good info...might try this...i cant stand my drone when the DoD kicks on...will this fix this?

 

You may be SOL with the DOD issue. The problem for you will be you will need two lengths, one for 8cyl. drone and the other for 4 cyl. drone.

 

Well, I think I fix it FINNALY!! I added about 6-7 inches to my resonation tubes and seems to have killed about 85% of the drone. Testing with my dB iPhone tool, seem about 78-80max dB at 75-80mph and 22-2500 rpm. it gets a little bit louder, but not in a drone kind of sound. SOOOO much nicer! Also is quieter around town too as it passes through the the 2-3000 rpm range. This was my last ditch effert before I ripped this whole exhaust off and putting a stock Duramax muffler and tail pipe on it. So now my resonation chamber tubes are about 31-32" long.

 

That's good news KB. Glad you got it sorted out. Guess I was lucky hitting mine on the 1st shot.

 

I didn't do a bunch of pipe fabricating or welding, but finally cured my drone 100% as well....

Ebay/OBX style longtubes
2.5" collector pipe to 3" ORY flange w/pretty much non-functional resonators (came on the Y pipe)
3" I pipe
Flowmaster 1 into 2 muffler (whatever the loudest one is, 40 or 44???)
Two 2.5" tailpipes

I originally ran it this way after longtube swap....
WAY TOO MUCH rasp and drone....which sounds like ****, IMO.
Put 3" Summit "Bullet" muffler inline in the I pipe/mid pipe
Helped tremendously, but still a fair bit of drone and rasp at certain rpm's
Today, swapped the Bullet muffler out for a Hooker AeroChamber muffler
Perfect sound IMHO...Thank God.

Can't barely even hear it at highway cruise rpm's, but still nice tone when accelerating, or in the upper rpm's...ZERO drone, and zero rasp, finally...even feels like it is a little torquey-er, lol.

 

that is awesome thank for posting should be a cheap fix

 

Interesting thread, I have some resonation around 2000-2300 rpm. I have an Aerochamber with the stock exhaust tube, but the pipe is cut after it goes over the axle and points down at a 45° angle or so.

I've been thinking on getting a turndown, but haven't done it as I'm afraid it will get worse.

 

jiar, just found this article that talks about tailpipe length and side branch resonator length to eliminate resonance, and is written pretty much in layman's language. The article was written about diesel generators, but exhaust is exhaust. http://www.enoisecontrol.com/related...rrier_wall.pdf
Scroll down to page 35 to go directly to tailpipe design. Seems that tailpipe length should be 1/4 the sound wavelength at the frequency where resonance occurs. The wavelength is the speed of sound/frequency. Guessing that tailpipe temperature is 250 degrees F, the speed of sound at that temperature is about 1300 fps. Frequency at 2000 rpm is 133.3, (2000rpm/60seconds/2 revs per fire)*8 cylinders. So...wavelength = 1300/133.3 = 9.75 feet. Ideal tailpipe length to eliminate resonance at 2000 rpm would then be 9.75/4 =~ 2.4 feet. One possible downside is that this length should resonate at 4000 rpm, although this would be much less annoying than at 2000 rpm.

Side branch resonator design starts on page 41. Side branch resonator length is calculated the same as taipipe length; the resonator length should be 1/4 the sound wavelength at the frequency where resonance occurs. Again, this would be ~2.4 feet at 2000 rpm.

I made my side branch resonator about 6" shorter than the calculation said it should be, but it still works. It's 2.5 feet long and it should be 3 feet; I ran out of room to extend it further because it hit the frame.

 

i was searching for exhaust drone cures and i came upon this thread. tons of great info here!! i'm asking for a little advice with my situation. i have a 03 silverado ss, stock manifolds, cats. the muffler is a dual 2.5" in dual out dynomax ultra flow x pipe muffler with 2.5 tailpipes both exiting under the rear bumper with 3" tips. when its cold its fine. as soon as the engine warms up it has a pretty loud and annoying drone from about 1500 to 1800 rpms. my question is this: would it be easier and have a better chance of eliminating the drone by adding the resonance chambers to each tailpipe, or by changing the tailpipes to both exit in front of the passenger side rear wheel. from what i read, shortening the length of the tailpipes helps change when the drone comes in, and to be honest, i hate having the tailpipes out the back. as it sits my drivers tailpipe is prob close to 7 ft. and the pass is noticably shorter. could this too be causing the drone? if anyone could give me some insite i would greatly appreciate it!!! thanks!

 

Gary, I've studied the formulas in the engine exhaust noise control article I linked in the previous post and think I understand, so here's my thoughts. The frequency of your exhaust sound at 1500 rpm is 100hz, at 1800 rpm it's 120hz. Depending on the exhaust temperature in the exhaust pipe, the speed of sound is somewhere between 1200-1300 feet/second. This makes the wavelength of the sound 12-13 feet at 1500 rpm and 10-10.8 feet at 1800 rpm. That article states that resonance occurs when the exhaust pipe length equals 1/2 the wavelength, so it's entirely possible you have the longer pipe (half of 12-13 feet) resonating around 1500 rpm and the shorter one (half of 10-10.8 feet) around 1800 rpm. Resonance doesn't just occur at exactly one frequency, but 300 rpm is a pretty wide range. Thus my thought that this is the result of the pipes resonating at different, but fairly close, frequencies. If you can make your tailpipes 2 or 2.5 feet long, that should move the resonant frequency up to 4500 or 3600 rpm respectively where it won't be as annoying...higher frequency plus you don't go there as often.
If you can take your tailpipes off fairly easily, you can buy some 2 1/2" ID flexible pipe really cheap and experiment to see if it works. Side branch resonators would work as well, but I suspect you'd need different lengths for each side somewhere between 2 1/2 and 3 feet long (1/4 wavelength). Length isn't super critical, but you have to be within a few inches.