What causes exhaust drone?

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BornInAFord

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Apr 22, 2005
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Muffler drone often is a result of the design of the baffles within the muffler, and the resulting harmonics rather than the size of pipes or whether one has an x or h or y or o pipe ;) or headers. The sound pulses (waves) from each cylinder's exhaust valve burst have a certain frequency that increases as the RPMs increase. At a certain RPM, the muffler enhances these frequencies much like a guitar case, shape, and opening will enhance the sound of the strings' frequencies. This phenomenon is called resonance, and can be annoying when using certain muffler systems. Many popular two and three chambered mufflers have some sort of resonance frequency, although many aren't noticeable because the frequency is either canceled by other baffles within the muffler or is at too low or high of an RPM to be obvious. Some, like the Flowmaster 40s and 50s (2 and 3 chamber mufflers) have a reputation for droning because their resonance frequency is around 2000RPM, which is the RPM at which many cars will cruise.
Make sense?
Daniel
 

jes72mustang

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Mar 31, 2005
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Good explaination, Thanks. I took my wife out in the mustang on Saturday and she was complaining about the drone. I have Flowmaster series 40s. What would be a good alternative when I do the new exhaust over the winter?
 

Jester67

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Sep 21, 2004
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BornInAFord said:
Muffler drone often is a result of the design of the baffles within the muffler, and the resulting harmonics rather than the size of pipes or whether one has an x or h or y or o pipe ;) or headers. The sound pulses (waves) from each cylinder's exhaust valve burst have a certain frequency that increases as the RPMs increase. At a certain RPM, the muffler enhances these frequencies much like a guitar case, shape, and opening will enhance the sound of the strings' frequencies. This phenomenon is called resonance, and can be annoying when using certain muffler systems. Many popular two and three chambered mufflers have some sort of resonance frequency, although many aren't noticeable because the frequency is either canceled by other baffles within the muffler or is at too low or high of an RPM to be obvious. Some, like the Flowmaster 40s and 50s (2 and 3 chamber mufflers) have a reputation for droning because their resonance frequency is around 2000RPM, which is the RPM at which many cars will cruise.
Make sense?
Daniel
This also applies to the cars body panels as that act like a drum and amplify the sound that is why the tarpaper sound deadener works.
 
Mine drones pretty badly within a tight RPM range, I don't know what RPM, because I don't have a tach yet, but it is at slower speeds

I switched from flowmasters, which were too loud overall for my taste anyway, to turbos..............but the drone itself is still there at the same RPM.

I'm blaming it on the cam because once I get to higher RPM's she just purrs like a kitten.

I'm trying to stop it from getting inside the passanger compartment with dampening material, but with the ragtop I don't think there is much I can do.
 

S-Car-Go

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Mar 25, 2003
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Good explanation but I still think there's more to it. Most mufflers drone at ~2,000 rpms. This is usually in the cruise range of the average car. Why would muffler companies design something with a resonant frequency right at the typical cruise RPM? That would be like designing a motor that was efficient except at 65-75 mph.. I just put in an entire exhaust system, headers to tailpipes. It has a noticeable drone when I'm by myself, but will blow your ears out if other people are in the car (more weight). I used Dynomax Super Turbos which supposedly don't drone. I put borla's on my 94 and they didn't drone anywhere near the Dynomax. I'm very disappointed to say the least. If this was a vette, new Shelby, viper, etc, the drone would kill sales, so why should we settle? Those cars have free flowing exhaust w/o the drone and can make more than 400 hp. I'd like to do the same (I'm building a 350 hp 289). Any ideas how to lose the drone without swapping mufflers trial and error?
thx,
Derek
 
I think there's a couple of other factors that we need more information on. First of all, do the pipes go all the way out the back or are they dumps? Also, do you have an H-pipe cross-over or an X-pipe? I rode in one Mustang that was really quiet. It had an X-pipe with the exhaust all the way out the back. Don't remember the muffler type, but it was a 2 1/2 inch exhaust. YMMV.
 

S-Car-Go

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Mar 25, 2003
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All ideas are welcomed!

The system is stepped long tubes 1 5/8" then 1 3/4" with 3" collectors reduced to 2 1/2". Then 2 1/2" intermediate pipes with an H pipe. Then it reduces again to 2 1/4. This is due to the 5 sp and the convertible "belly plate". It's really tight in there. Anyway, 2 1/4" offset/offset Dynomax with 2 1/4" tailpipes all the way back. Turndowns out the valance. I was gonna swap out the turndowns for straight tailpipes, but the turndowns exit at a 45* angle, so I don't think they'll cause resonance under the car like dumps. It's a real clean job with rubber hangers also.

I'll look into Dynomat for under the rear carpet. That sounds like a good idea.
 

enferno

Member
Jun 20, 2006
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S-Car-Go said:
Good explanation but I still think there's more to it. Most mufflers drone at ~2,000 rpms. This is usually in the cruise range of the average car. Why would muffler companies design something with a resonant frequency right at the typical cruise RPM? That would be like designing a motor that was efficient except at 65-75 mph.. I just put in an entire exhaust system, headers to tailpipes. It has a noticeable drone when I'm by myself, but will blow your ears out if other people are in the car (more weight). I used Dynomax Super Turbos which supposedly don't drone. I put borla's on my 94 and they didn't drone anywhere near the Dynomax. I'm very disappointed to say the least. If this was a vette, new Shelby, viper, etc, the drone would kill sales, so why should we settle? Those cars have free flowing exhaust w/o the drone and can make more than 400 hp. I'd like to do the same (I'm building a 350 hp 289). Any ideas how to lose the drone without swapping mufflers trial and error?
thx,
Derek


in terms of efficency of gas mileage, almost every gasoline engine has a non linear, exponential scale of consumption.

when comparing speed to consumption, maximum efficency peaks at around 55 mph, and drasitacly increases at anything higher than that.




i also believe that your theory that it's not a flaw in the design in the exhaust that causes the drone, but rather the engine etc. is false. one could use the same argument that you did . . why make an engine that has drone around 2k rpm?
 

D.Hearne

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Sep 29, 2000
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I've got Dynomax's on my Ranger and they don't drone. But I've also got a homebuilt "equalizer pipe" ahead of them. Shelby type Tri-Y's into 2.25" pipe, into a single 4" dia. X 12" long pipe, back into dual 2.25" into the Dynomax's then out the rear of the truck with 3" angle cut ends.
 

mustangdave

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Feb 26, 2002
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The answer to the problem is under most 80's Mustangs with factory exhaust. Stock mufflers had the same problem so Ford shortened the pipe on one side so that the mufflers were offset--no more drone!!!!!! Nothing to do with muffler or engine design...it just is.
After installing an aftermarket kit on an 86 the drone was there due to equal length pipes to the mufflers: I had the muffler shop cut one shorter and weld in a new piece and the drone was gone.
 

Hack

15 Year Member
Mar 23, 2004
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mustangdave said:
The answer to the problem is under most 80's Mustangs with factory exhaust. Stock mufflers had the same problem so Ford shortened the pipe on one side so that the mufflers were offset--no more drone!!!!!! Nothing to do with muffler or engine design...it just is.
After installing an aftermarket kit on an 86 the drone was there due to equal length pipes to the mufflers: I had the muffler shop cut one shorter and weld in a new piece and the drone was gone.
The drone/resonance has to do with the length of the pipe and its relationship to the length of the sound waves generated by the engine. I don't have the time to do the math, but I think if you do you will see that for about 2,000 rpm the length of the sound waves are about the same as the length of the tailpipe.

This is why Ford changed the pipe length. Your solution is either go to a quieter muffler or change the length of the exhaust.
 
mustangdave said:
The answer to the problem is under most 80's Mustangs with factory exhaust. Stock mufflers had the same problem so Ford shortened the pipe on one side so that the mufflers were offset--no more drone!!!!!! Nothing to do with muffler or engine design...it just is.


After installing an aftermarket kit on an 86 the drone was there due to equal length pipes to the mufflers: I had the muffler shop cut one shorter and weld in a new piece and the drone was gone.

Are saying that the problem is because the pipes coming out of each end of the muffler are the same lenght :shrug:

after the new piece was welded in, didn't it now become just as long as it was before it was cut short???

i'm totally confused........
 

302 coupe

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Mar 2, 2000
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I remember a post on a here a long time ago where a guy had added some piping to his tailpipes, kinda like an H pipe but it was capped off instead of connecting the pipes. His theory was that it worked as a cancellation chamber and the soundwaves would go into the faux H, then bounce back out and cancel each other. My car drones like mad at 2200 rpm, fortunately at 55 mph I'm doing 3200 rpm, lol.