Side Branch Resonators on our 60v6's?

[TGP37]

I was reading about different mufflers and other exhaust items. Came across the side branch resonator. It seems like a great way to reduce any drone. I did notice the 60v6 doesn't have a real drone "problem" but any drone reduction can only help more versus the flow restriction applied.


Example pic

I see they need to be tuned, 1/4 wavelength at the desired RPM. Which is simply increasing or decreasing the length of the side pipe.

Any one try or use one of these and if so, what is the best pipe length?

Or how do I go about calculating the required length?

Opinions, advise, discussion...please.

[pocket-rocket]

Those are strange looking...

I would go about finding the manufacturer to find out how to calculate the length.

[SappySE107]

http://www.amazon.com/Scientific-Exhaust-Systems-Engineering-Performance/dp/0837603099

This is what you want. I wish I still had my copy, but it was never returned. It explains those and examples/results as well.

[ericjon262]

http://www.amazon.com/Scientific-Exh.../dp/0837603099

This is what you want. I wish I still had my copy, but it was never returned. It explains those and examples/results as well.

I've been reading that on my lunch breaks at work, good read, lot's of info in there.

[Mars]

I don't know, keep it simple, go with a perforated core muffler same size as the rest of the exhaust and voila. Dynomax, Borla, Magnaflow....

[slw240sx]

since your V motor uses a single pipe exhaust you really wont get the drone like a V motor using dual pipes all the way back. I don't think you need these at all. the turbo also will do away with a V motor drone it acts as a resonator to remove drone. for your setup being a turbo car you will want the least restrictive exhaust possible a straight through muffler like magnaflow, which i recommend over all other brands due to their throaty tone. if those are too loud for your taste a good straight through resonator is needed. The longer the quieter it will be. also pipe size and wall thickness will play a big part in the note and drone created. thinner wall material will be louder. as larger diameter will be more raspy and drone. the need to throw a 3" exhaust on a turbo car is very common, but for power levels in the 250-300hp range i suggest staying around 2.5" to help reduce the raspyness.
you can also experiment with making your own mufflers by buying all the stuff from mcmastercarr.com. they have the perforated mesh, the filler stainless steel or fiberglass batting, and sheet metal to make the case. I am really wanting a customer wanting to experiment with aluminum exhaust so i can build an aluminum muffler using stuff from there.

I have recently had a 350Z in that had a similar setup to these but they where pre muffler, they were to help do away with drone cause by the uneven exhaust pressures because its a true dual exhaust. the exhaust also had a cheesy H pipe up front before the cats. the customer had done some research and they have found the only way to cure it is to remove the H pipe and put in a X pipe to equalize the exhaust pulses. I built the Xpipe and it worked amazingly.

[Mars]

On my turbo car, the absolute BEST it ever sounded was no muffler, just 3" tailpipe with a 3" hi-flow cat in the stock location. it sounded WICKED.

I removed it though, cause I'm almost certain I would plug a cat real quick just by how rich the car is running, but if I could, that's what I would do...

[TGP37]

Thanks guys.

I did a spectrum scan and found two higher frequencies in the exhaust note. At 1.95 KHz and 2.27 KHz.
I figured a side branch resonator would do well to cancel out those frequency ranges. They will each be less then 1' each due to the frequency.


The yellow lines on the left are the boundary lines of the dominant exhaust note, the meat behind the tone. When rev'ed up the exhaust has a mild rasp like any exhaust does. This analysis is showing me there are two frequencies at a higher amplitude. Probably the result of exhaust harmonics in the pipe or what ever. But now I can neutralize them.

The software is easily found, sound analysis has dozens of great programs out there. With a good microphone, anyone can record their exhaust tone and look for any high pitch frequencies standing out. The side branch resonator can be determined what length needed to neutralize the specific frequency.

[SappySE107]

I will be checking that out

[TGP37]

Okay, I found the formulas and did the math. I happened to stumble upon what I think is a great idea for a side branch resonator.

Example, current exhaust is 2.5" diameter and we are going to neutralize the frequency range 1.97-2.27 kHz. A straight pipe with a capped end will return a frequency dependent of the tubes length. EGT in this example are 900°F.

Speed of Sound (ft/sec) = C = 49.03*(460+EGT°F)^0.5 = 1808.14 ft/sec
Frequency Hz = F

Wave length in feet = C/F
wave low = 1808.14/1970 = 0.918 feet = 11 inches
wave high = 1808.14/2270 = 0.797 feet = 9.5 inches

Side Branch Length = wave length / 4 = 11/4 = 2.75 inches
Side Branch Length = wave length / 4 = 9.5/4 = 2.37 inches
---OR---
Resonator Length = (λ)*3 / 4 = 33/4 = 8.25 inches
Resonator Length = (λ)*3 / 4 = 28.5/4 = 7.125 inches

So now we have 4 lengths to cancel out the frequencies we picked out. A straight pipe will only reflect 1 specific frequency due to the reflecting cap being parallel. Leaving gaps in between the 4 lengths we have.

But a curving branch has a short side AND a long side which means it will cover a range of frequencies. Using a 2.5" mandrel bend with the bend at 12" diameter, and measuring the length from the center of the bend, a curve of 24° will cover the range of 2.8" to 2.3". 2.5" being the center line. The bad frequency we are trying to eliminate needs a resonator from 2.75-2.39 inches. the 24° side branch resonator will cancel out the spectrum.

Another one can be made with the 8.25-7.125" branch length. A degree of 72 to 76 degrees will cancel out 8.25-7.125.

So a mandrel bend of 24° and 74° will cancel out the frequencies noted in the spectrum analysis. Some said place them in the center of the exhaust piping, but many more suggest it just up stream from the mufflers.

I read these resonators can cancel out 20-30 dB's if built well enough.

So now we can record our exhaust sound and pin point problematic rasp of other annoying frequencies. Not sure how many can be used but I am planning on one for each dual exhaust, one 24 and the other 74 degrees.

If I wanted to reduce a lot more of sound during idle at 800 RPM's, a mandrel bend of 151° will reduce the dominant engine frequency at that idle speed.

Also, another resonator length that is effective is (λ)*5 / 4. So if each resonator was well tuned, welding up three different lengths targeting on specific area could really reduce the sound.

I hope it works as it should.

[SappySE107]

I think some pictures would go a long way with all those numbers. Very cool indeed, and placement would depend on temperature right? have you tried using other temps to see what kind of difference that makes?

[TGP37]

I think some pictures would go a long way with all those numbers. Very cool indeed, and placement would depend on temperature right? have you tried using other temps to see what kind of difference that makes?

Not yet, I am refining the excel sheet for now. So I can just insert a few numbers and get the result. Nothing carved in steel yet but there will be. Once the sheet is done I will see what a variation in exhaust temp does to the resonators range of effect.

As far as I know, the exhaust temp will change the speed of sound. Which should skew the dominant frequencies but not by a whole lot.

lets see, the speed of sound is calculated .....
c = 49.03 * Tr ^0.5
Tr = 460 + egt°F

one temp of 1400°F and another of 400°F

c = 49.03 * (460 + 1400) ^0.5
c = 2114.5 ft/sec

c = 49.03 * (460 + 400) ^0.5
c = 1437.87 ft/sec

2000 Hz is the target frequency
@1400°F branch is 3.17"
@400°F branch is 2.15"

0.01" per 10°F
So the difference of 1,000°F caused a needed shift of 1". Which is quiet manageable. As long as the average EGT's are known it wont be an issue after install.

This document provided a LOT of information. Side branch resonators are on page 42.

LINK

It also depends on the situation. Either tune to resolve harsh tones at WOT, or maybe drone at cruise, or even at idle.

I'm not going to build and install these side branch resonators until I get the car on the road and record the exhaust note. But when I do, I'll take pics so others can do the same if all works out well.

[Z3500]

That sounds really cool (but complicated) if it works.

I am working on tweaking my own way to cancel out the drone...

[pocket-rocket]

That sounds really cool (but complicated) if it works.

I am working on tweaking my own way to cancel out the drone...[ATTACH=CONFIG]6058[/ATTACH]

At work last week they did the standard annual hearing tests...

I have moderate hearing loss in my right ear, and higher frequencies are my worst ones in both ears from the same theory you are working on (Polk audio tweeters in the side mirror panels... ), lol.

[TGP37]

Hey check this out. I was looking at the spectrum scan of my exhaust note and it dawned on me the pipe to the waste gate will act like a branch resonator while closed. So I went to measure and it was roughly 7" long. Did the calculations and I found it's primary resonance was 900 Hz. I looked at the spectrum scan and sure enough the area around 900 Hz is much quieter. You can see the resonance of the pipe canceling frequencies around 800-1000 Hz.

So I suggest to run the waste gate dump pipe at a length that will help reduce any drone at cruise. Or idle muffling if there is no drone.

Who would have thought!