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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...
This is what I am trying to work out. It is a FI engine but the induction of compressed and cooled air is only being pushed into the intake header once the turbine is spinning the compressor. Does it not act partially as a NA engine until then and consequently rely on cylinder evacuation and a degree of vacuum in the exhaust?
 

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Your comparing 20+ psi of boost to a couple inches of vacuum

Turbo header design is all about getting the hottest exhaust gasses to the turbo as quickly as possible (massively short headers or integrated into the block or manifold). Turbo exhaust is all about getting rid of backpressure working against the turbo.
 

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Discussion Starter #23
Progress Report:

Looking at the internal construction of this muffler, I have identified three (3) points of potential flow restriction which I wish to address. I'll review the first two (2) of those points here, saving the third restriction for a later posting.

1) You may recall that there are three (3) dividers [which I had labeled as (B) in the diagram in my first post in this thread] which partition the internal area of the muffler into four (4) chambers. The first and the third (B) dividers appear to be of secondary significance as they are not in the direct path of the internal flow. However, the middle (B) divider is key, as all of the entering gas needs to pass thru this area in order to be able to exit the muffler. As can be seen in the picture below,

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this partition already has a 2-7/8” diameter thru hole, with perforations above and below it. Even though this hole has a pretty large diameter, I felt that I could further improve the flow thru this partition by cutting out the top area of perforation. This creates a second fairly large passageway, located above the existing round hole. I left the bottom area of the divider's perforation (the area below the factory hole) alone, simply because this location was too hard to reach. Plus, I felt that my new upper hole, combined with the existing factory round hole, seemed to offer plenty of gas flow area. (Pictures to come, below).

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2) The largest flow restriction which I observed was the perforated fan shaped “filter” / diffuser at the entry point of the inlet pipe. [Again, referencing my original post's diagram, I labeled this item as (A)]. This diffuser is pinched closed on the end, so -all- of the entering gas is forced to pass out thru the various shown perforation holes:

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My work involved cutting away this diffuser, one portion at a time. This work took me quite some time to do cleanly, since free and clear access to this area is quite limited. Nevertheless, at the end of the day, I now have this:

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A word on the actual material removal: All cutting work which I have done to date has been done using a common Dremel tool and a container of their no. 409 grinding discs:
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Dremel offers a 90 degree adapter for their tool, shown fitted in the picture, which is -invaluable- for reaching all of the hard-to-reach places.

The thickness of all material encountered thus far in this project, both the external casing and all of these internal bits, is approx. 1 mm (0.040”) thick. While the Dremel no. 409 discs cut thru this material quite easily, you'll end up using a lot of discs – these discs come in a container of 36 pieces, and so far I've gone thru nearly a complete container's worth.

So … that's where I am at the moment.

Stay tuned to this thread for my upcoming third step!

Thanks - DM
 

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I think that you are going to be surprised with the change in sound, which may or may not be bad. The change to that inlet cone thing is going to have a large impact on the flow IMO, and the perforations you removed from the divider is going to change the sound. I can't wait to hear the difference! Please do an apples to apples comparison, with the same mic, same vehicle location, mic placement (distance, angle, etc). Too many comparisons are not done right. One sound clip is in a garage, and then the after is outside. Don't do that. :D
 

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More great work!
Have you though to do this (it will require a bit of working back, maybe using some duct tape)? Using a home vacuum cleaner and a vacuum gauge installed in-line, test for flow restriction before (stock), and after each modification. This may show obvious differences.
 

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Did you figure out a means to measure the before and after pressure loss through the muffler? Without that it is very difficult to say if your hard work has yielded a benefit.
 

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Ah! I didn’t realize the inlet diffuser was closed on the end; opening it seems like a major change in flow characteristics, so it will be very interesting to see what that means to the sound!
 

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Lockem and Triumph23 are correct that what we are looking at are a series of acoustic filters which are clearly very carefully engineered to meet SPL regulations (probably the stricter EU regs) while minimizing any performance loss. The pipe lengths, chamber volumes, and various size apertures are all engineered to reduce the amplitude of certain frequencies. Sound waves have alternating positive and negative pressures and these occur with frequency that is dependent on engine RPM. Resonant peaks at certain frequencies can be damped by delaying the frequency to cause the positive and negative pressure zones to time align and cancel each other out.

As pointed out in this thread the art of tuning a naturally aspirated engine involves the length and diameter of both the intake and exhaust, and when combined with valve overlap, can create a supercharged or ram effect at a fairly narrow RPM range. When done well it creates a surge of power, usually in the upper RPM range where the ram effect is most efficient. Turbo cars take a different approach to pressurizing the combustion chambers, but I can imagine intake and exhaust resonances affecting the spin up time of the turbo. But the turbo spins at such a high frequency that it isolates the intake and exhaust tuning.

Applying my moderate knowledge of acoustics (I work for a loudspeaker company) when looking at the muffler internals I would predict that you will change the sound but not the flow characteristics of the stock muffler. For example, the small holes in the dividers allow some frequencies to pass freely while other frequencies face a high resistance - but I think you will find that if you add up the area of all the holes in a divider it will equal or surpass the area of the entering exhaust pipe. The different length tail pipes are each tuned to a different frequency to spread the energy in a certain range of frequencies. The end chambers with the fiberglass filler are acoustic dampers that attenuate certain frequencies. Anything you change inside the muffler will subtly change the tone and apparent loudness by allowing some resonant peaks to pass with less attenuation.

You can measure the acoustic before-and-after with a smart phone using an app like Studio Six 'Audio Tools' to measure the acoustics of the system. You can use the SPL Meter (use C weighting) to measure the overall loudness and the FFT to see the SPL at specific frequencies. Prediction: the stock muffler will have a smoother looking FFT and the modified muffler will have more peaks and look a bit jagged. It will sound raspier and subjectively louder.
 

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Discussion Starter #29
The Latest Progress Report (and with apologies for the delay in posting):

As I noted earlier, there were three (3) internal potential flow obstructions which I wished to clear. To review, the first one was to enlarge the passage holes in the vertical middle (B) partition wall, located between chamber nos. 2 and 3. The second one was to remove the closed perforated inlet diffuser at the end of the inlet pipe, as located in chamber no. 2.

Since both of these tasks are now done, here's the third and final obstruction which I have chosen to address.

The inlet pipes for the pair of exit tail pipes are located in chamber no. 3. As can be seen here:

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These inlet pipes have some pretty long extension pieces which protrude deep into chamber no. 3. Therefore, notice the long path that the gas needs to travel in order to enter this pair of exit pipes – the gas has to go “out and around” the end of these pipes. And with the gas “fighting” to be able to enter these pipes, it certainly seems as if there's a high potential for flow turbulence. So, in order to improve the flow thru this chamber, I've simply cut a couple of inches off of the end of each of these pipes, as shown:

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Note that, to cut these pipe ends, you are working within a pretty tight space here, and it is a challenge to be able to cut off each pipe evenly and cleanly. So patience is your friend here.

The end result of this trimming of the pair of exit pipes has created a fairly large empty plenum within chamber no. 3. Seems obvious that this path modification will aid the clean flow of gas from the inlet to the outlet, from one side of the chamber to the other.

Some measurements for reference: The single inlet pipe (the one which had the OEM fan-shaped diffuser, since removed) measures about 2.6 inches in inside diameter, for an inlet flow area of roughly 5.3 sq. inches. The existing round hole in the middle vertical divider (B) measures about 2.8 inches in diameter, offering about 6.2 sq. inches of flow area. Note that since this existing OEM hole offered a greater flow area than does the inlet pipe, one could perhaps argue the point that my earlier enlargement of the perforated area in the (B) divider above this hole was not really needed. Nevertheless, I'm pretty comfortable that my removal of the (B) perforations has furthered the cause.

The inside diameter of each of the pair of tail pipes measures roughly 2.2 inches, each thus offering a flow area of about 3.8 sq. inches. So this pair, combined, yields a cross sectional flow area of about 7.6 sq. inches. Therefore, this modified stock muffler now allows a inlet-to-outlet flow area comfortably above the limiting cross-sectional area of the inlet pipe. And my keeping the rest of the pair of the outlet pipes as they are should offer enough flow path restriction to differentiate my DIY approach from an aftermarket no-muffler straight thru pipe system – check my diagram in the first post to see the U-shaped flow path which these exit outlet pipes offer, and which I am intentionally not going to modify.

All in all, in my book, mission accomplished.

The final task to be done will be to weld the outer casing hole access patch closed. Then to be installed onto the car in lieu of the stock system.

As noted, I will post my subjective reactions accordingly.

Thanks - DM
 

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@Dave80gtsi please dont forget to take a sound clip before with current muffler and after with modified muffler using the same recording device in the same location. Also note if there is any subjective change in low down torque.
 

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The Latest Progress Report (and with apologies for the delay in posting):

As I noted earlier, there were three (3) internal potential flow obstructions which I wished to clear. To review, the first one was to enlarge the passage holes in the vertical middle (B) partition wall, located between chamber nos. 2 and 3. The second one was to remove the closed perforated inlet diffuser at the end of the inlet pipe, as located in chamber no. 2.

Since both of these tasks are now done, here's the third and final obstruction which I have chosen to address.

The inlet pipes for the pair of exit tail pipes are located in chamber no. 3. As can be seen here:

View attachment 95513

View attachment 95514

These inlet pipes have some pretty long extension pieces which protrude deep into chamber no. 3. Therefore, notice the long path that the gas needs to travel in order to enter this pair of exit pipes – the gas has to go “out and around” the end of these pipes. And with the gas “fighting” to be able to enter these pipes, it certainly seems as if there's a high potential for flow turbulence. So, in order to improve the flow thru this chamber, I've simply cut a couple of inches off of the end of each of these pipes, as shown:

View attachment 95515

View attachment 95516

View attachment 95517

Note that, to cut these pipe ends, you are working within a pretty tight space here, and it is a challenge to be able to cut off each pipe evenly and cleanly. So patience is your friend here.

The end result of this trimming of the pair of exit pipes has created a fairly large empty plenum within chamber no. 3. Seems obvious that this path modification will aid the clean flow of gas from the inlet to the outlet, from one side of the chamber to the other.

Some measurements for reference: The single inlet pipe (the one which had the OEM fan-shaped diffuser, since removed) measures about 2.6 inches in inside diameter, for an inlet flow area of roughly 5.3 sq. inches. The existing round hole in the middle vertical divider (B) measures about 2.8 inches in diameter, offering about 6.2 sq. inches of flow area. Note that since this existing OEM hole offered a greater flow area than does the inlet pipe, one could perhaps argue the point that my earlier enlargement of the perforated area in the (B) divider above this hole was not really needed. Nevertheless, I'm pretty comfortable that my removal of the (B) perforations has furthered the cause.

The inside diameter of each of the pair of tail pipes measures roughly 2.2 inches, each thus offering a flow area of about 3.8 sq. inches. So this pair, combined, yields a cross sectional flow area of about 7.6 sq. inches. Therefore, this modified stock muffler now allows a inlet-to-outlet flow area comfortably above the limiting cross-sectional area of the inlet pipe. And my keeping the rest of the pair of the outlet pipes as they are should offer enough flow path restriction to differentiate my DIY approach from an aftermarket no-muffler straight thru pipe system – check my diagram in the first post to see the U-shaped flow path which these exit outlet pipes offer, and which I am intentionally not going to modify.

All in all, in my book, mission accomplished.

The final task to be done will be to weld the outer casing hole access patch closed. Then to be installed onto the car in lieu of the stock system.

As noted, I will post my subjective reactions accordingly.

Thanks - DM
Any updates?
 

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Discussion Starter #33
Any updates?
Not to worry - I am still here and have not forgotten!

I've simply had some work slow down delays in the welding of the cut-out patch back onto the muffler body. The delays were all just time / schedule related; nothing at all to do with the project's technical details.

At this moment, the muffler is all welded back together and is completely ready for installation. Schedule permitting, I hope to be installing it within the next couple of days.

I will be doing at least two (2) more postings here - the first one will review the actual muffler installation procedure itself, with details and any little tips which I might encounter along the way that I would hope could help the work go smoothly for the next fellow to try this project. The second posting will contain carefully controlled acoustical measurements along with my summary / subjective reactions.

By the way - Assuming a successful outcome, the whole intent of my taking the time to finely detail this whole project as I have done has been to give to others the step-by-step confidence that they, too, can go out and do this work themselves!

So … Stay tuned! - DM
 

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Not to worry - I am still here and have not forgotten!

I've simply had some work slow down delays in the welding of the cut-out patch back onto the muffler body. The delays were all just time / schedule related; nothing at all to do with the project's technical details.

At this moment, the muffler is all welded back together and is completely ready for installation. Schedule permitting, I hope to be installing it within the next couple of days.

I will be doing at least two (2) more postings here - the first one will review the actual muffler installation procedure itself, with details and any little tips which I might encounter along the way that I would hope could help the work go smoothly for the next fellow to try this project. The second posting will contain carefully controlled acoustical measurements along with my summary / subjective reactions.

By the way - Assuming a successful outcome, the whole intent of my taking the time to finely detail this whole project as I have done has been to give to others the step-by-step confidence that they, too, can go out and do this work themselves!

So … Stay tuned! - DM
Depending on your results I may do this exact thing. I may not go quite as wild as I you did, only very minor alterations, but your work will definitely guide my path. I appreciate the work you're doing, thanks a bunch!

On a bit of a side note. I have contemplated completely gutting the muffler and enclosing a perforated Y pipe and packing the muffler with fluff (e.g. rockwool, or other absorption material). It would be very similar to resonator/glass pack, but a much larger chamber.
 

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Discussion Starter #35
Update: OEM Muffler Removal and Reinstallation

Once the modification access hole in the muffler has been welded shut, you are ready for the installation work.

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Overall, the work required to remove, and then to reinstall, the OEM muffler is generally straightforward. Anyone who has fiddled about with this sort of exhaust pipe and muffler work in the past should comfortably be able to handle it. But there's a couple of Handy Hints which I discovered, and which I thought that I'd pass along.

The rear of the car does need to be elevated a little bit if you intend to work from floor level (vs. using a lift to elevate the entire car). What I found to work well was to jack up each rear corner and to place a couple of 4x4 wood pieces beneath each tire, so that the weight of each tire is firmly on the wood pieces. This allows for plenty of working height, with only the front inlet pipe hanger requiring a bit of an arm stretch. (Plus, I'm not a big fan of working underneath a car when the car is elevated only by a pair of occasionally wobbly jack stands.)

Once you are lying beneath the rear of the car and looking up at the rear underside, you will find three (3) plastic access shrouds which must be removed: The bigger center one which covers the muffler, and a pair of smaller “boxes”, one of which is located behind each rear wheel. All fasteners require either a 10 mm socket or else a Torx screwdriver bit. Easy.

Note that you do -not- need to remove the rear bumper fascia. Nor do you need to remove that black diffuser-looking piece which is beneath the bumper fascia. But be aware that the eventual removal of the muffler will require you to have to bend this black diffuser downwards quite a bit. Since this diffuser is plastic, it does flex and bend, but be forewarned that you will be needing to bend it down a little bit more than your Comfort Zone might usually allow.

The muffler is held to the car via five (5) rubber isolation hangers. The first one is located at the middle of the car, near the opening in the inlet pipe. This one is very awkwardly placed, making it really hard to reach when the car is on the ground. The Path Of Least Resistance here is for you to just cut away the old rubber hanger element for removal, buying a replacement hanger element for your future use at installation. The rubber hanger element carries A-R part no. 50533703 and it's pretty cheap. Trust me on this – you will save yourself a ton of grief if you do what I've just indicated and buy this part beforehand. Otherwise, you'll be frustratingly wrestling for hours with the old hanger element to remove it, and life is too short to spend your time doing that. And since you are already lying there under the center of the car, go ahead and loosen the band clamp nut at the inlet pipe's entrance point, which curiously takes a relatively uncommon 15 mm socket.

The second and third muffler hanger elements (the front pair) are easy to remove, as the removal of the pair of those plastic “boxes” mentioned earlier gives you direct access to them. Use a 10 mm socket to remove the brackets.

The fourth and fifth rearward muffler hanger elements are also easy to remove once you know the “double-secret-probation” ( … old movie quote) hint. These last two hangers are held to the car with brackets, which are also secured by 10 mm nuts. The tricky bit is that these nuts cannot be reached without the temporary removal of the pair of decorative chrome exhaust tips which are mounted to the backside of the black diffuser fascia panel. Each chrome decorative tip is held to the black diffuser via a pair of Torx head screws, as well as a single Allen head bolt, and you can reach these by working from the inside of the diffuser. Once these tips are loosened and slid back, the fourth and fifth muffler bracket nuts are now exposed – See picture below, and note this nut looking right at you at roughly the 1:00 o'clock position relative to the pipe. That's the one. Using a swivel extension on your ratchet, you can now easily reach and remove these nuts by working from the outside of the car, poking your wrench in through the oversized exhaust holes in the diffuser.

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At this point the muffler should be loose from the car and is ready for removal. You will first need to push the muffler back towards the rear of the car by about 2 inches so to be able to remove the inlet pipe from the sliding slip connection. This part of the job is not much fun, as you will be supporting the surprisingly heavy muffler on your chest whilst wiggling and twisting it, pushing it towards the rear of the car. Patience is your friend here, as this part takes a while. But eventually you will hear that satisfying 'clunk' as the inlet pipe falls out and hits the floor.

Finally, remove the rear muffler portion from the car via some twisting and maneuvering. As noted earlier, you do have to bend the black plastic diffuser downwards so to allow the various dangly muffler brackets and bits to pass, so take your time here and be certain not to damage anything. And at last, you will have freed the muffler from the car.

As that old cliché goes, “Installation Is The Reverse Of The Removal”. There's really nothing else to mention here which is particularly unique or tricky concerning the installation procedure, and I personally found this part of the work to go far easier than did the removal.

So, there we are. At this point the new modified muffler should be installed, and you are finally ready to fire up the engine and to hear the results of your work.

Which I will discuss here in my final DIY post installment, tomorrow.

Thanks - DM
 

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Discussion Starter #37
Final Update: “So … Was It Worth All Of The Effort?”

The modified OEM low restriction muffler has now been mounted to my car, and I've taken the car on a number of short trips, keeping an open ear (as well as an open mind).

Overall, the audible change due to the installation of the modified muffler has been fairly modest. And that's exactly as I had wanted it to be.

I recorded before-and-after sound pressure level readings, under carefully controlled environmental conditions, such that the only variable between the before and the after readings was the muffler itself. And I was a bit surprised to see that, at idle, there was just a fairly small 1 to 2 dB increase in the sound pressure level due to the modified muffler. Idling, it would be hard for anyone to subjectively tell any difference between my modified muffler and the OEM system. Those of you who feel that most of the various wide-open aftermarket exhaust systems are a bit too loud for your taste would be pleased with the results of this project.

I won't be posting any sound clips here, as the ones that I've done don't really capably demonstrate any before vs. after contrast.

But what has changed has been the subjective sound of the engine, as sensed from the driver's seat, whenever you press down on the accelerator pedal. The new modified muffler generates a little bit more of a satisfying louder 'growl' whenever the accelerator is pressed. Easing off of the pedal reduces this sound, so that when you are driving at a steady engine rpm (like on the freeway) there's no perceived audible difference from stock. Therefore, there is no more droning with this system than there might be with the OEM system.

Again, subjectively, there does seem to be a little bit quicker acceleration which accompanies this new growl. But I emphasize that this would be quite hard to measure objectively without the use of test equipment. And it honestly could simply be a placebo effect due to the slightly louder sound.

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So … The wrap up.

I am pleased with the end results of the DIY modifications which I have documented in this thread. I did not want a super-loud system, and I did not end up with one. What I now have instead is a modified muffler with very minimal flow restrictions. And a little bit louder sound … whenever I want it to be so.

Plus, for those of you who might prefer something louder, I can recommend that you follow the same steps which I have documented here, and then take it one step further. That final step would be to cut away the pair of internal tailpipes, so to bypass the OEM “U” bends for each pipe. You would accomplish this by cutting out and removing the solid sections of these pipes which pass thru chambers 2 and 3. This plan would remove almost all of the flow restrictions which are present in the OEM muffler, giving you basically an empty can. Seems like this should give you the increased roar, if that's what you might be seeking.

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Finally, the big question: Why go to all of this trouble when you can just buy an aftermarket system and be done with it? That's a fair question to ask, and your answers might easily differ from mine.

For me, the advantages of this DIY modification, contrasted against the aftermarket approach, are:

1) The tinkering fun and resulting pride of the whole concept by having at some DIY hobby work;
2) The ability to tailor the end sound to be as quiet, or as loud, as you might wish;
3) It's cheap, since you already own the muffler!
4) It's the ultimate stealth system. Since the muffler looks stock from the outside, no one else will ever be the wiser.

And, for me, by far the most important reason:

5) You are assured of a 100% perfect fit. As I've noted earlier on, I've wasted far too many frustrating hours of my life attempting to make aftermarket exhaust systems fit exactly right … and they almost never do!

Thanks for tagging along!

Cheers - DM
 
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