when I completed this work, I was greeted by this:

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I felt that the easiest way to show what is going on here (since the black sooty internals do not always photograph really well) was for me to make a flow diagram. See below:

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To walk you thru this, the muffler is internally divided into four (4) chambers via a series of three (3) perforated dividers, which I have labeled as (B) in my drawing. Labeling these chambers as 1 to 4 (left to right), chambers 1 and 4 (on the left and right ends) are filled with a fiberous spun cloth, so as to discourage flow thru these chambers. So the most interesting chambers are the pair of 'inside' chambers nos. 2 (left middle) and 3 (right middle), which match the casing discoloration.

The muffler inlet port occurs in chamber 2, and the inlet pipe terminates in a fan-shaped perforated 'filter' which I have labeled as (A) in the drawing. The center (B) divider (2 of 3) is not only perforated, but also has a large passage hole, as you can see in one of the pictures. It is thru this hole that most of the exhaust gas enters into chamber 3. Once the gas is in chamber 3, it then exits thru a pair of pipes, which are stacked vertically one above the other. In my drawing, this path is illustrated by the blue (the top pipe) and the orange (the bottom pipe) flow arrows. And finally, this pair of pipes each go thru a U-bend (within chambers 1 and 4), and from there, exit via the twin exhaust pipes.

So, this is where I am at the moment. I am going to take a pause now in my work as I try to determine exactly my next step and how far I wish to take this.

I will update this thread with further developments as they happen.

Thanks - DM
 

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Doing it by trial and error would be tedious but the obvious first step would seem to be removing the wadding in the end chambers, welding it back up and listening to the result.
 

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Can you not just remove all the internals and create a Y straight pipe? This way it would look like you had a muffler but sound very loud?
 

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You got into that quickly, @Dave80gtsi !

I assume that getting exhaust gas into pipes A and B as quickly as possible would increase sound volume, is that right? If so, would adding pickup holes to those pipes in compartment 2, right aft of the inlet diffuser/filter make sense? Or perhaps the same but immediately above the inlet terminal in pipe A?

Another question: do the terminal lengths of pipes A and B, i.e. the lengths which the pipes extend from the perforated dividing walls defining chamber 3, have anything to do with gas velocity or resonant frequency? Is foreshortening them a viable strategy to get the muffler flowing faster and sounding louder?

Do keep us posted on decisions and progress, Dave!
 

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Can you not just remove all the internals and create a Y straight pipe? This way it would look like you had a muffler but sound very loud?
The objective (see post 1) is to improve flow without substantially increasing the sound level. That sounds like a good, but difficult cause.

The muffler clearly contains a series of resonant chambers. I have a "Theoretical Acoustics" by Morse and Ingard, which gives ways to model such systems. However, said models have issues due to the structure dimensions being multiple wavelengths, plus things like flow resistance generally are not considered. Things like an acoustic low pass filter for a telephone microphone are examined, and most such microphones have such a filter.

One study found that making the shape of the cavities irregular improved the accuracy of the model considerably. This was done with the microphone filter by adding random irregularities to the cross section of each cavity. The hypothesis being that the irregularities break up standing waves within the cavity. That would seem to imply that the neat oval outer shape of the muffler is not ideal.

I do not think that the cavities on the ends, packed with fibers, have anything to do with restricting flow. The flow does not go through those cavities.
I also notice that the looped around pipes have small holes where they cross through the center cavities. This is clearly some tricky bit of design that is above my pay grade.

As first guess for the OPs objectives, I would remove that "fan" or enlarge the holes in it, then allow more flow into those small holes where the looped back pipes cross the center cavities, and finally put some irregular dents into the "lid" before welding it back on. All total guess work.
 

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I read a lot about exhaust and muffler design when I was looking to upgrade my exhaust. I found a very useful article which I have linked below.

Looking at it, this is a typical Reactive-Absorptive muffler design. The outer chambers offer sound absorption using the fibreglass packing with the gases entering these chambers via the perforations in the baffles. These have no effect on exhaust gas velocity or back pressure. The perforations in the pipe are for scattering the sound pulses inside the chamber in what is called destructive interference. Not sure if the exit pipes also have perforations under the fibre packing.

https://www.acoustics.asn.au/conference_proceedings/AAS2005/papers/34.pdf
 

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Great work @Dave80gtsi . IMO, this muffler is engineered! Certainly one of the goals is to reduce noise. Is another goal, with the looping pipes, to increase the total length (path) that the gasses travel? I am not an expert, this is pure conjecture. Race cars (whose engines are tuned to create peak power at higher RPM's) generally (if allowed) use a short exhaust, exiting in front of the rear wheels. Street engines are tuned for low-mid range peak power. Was the looping of the pipes engineered to enhance low and mid range power? Perhaps keep this in mind when you work towards your goal of reducing restriction. Great work!!
 

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Discussion Starter #10
Thanks, folks! I appreciate all of the comments and questions!

I'm starting the next step of the project now, and I'll document the play-by-play with pictures and commentary over the next couple of days. Chances are that those of you with questions will find them addressed there, so just stay tuned!

Cheers - DM
 

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Great work @Dave80gtsi . IMO, this muffler is engineered! Certainly one of the goals is to reduce noise. Is another goal, with the looping pipes, to increase the total length (path) that the gasses travel? I am not an expert, this is pure conjecture. Race cars (whose engines are tuned to create peak power at higher RPM's) generally (if allowed) use a short exhaust, exiting in front of the rear wheels. Street engines are tuned for low-mid range peak power. Was the looping of the pipes engineered to enhance low and mid range power? Perhaps keep this in mind when you work towards your goal of reducing restriction. Great work!!
@AlfaAndy, your comments bring to mind a Chrysler V-8 fifty years ago that had very long intake runners, so long that the left bank 4-barrel carburetor was outboard of the right bank and vice versa. The very long runners caused resonance at midrange rpms that had a ram effect due to the closing and then re-opening of the intake valve. Of course this only worked in a fairly narrow rpm range and I bet there were problems with fuel pooling in the cold runners. The point here is that the same thing happens in an exhaust system with the opening and closing of the exhaust valve and a properly tuned exhaust will have a scavenge effect in a certain rpm range, thus improving flow beyond what can be obtained with a drop to normal atmospheric pressure. The downside, of course, is that outside of that rpm range the resonance won't exist and the length of tubing may actually be detrimental when out of the sweet spot. Thanks for this thread @Dave80gtsi !
 

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After posting the above I got curious about the Chrysler V-8 I mentioned. It was the 300F V-8 of 1960 - so 60 years ago, not 50. How time flies when you're having fun!
 

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@AlfaAndy, your comments bring to mind a Chrysler V-8 fifty years ago that had very long intake runners, so long that the left bank 4-barrel carburetor was outboard of the right bank and vice versa. The very long runners caused resonance at midrange rpms that had a ram effect due to the closing and then re-opening of the intake valve. Of course this only worked in a fairly narrow rpm range and I bet there were problems with fuel pooling in the cold runners. The point here is that the same thing happens in an exhaust system with the opening and closing of the exhaust valve and a properly tuned exhaust will have a scavenge effect in a certain rpm range, thus improving flow beyond what can be obtained with a drop to normal atmospheric pressure. The downside, of course, is that outside of that rpm range the resonance won't exist and the length of tubing may actually be detrimental when out of the sweet spot. Thanks for this thread @Dave80gtsi !
I believe that effect is limited on a car with a turbo charger and catalytic converter.
 

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I believe that effect is limited on a car with a turbo charger and catalytic converter.
The intake ram effect is certainly useless with a turbo providing much denser charge than ram effect could ever do. But if you can hear exhaust pulses downstream of a cat, and not just a whoosh like a turbine, then there is scavenge to be had if you want it. Probably not as strong as without the cat, though, you're right there.
 

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@lockem , for the discussion of exhaust scavenging, why would a catalytic converter be any different than a resonator or muffler? @alfan , In years past engine designers could only tune their intake for a certain (narrow) power band. Shorter runners for high RPM power, longer for low RPM torque. Then, in the late 80's(?), 90's(?), intake manifolds were designed to be variable. Using shutters, vanes, trap doors, etc. and vacuum or electric solenoids, the path of the air could be directed through a set of shorter, or longer runners to suit the power demands and current RPM's.
 

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On the topic of exhaust scavenging, I definitely think the exhaust pipe diameter and the flow restrictions/design have an impact on power especially low down in the rpm range. Most people seem to concentrate on having as open an exhaust as possible without thinking of low down usability. I have an axle back exhaust with slightly larger 2.75” pipe as opposed to the 2.5” OEM pipe. Initially I fitted a 2.75” fully open front section pipe without a resonator to replace the OEM resonated front section with the crushed section of tube in the middle. Immediately upon fitting this system, I noticed there are very noticeable drop in power and reduced throttle response in the very low rpm range during take-off and initial acceleration. I felt I had lost some bottom end torque. I discussed this with various people including the exhaust shop who all were insistent that a fully open exhaust is the best for a Turbocharged car. While this may be true once the engine is working in the turbo operating envelope i.e. >2.5k rpm, low down where the turbo is yet to start working, I judged the exhaust gases were not being evacuated/scavenged quick enough due to combination of increased pipe diameter and loss of resonator causing a drop in exhaust gas velocity. Higher up the rpm, the engine still felt powerful as before. This low down torque drop bugged me and I decided to put the front resonated OEM pipe back on mated with the sport axle back aftermarket exhaust. Lo and behold, the low down torque was back and I could feel the engine responding to throttle input more readily. The open front pipe is still in my garage as a result. Despite what people may say either one or a combination of the diameter/design/lack of flow restriction of the aftermarket front pipe caused a bottom end loss of power.

Surely , even a turbo engine will act in part as a NA engine until there is enough exhaust gas volume to start spinning the turbine in its working envelope? Therefore, exhaust gas scavenging and the resultant tuning for this is important for cars that are driven as daily users frequently accelerating from stand still or slow engine speeds rather than open exhausts fitted to race cars which will always have the engine speed high enough to keep the turbo spooled. Even our cars in D mode the strategy is to keep the gear selection above the boost threshold so that the engine is eager to respond. Noise suppression is an added compliance factor for factory exhausts making exhaust design an art in itself.
 

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Your not going to get exhaust gas resonance effects bouncing from the muffler up through a cat, resonator, crushed piping section, and most importantly a turbo
 

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If my primary goal was to improve flow, I would turn it into a Y-pipe by removing the center divider to create a large chamber in the middle, and cutting the outlet pipes so they start inside that chamber. The U-bends would become surplus except that you could fill them with insulation to tune the sound.
 

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@lockem , for the discussion of exhaust scavenging, why would a catalytic converter be any different than a resonator or muffler?
That is almost the point. The cat acts like a restriction plate in a muffler (it models like an inductor in an electric circuit) rather than like a whole muffler and the cat is located near the engine. You put your resonator downstream of said cat and your exhaust pulses are first attenuated by the cat, then your now weak reflected pulses get weakened by a second trip through the cat before reaching the engine for scavenging. Thus, some pressure drop makes it back to the engine but the effect is small compared to what you got "back in the day" with a cat-less turbo-less engine.

The turbo, with its small cross sectional area acts like a crushed section in the exhaust pipe and has a similar acoustic model to the restriction plate and/or cat.
 

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... Not to mention it's a forced induction engine so we aren't using vacuum pulses to help pull more air into the cylinder...
 
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