[MFE]

This isn't installed, and these are fixed-length pipes, but it's virtually identical to this. The only thing unique about the G8 setup is that they utilized some free space that used to be taken up by the gigantic factory mufflers. They don't have to be mounted here, and as you know they don't have to be J-shaped. More pics in this thread

 

[Michael Yount]

Nice -- did you measure the length of the J-tube...

 

[MFE]

No, I forgot to, but by my calculations they should be about 25 inches.

 

[buffhomer]

Damn, 10 decibels. That's incredible, although I completely know the feeling!

I tried to get an app on my Galaxy to measure the difference in my car but none of them seemed to work correctly. Good to know the true difference.

 

[Ascaris]

BTW, that 10-decibel difference is huge.

I went back and reread my entries from earlier in the thread (before it was merged), and I see that I got the same result. I used a digital sound level meter from Radio Shack.

On my 1990 LX 5.0, stock exhaust other than Super Turbos welded in the stock location, I got a dB(a) reading of 79 to 80 on flat ground at 2000 RPM in fifth (about 68 mph), but the slightest bit of throttle (to go up a slight hill etc.) brought the spl up to 85 dB(a).

The quarter wave tubes brought that down to 75 dB(a), on flat ground or going up a slight hill (I used the same length of freeway in the city to test).

So, 10 dB change for me as well. 75 was only one dB higher than the reading with the stock mufflers at the same speed on the freeway. (The stocker had a few rust holes in the bottom, but I had patched them. It was quieter, but I did not have any confidence in the longevity of the repair.)

My use of dB(a) simply means I used the "a" weighting scale on the meter, rather then the "b" scale (the other option I had).

 

[Reverb]

I can't remember if there was an answer to the question of the diameter of the tube - I know someone used 2 1/2 for their 3" system, but how small can you go before it effects the wave?

 

[VORTECD]

I have 3" exhaust and used 2" pipe and it worked fine

 

[Reverb]

Did you adjust the formula at all? The wave would have a higher frequency if it went from 3" to 2" at the same speed.

 

[the_tool_man]

The speed of the sound waves doesn't change with tube diameter. A smaller diameter tube should reduce the amount of attenuation, because it transmits less sound energy back into the exhaust.

 

[Reverb]

So the diameter doesn't matter? Can I use a 1" pipe? Or what you are saying is it won't quieten as much with a smaller diameter

 

[TrickVert]

Smaller diameter pipe will give less attenuation (meaning yes, it will have less "quieting" effect). A small deviation would probably not be very detrimental - using 2-1/4" pipe on a 2-1/2" system, for instance - but 1" pipe would likely be nearly ineffective.

Andy

 

[Saleen9090]

I forgot how touchy some people on forums can get. By NO means was I trying to take away or deny anything that anyone else has done on this thread. Please do not take offense from my post because I did not mean to disrespect.

My point was, if you look at the Flickr photo I posted, NONE of the amplitudes at any frequency were canceled out which I don't know how to explain. Could it be because I installed midway down the exhaust? I can't install at the tips due to space so I had to install at the midpoint of the exhaust. I installed immediately before my single 3" splits into a dual 2.5". I wonder if this SBR had no effect because the dimensions of the dual exhaust could be causing the drone? I have below a picture of the setup for all to see.

Would anyone like to offer help on the calculation? Isn't it incorrect to use the speed of sound at ambient temperature like so many people have been using? I may be wrong but I would assume exhaust be around at least 500F. So I did 1518ft/s divided by 173Hz to get 8.77 feet and a quarter wave length of ~26 inches. Now my current setup of 30" will cancel out a frequency of 152Hz assuming 500F temperature. I feel this should have changed the chart I posted in my last post to reflect an amplitude change at 152Hz. I'll let the experts chime in though. I do appreciate the help though guys.

IMG_3837 | Flickr - Photo Sharing!

 

[93Cobra#2771]

The Jpipe on my solo performace exhaust is actually right after the cat, well before the muffler and probably in the first 1/3 of the exhaust...

 

[buffhomer]

No, the placement doesn't really matter. It's not that anyone is touchy, it's simply because your setup is the incorrect length.

Not sure how you came up with 173hz? That's nearly 2600rpm, not 2100.

*edit* Nevermind, couldn't see your chart on my work computer but I got it on my phone.

 

[MFE]

Would anyone like to offer help on the calculation? Isn't it incorrect to use the speed of sound at ambient temperature like so many people have been using? I may be wrong but I would assume exhaust be around at least 500F. So I did 1518ft/s divided by 173Hz to get 8.77 feet and a quarter wave length of ~26 inches. Now my current setup of 30" will cancel out a frequency of 152Hz assuming 500F temperature. I feel this should have changed the chart I posted in my last post to reflect an amplitude change at 152Hz. I'll let the experts chime in though. I do appreciate the help though guys.

You shouldn't assume the air in the tube is 500 degrees. Since they don't see any actual exhaust flow, they actually stay relatively cool.

If your drone is 2100 RPM you should be thinking more in terms of 140 Hz, which would indicate a tube length more like 23-24 inches. Your 30-inch tubes are attenuating more in the 1600 RPM/110Hz range.

 

[Saleen9090]

And I have an inline 5 so both the equation and real frequency analysis match.
5 cylinders * 2100 RPM * (1/60)s = 175Hz. Physics works!!!

So can somebody tell me where I went wrong? Was it wrong to not use ambient air temp? So instead should I have 1142ft/s divided by 175Hz equal a 19.35" SBR? Did everyone else use ambient air and was it successful?

 

[MFE]

But aren't you trying to cancel the wavelength inside the flowing exhaust? If so, you want to find the quarter length for the flowing exhaust which will be hotter than ambient. Shouldn't you be basing your equation from variables of the flowing exhaust and not the resonance chamber?

No. You're trying to calculate for the frequency inside the capped tube. That's where the sound wave enters, hits the end, and bounces back out just in time to meet the next one in the main pipe. That sound wave doesn't care if the exhaust is 100 degrees or 2500 degrees, it only has to travel up that capped tube and back, and there is no exhasut flowing through that tube.

 

[Saleen9090]

Just realized something incredible. The small charts I uploaded don't really show this, but when I crunch the numbers, I may have actually changed my exhaust note without knowing.

So my actual SBR is 32" which is 2.667ft which will cancel out a wave of 10.667ft. Using ambient air temp, the frequency being canceled would be 1142ft/s divided by 10.667ft which equals 107Hz.

Now looking at my charts, 107Hz is actually a valley, not a peak, which is why I noticed nothing.So I actually made a quiet frequency more quiet which is undetectable to a human ear.

I took the magnitudes of the max peak (175Hz) and compared it to the magnitude of the valley at 107 Hz for the pre-SBR and post-SBR charts. The results are as follows:

Pre-SBR: 107Hz Valley=-36dB 175Hz Peak=-16.4dB delta=19.6dB
Post-SBR: 107Hz Valley=-38dB 175Hz Peak=-11.6dB delta=26.4dB

The magnitudes are all different because I used different sound recording devices with different sensitivities. The important information is the delta! After the SBR, the valley at 107Hz actually became more of a valley causing the delta between the peak and valley to increase from before the SBR was installed. The delta actually increased with the SBR 7dB which is a good sign! This means I did successfully cancel drone at 107 Hz.

Does everyone agree with this analysis? If so, I will proceed to shorten but still need advice on what to shorten to. I would calculate 1142ft/s divided by 175Hz which is a wavelength of 6.525ft for a quarter length of 1.63ft or 19.577 inches.

I would like to hear opinions from the experts and I thank you for being patient with me.

 

[MFE]

for ####'s sake our ESP capabilities failed us, and we had no idea until just now that you're dealing with a 5-cylinder engine.

ANYway, the math for a 5-cylinder works like this:

2100 RPM * 2.5 pulses per revolution = 5250 pulses per minute, or 87.5 pulses per second (Hz)

1075 ft per second (113 degree air) / 87.5 Hz = 12.3 foot wavelength, or 147.4 inches. 1/4 of that is 36.9 inches.

But that Hz doesn't seem to correspond to your largest drone volume, which makes me wonder if your exhaust is exciting something else in the structure and making a resonance at 175 Hz, which is exactly 2x the exhaust drone of 87.5 Hz. Kill the exhaust drone at 87.5 Hz and you'll likely slay the dragon.

 

[the_tool_man]

One important detail to remember. All of the theory about noise cancellation assumes even sound pulse spacing, which implies an even-fire engine. Many 5-cylinder engines and some V6's are odd-fire. I'm thinking the noise cancellation would not be as effective on those engines.