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Curing exhaust drone - Helmholtz resonator
- Thread starter steveg
- Start date
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OP
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I had considered that and, while searching for same, came across lots of discussion and youtube videos of people curing aftermarket-exhaust droning using these devices. One uses a phone app to measure the frequency of the drone @ rpm, then plugs it, along with engine size, cylinders, etc., into a formula which tells one how long to make the j-pipe.
The smallest resonators I found are 12" x 3" which will fit my 1-3/4" pipes and would probably be OK ground-clearance-wise if placed directly under the axle. Whether they worked would be guesswork, whereas the helmholtz resonator would eliminate or greatly alleviate the drone if I do the math correctly.
The length is critical in order to create a sound wave that's 180 degrees out of sync with the main drone, therefore cancelling it.
OP
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The existing single muffler (non-BJ8) is the Moss Tourist Trophy, so yeah the resonance originates there - though it is not a Helmholtz resonator. I had the same problem with my Monza and earlier stock muffler. According to the youtube videos, all aftermarket mufflers have drone points to some degree.
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I always learn something new from your posts, Steve. Let us know how it turns out.
Bob_Spidell
Yoda
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I think 2,500RPM is about 60MPH in fourth gear; could you tune the scuttle shake to cancel out the drone (or, better yet, vice versa)?
OP
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Sorry, with 4 whl discs have no scuttle shake to work with.
2500 is freeway cruising speed - 70+ in 5th.
OP
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Thank you for the complement, Rick.
It'll be a while. Will be taking a welding class and researching a welder so I can do the project myself.
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"Sorry, with 4 whl discs have no scuttle shake to work with."
Shameless self-promotion...
Shameless self-promotion...
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OP
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My tourist trophy exhaust has only around 1000 miles on it and has had the drone since installation.
Bob_Spidell
Yoda
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Begs the question: Why do these things drone? We put a Falcon SS system on our BN2--which I colored black because I don't think shiny, chrome-y exhausts look right on a Big Healey*--and it sounded a bit tinny for a while but as soon as it sooted-up it mellowed out, but no drone. OTOH, I put an aftermarket stainless exhaust on my Ranger and regretted it almost from the first minute, as it drones across the RPM band. Is it caused by too thin metal? Diameter of the pipes?
* My opinion only. Have at.
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I put a Bell system on my BJ8 and have no drone. I do recall a drone on certain mufflers sold back in the ‘70’s for Healeys.
What about installing a crossover pipe between the downpipes in front of the muffler?
What about installing a crossover pipe between the downpipes in front of the muffler?
Keoke
Great Pumpkin
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Begs the question: Why do these things drone?
HI BOB:
HI BOB:
My experience is the outer casing metal used in the manufacture of these mufflers is so thin or the welds to the internal baffles break due to the drone vibrations in the case
.
BEIN Cheep:
One I cured years ago I simply added three stainless steel hose clamps equally spaced around the mufflers outer case no more Drone.
Patrick said he uses the Bell system hardware ,Me too and not a peep----LOL
Last edited:
Keoke
Great Pumpkin
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Well Steve:
It seems like if they make the under floor muffler cases thin as a dime they gonna whine.
However, the rear BJ8 resonators generally had sturdy thick cases and did not contribute to the problem.
OP
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Well, Keoke, I may have a whine but can't hear it as my hearing loss is in the upper ranges. However the drone is much lower and makes my skull resonate along with the car.
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