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Old 01-16-2017, 10:46 PM   #1
spock345
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Default Severe drone at 2.5k rpm after middle muffler delete.

I replaced my middle muffler with a straight pipe after it fell apart on me due to rust. Now whenever the engine hits 2.5k rpm I get a really bad drone. I rather like the difference in sound most of the time but need to find some way to at least reduce the drone as I will be driving cross country soon and I think this will drive me insane. Is there an inexpensive way to reduce the drone?
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Old 01-16-2017, 11:26 PM   #2
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A middle muffler.


You asked for it
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Old 01-16-2017, 11:29 PM   #3
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Higher speed/higher rpm will get rid of the drone.
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Old 01-17-2017, 12:13 AM   #4
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Higher speed/higher rpm will get rid of the drone.
I would do that but the drone happens at 2500 rpm, which is where my automatic transmission likes to live while cruising at 60-65 mph. If I go below that or above 3000 the drone lessens or disappears.

I am just curious if there is a modification I can perform to the exhaust that will cost less than a $40 resonator.
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Old 01-17-2017, 12:40 AM   #5
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24.95 at amazon, and also good for working with tools.

40$ is a cheap fix, engineered by Volvo engineers, you aren't going to get better for cheaper.
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Old 01-17-2017, 12:55 AM   #6
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What type of rear muffler is being run?
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Old 01-17-2017, 01:09 AM   #7
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Google "¼ wave resonator". It's basically a tuned length of pipe attached at 90° to the exhaust pipe.
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Old 01-17-2017, 03:08 AM   #8
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What type of rear muffler is being run?
The stock rear muffler. I am intrigued by the 1/4 wave resonator. Might as well try that. I wonder if changing the weight distribution of the exhaust at all would change the resonant frequency?

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Old 01-17-2017, 03:21 AM   #9
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It's cheaper than a muffler and supposedly doesn't rob any HP's.
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Old 01-17-2017, 03:23 AM   #10
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Some people with sport exhausts install vibration absorber which can be found on a lot of modern cars (BMW, Audi) and they seem to work.

Example No 10
http://www.etkbmw.com/bmw/EN/search/...61/18307795604

BMW 330i No 20
http://static.bmwfans.info/images/epc/MTU2MDY5X3A=.png

Installed one a few days ago but test drive not yet done...
http://www.bilder-upload.eu/show.php...1484637764.jpg
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Old 01-17-2017, 07:25 AM   #11
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Installed one a few days ago but test drive not yet done...
http://www.bilder-upload.eu/show.php...1484637764.jpg
Nice. Curious about the results. Need to replace the exhaust cat-back on mine, will do stainless steel, maybe this helps if i run into a small drone problem. I imagine the Helmholtz (1/4 wave) resonator is more effective then.
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Old 01-17-2017, 08:54 AM   #12
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I´m also looking forward to the first test drive (actual troubles to get the car running normal after 1 year standing and wasted spark modification).
The resonators are quite heavy. Got them new for 10 Euro so no tragedy if they are useless...
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Old 01-17-2017, 09:25 AM   #13
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Exhaust weights for drone/harmonic issues are quite common. I have the same condition as the OP at 2300, but it doesn't occur enough for me to have addressed it, so I can't speak for personal results. If I were going to try to remediate it, however, it's the only approach I'd take. Otherwise, I'd say F it.
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Old 01-17-2017, 09:35 AM   #14
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1/4 wave resonators work, and work well. However, there's some science involved. If you decide to make one, make it adjustable so you can tune it appropriately. I can promise you won't get it right on the first try. Pipe diameter and length are crucial.

Or just buy a vibrant ultra quiet resonator and be done with it.
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Old 01-17-2017, 10:44 AM   #15
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honestly, I'd ditch the rear muffler and put the resonator back on there. aside from the weight it's not a very restrictive piece.. and I doubt OP is at a point where a couple of lbs makes any difference whatsoever in the performance of the vehicle.
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Old 01-17-2017, 03:16 PM   #16
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Remove all of your exhaust and just make a series of 1/4 wave resonators, so it looks like a pipe organ down there....you'll eliminate all the frequencies.
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Old 01-17-2017, 05:25 PM   #17
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Long thread but lots of good info here including how to calculate length of tube needed.....the fewer the number of cylinders, the longer the needed tube is.... http://forums.corral.net/forums/gene...tely-gone.html
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Old 01-17-2017, 05:33 PM   #18
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Longer rear muffler will result in less drone.
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Old 01-17-2017, 05:45 PM   #19
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Thank you all for the input. This is definitely enough to keep me busy for a while. I will just have to start experimenting with my exhaust setup. Perhaps try experimenting with a few weights on the exhaust or the vibration absorbers to see what that does, then a 1/4 wave resonator if the previous approach doesn't work (I can actually leverage those college physics classes now). If all else fails I will experiment with mufflers, although I would rather not as I like the sound my exhaust makes right now, other than the drone.
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Old 01-17-2017, 05:59 PM   #20
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What car is this for?
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Old 01-17-2017, 06:17 PM   #21
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What car is this for?
My '89 244.
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Old 01-17-2017, 10:53 PM   #22
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Google "¼ wave resonator".
A pretty interesting rat hole you sent me down.

For best results google "quarter wave resonator exhaust" It appears that there are some other applications for the principle.

Top result seems pretty informative, gives several examples of calculation for size
.
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Old 01-18-2017, 01:15 AM   #23
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Quote:
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...experimenting with a few weights on the exhaust or the vibration absorbers ...I can actually leverage those college physics classes now..
It interested me to see in the link by Trieb that the part number for the exhaust dampers had a Hertz frequency listed.


I see those weights all the time working on modern cars. They even have em dangling off the back of transmissions sometimes. I have seen em on Fords and Nissan, I think.... and on BMW exhausts from early 2000's.


Here is my regurgitated google link during 10 seconds of searching. Talks about sound frequency of exhaust notes:

http://www.caranddriver.com/features...ferent-feature
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Old 01-19-2017, 06:04 PM   #24
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The factory resonator was in there to reduce that drone.
Easiest way to fix it is to put it back in there. You won't notice the .25hp it cost at the wheels.
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Old 01-19-2017, 10:10 PM   #25
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What about a Test Pipe+Turbo Muffler, that would give a good tone while not being too loud right?

Specifically flowmasters turbo mufflers are pretty good at quieting the exhaust. You can basically put a test pipe all the way to the muffler.

Last edited by Fumomike; 01-21-2017 at 12:23 AM..
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