Quarter Wave Resonator vs Drone

[Dust Buster]

That looks correct. I ran a gutted CAT / DPF for about three years. They may have acted like an expansion chamber.

[ptuomov]

That looks correct. I ran a gutted CAT / DPF for about three years. They may have acted like an expansion chamber.

Cool. Approximately how much power does the car make now?

[ptuomov]

And what’s inside this stock rear muffler?

1981E7D0-FA80-4EC6-BADB-ABBFCDDE3440.jpeg

[Dust Buster]

Cool. Approximately how much power does the car make now?

I'm never sure how to answer this question. I use a dyno for comparative purposes, but here's a stab.

OEM 80 kW at 4000 rpm and 240 Nm at 2500 rpm at the engine.

Now I'm spooling up two seconds quicker:
on an Eddy Current Chassis Dyno (J1349), I'm getting 91.2 kW at 4224 rpm and 264 Nm at 2601 rpm at the wheel or 109.4 kW and 316.8 Nm on an Inertia Dyno (J1995). With friction losses, that's probably 126.4 kW to 151.7 kW and 362.2 Nm to 434.6 Nm at the engine.

Small numbers in this company, but there it is.

[Dust Buster]

And what’s inside this stock rear muffler?

No idea as I didn't cut this one open. When I have cut other ones open, they look like the usual chambers with interconnecting tubes and no absorptive material. However, this one provided too much backpressure.

[Dust Buster]

I did test drive the roofing product on top of the boxes' heat shields today, but the road was louder than my normal testing strip, probably due to more gravel than bitumen showing. The average drone registered 168 Hz. My co-pilot didn't notice anything different, but she liked the growl under load. Maybe it's time I had my ears retested, but the app also picks it up even with more background noise.

[Rick!]

Maybe perform a simple speed sweep recording of engine/exhaust noise at the highest recording rate possible, in neutral. Use a 30 second recording while smoothly and steadily increasing RPM from idle to as fast as you want to rev the engine.
Perform the FFT or PSD with at least a 4096 or 8192 hanning window and see what shakes out.
Then maybe post the screen shot of the graph with Hz on the abscissa and magnitude on the ordinate axes.
Create one recording from inside the vehicle and one or two from outside the vehicle.

The other test you can do is to "scan" the interior while driving in the drone zone. Since the phone mic is somewhat directional, you can maybe identify an area of the interior that is most "sympathetic" to the drone frequency.

If we are to understand you correctly, you removed the heat shields, tarred them and reinstalled them?
Before one could make a judgment to the effect of the treatment, a few more sound "runs" would be necessary.
If "tin" is rattling around, that would need to be addressed first before fitting test solutions.

[ptuomov]

So 1.5x-2x stock engine power? Increasing the pipe diameter by a factor of 1.25-1.4 makes sense then.

[ptuomov]

What are the dimensions of the new rear muffler and old rear muffler? What is the largest volume muffler you could fit as the rear muffler?

43.5Mm -> 57mm diameter is 31% increase in diameter and 72% increase in area. That’ll reduce back pressure for sure and makes sense with 50%-100% peak power increase. However, it does mean a much louder car unless the muffler external sizes are increased correspondingly. I had this problem when starting with dual 3” car that was too loud and trying to turn it into a dusk 3.5” car that is quiet at cruise.

One thing you might want to try is temporarily constraining the tail pipe with a reducing tip. How much can you step down at the very end of the tail pipe without losing power? The exhaust gas has cooled and is denser so maybe you could do with 50.8mm tail pipe or even rear muffler. But the only way to know is to try.

[Dust Buster]

Maybe perform a simple speed sweep recording of engine/exhaust noise at the highest recording rate possible, in neutral...

If "tin" is rattling around, that would need to be addressed first before fitting test solutions.

My chosen app settings are in bold.
For FFT size the app allows from 128 to 16384 (8192 is available, but 2048 fills the graph better)
Range 22kHz to 1.1 kHz
Window none, Bartlett, Blackman, Flat Top, Hanning (is available), Hamming, Tukey, Welch
Weighting: A, C, none
Octave bands: octaves to twelfth octaves (three)
X-axis (magnitude, ordinate): log or linear
Y-axis (abscissa): decibels, log or linear amplitude / intensity / custom
Autoscale: off, x, y, x&y
Averaging: off, on, reset
Max & Min: Max, Min, Max & Min
Interval: fastest, 250ms, 500ms, 1000ms
Cursor: off, free, snap to peak
Save: CSV, Store setup, recall setup [I hope that I can gain access to the saved file.]

I scanned part of the interior for dB (A), but will need an assistant to complete this and do the exterior recordings. The cabin noise is slightly more from the driver's than PAX and even less in the rear seat. To me, this points to the front heat shields. The first box is just above the rear PAX side.

I did make a sound recording, but it is in .m4a format, which I can't upload here. Anyway, the recording is way down on dB (A) and the Hz are not the same as the app was picking up live. However, from a cursory glance at the app using the same settings above (as done previously), I think I'm already seeing less drone.

The heat shield above the first box provides a sufficient gap for pushing strips of the tarred aluminium through and pressing them into place. However, the front two heat shield sections will require their removal. I'll rotate this vehicle out of service for that project as I want to secure the “French” clips with tar after removal as they act like once-off clips onto what looks like the sharp end of self-tapping screws. I lost a fender liner to these silly clips already.

I agree that I must stop the “tin” rattling. I'm glad for the challenge that resulted in its discovery. I'd be happy if sound deadening the front two heat shields stopped the drone. Then it would appear that the new pipe's harmonics set off the heat shields to vibrate which the cabin then picks up.

[Dust Buster]

So 1.5x-2x stock engine power? Increasing the pipe diameter by a factor of 1.25-1.4 makes sense then.

Correct on the power increase. I was hoping to use 50mm instead of 57mm, but I'm supposedly just over the 50mm and it would eventually limit me from further gains.

[ptuomov]

So 1.5x-2x stock engine power? Increasing the pipe diameter by a factor of 1.25-1.4 makes sense then.

Correct on the power increase. I was hoping to use 50mm instead of 57mm, but I'm supposedly just over the 50mm and it would eventually limit me from further gains.

I believe that if you have 57mm downpipe, the appropriately matched tail pipe end might be 50mm. The gas cools and the velocity might be the same that way. The good news is that you can test this and reduce the tail pipe opening to see if power goes down or not when reducing the tail pipe to 50mm.

50mm tail pipe is easier to muffle than 57mm tail pipe.

[Dust Buster]

What are the dimensions of the new rear muffler and old rear muffler? What is the largest volume muffler you could fit as the rear muffler?

43.5Mm -> 57mm diameter is 31% increase in diameter and 72% increase in area. That’ll reduce back pressure for sure and makes sense with 50%-100% peak power increase. However, it does mean a much louder car unless the muffler external sizes are increased correspondingly...

One thing you might want to try is temporarily constraining the tail pipe with a reducing tip...

Both new mufflers are 10cm x 18 cm ovals x 39 cm in length and the largest sized 57mm (internal bore) mufflers that they sell (they said that they were quieter than their smaller 57mm versions). From the contortions to reach the back of this muffler, I'd say that it couldn't be much, if any, longer. Upon reflection, I think that I should have requested one to be smaller than the other for Helmholtz purposes. I didn't measure the old rear muffler, but it seemed more rotund and shorter.

That larger pipes are louder is one of the reasons that I was thinking of the 50mm vs 57mm, but these fabricators don't do mandrel bends.

I did manage to get the old tip off the section of cut pipe that they gave me (upon request). I can try adding that after deadening the front two heat shields.

I also placed a narrow strip of the roofing product along the length of the first muffler. Now that I see it won't leave a flaming drip trail on the asphalt, I will do the same for the rear box. It might just be that the boxes are stuffed tight with packing and thus, don't flex in my hands, but are still vibrating. I should try this as a separate experiment.

[Dust Buster]

My bad - <Ctrl>B doesn't bold in hypertext

My chosen app settings are in bold.
For FFT size the app allows from 128 to 16384 (8192 is available, but 2048 fills the graph better)
Range 22kHz to 1.1 kHz
Window none, Bartlett, Blackman, Flat Top, Hanning (is available), Hamming, Tukey, Welch
Weighting: A, C, none
Octave bands: octaves to twelfth octaves (three)
X-axis (magnitude, ordinate): log or linear
Y-axis (abscissa): decibels, log or linear amplitude / intensity / custom
Autoscale: off, x, y, x & y
Averaging: off, on, reset
Max & Min: Max, Min, Max & Min
Interval: fastest, 250ms, 500ms, 1000ms
Cursor: off, free, snap to peak
Save: CSV, Store setup, recall setup [I hope that I can gain access to the saved file.]

[CMIdiesel47448]

This works better than I could have possibly imagined. ~28” in length, diameter is really of no consequence. 302 ford with 2 chamber flowmasters. Unbearable 2000 rpm drone before, absolutely none after. As an unexpected fringe benefit, had the same impact on idle drone.