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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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