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Curing exhaust drone - Helmholtz resonator

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steveg

steveg

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My exhaust has a heavy drone around 2500 rpms. I'm considering installing a j-pipe.

What I'm wondering is - can I get by with a single j-pipe if I run it as an extension off a balance tube between the two pipes?

screenshot.1403.jpg
 
Why not just add small resonators ??
 
Keoke said:
Why not just add small resonators ??
Click to expand...

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.
 
well Yeah theoretically that is correct.

However, the existing mufflers which by definition is also a Helmholtz resonator could be the culprit???
 
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.
 
it is not a Helmholtz resonator,?

Yeah they are since this is just an additional exhibit of what happens when Exhaust/ air pass through a cavity.​
 
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)?
 
"Sorry, with 4 whl discs have no scuttle shake to work with."

Shameless self-promotion... :smile:
 
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I had the same drone with the tourist trophy exhaust, fortunatly Moss honored the 5 year warranty and sent new ones. I suspect these will also burn out and the drone will return.
 
bdcvg said:
I had the same drone with the tourist trophy exhaust, fortunatly Moss honored the 5 year warranty and sent new ones. I suspect these will also burn out and the drone will return.
Click to expand...

My tourist trophy exhaust has only around 1000 miles on it and has had the drone since installation.
 
steveg said:
My tourist trophy exhaust has only around 1000 miles on it and has had the drone since installation.
Click to expand...

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.
 
I put a Bell system on my BJ8 and have no drone. I do recall a drone on certain mufflers sold back in the ā€˜70ā€™s for Healeys.
What about installing a crossover pipe between the downpipes in front of the muffler?
 
Back in the day, I replaced my TR3 muffler with a Smithy's (which gave our Fords that deep mellow tone) and cured the same problem. Just a thought.
 
Begs the question: Why do these things drone?

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
 
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steveg said:
I'm surprised, with 4 mufflers, that you BJ8 folks had a drone at all.
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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.
 
Keoke said:
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.
Click to expand...
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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