What causes exhaust drone?

[mudbilly]

i'm still confused

even if you cut the entire pipe off of one side of the muffler, you still have to attach it to the exhaust system. so, maybe the muffler would move 6 inches toward (or away from) the engine, but it is still pipes on both ends

 

[mudbilly]

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.

same hear, i estimamte that is close to the RPM that mine drones at, it is only annoying when i am doing a slow cruise by looking at eye candy.........i guess i should just drop er into second gear

 

[302 coupe]

problem solved, lol....

 

[D.Hearne]

i'm still confused

even if you cut the entire pipe off of one side of the muffler, you still have to attach it to the exhaust system. so, maybe the muffler would move 6 inches toward (or away from) the engine, but it is still pipes on both ends

But the distance between the engine and mufflers is changed. I can see this working, but an X or H ( or my "eqalizer" chamber") to me would be a better, more efficient solution.

 

[mustangdave]

To clarify, D. Hearne got it: the pipe is made shorter on one side between the engine and one muffler, moving one muffler closer to the engine and changing the frequency to a range the engine spends very little time at, making it unnoticeable.

 

[S-Car-Go]

Interesting stuff. What I've learned:

1) Droning is the resonant frequency of the muffler or debatably the muffler and system.

2) The car/panels can act as an amplifier (Jester/Bottlefed, I think you got something here)

3) I’ve got the only Dynomax super turbos that drone (joking)

So if it's as simple as making one side shorter than the other, I would think this would be standard practice when building a system from scratch. It does make sense though. I'm surprised they didn't use two different sized mufflers on the 80's mustangs. When I get a chance I'm going to plug one tailpipe, forcing one exhaust bank through the H pipe and see what happens at 2,000 rpms. There should be no drone.

I was thinking "why not tack steel plates to the mufflers, adding mass and raising the resonant frequency of the mufflers?". Then I was seeing a bunch of Dynomat in my future for the floorboards, etc. I got to thinking "what would happen if you wrapped the mufflers with Dynomat (or equivalent). Or just one muffler? I don't think my mufflers get over 300*F. That MIGHT raise the droning RPM out of my cruise RPMS's.

Other ideas would be a 2 1/4 muffler on one side and a 2 1/2 on the other. I wish I had the time, money, and resources to play with this. I bet you could come up with some interesting combinations. Especially if I could get an X pipe under there.

 

[Hack]

I was thinking "why not tack steel plates to the mufflers, adding mass and raising the resonant frequency of the mufflers?". Then I was seeing a bunch of Dynomat in my future for the floorboards, etc. I got to thinking "what would happen if you wrapped the mufflers with Dynomat (or equivalent). Or just one muffler? I don't think my mufflers get over 300*F.

.

I think it's the tailpipes that resonate, not the mufflers.

I would be willing to bet that your mufflers get hotter than 300F, especially if you wrap them with something. The Dynomat would turn into Dynomush!

 

[302 coupe]

interesting idea on welding plates to the mufflers, its cheap and easy, plus, it kinda seems like it might work.

 

[Applejack]

There is a difference between the acoustic resonsant frequency (a function of pipe length and volumes) and the natural frequency of vibration of the parts (a function of mass, location and stiffness.

Welding mass to pipes and mufflers is a common band-aid fix to OEM systems, but it's only used when system vibration is making it's way up through hangers and into the body causing the shell to 'light off'. It will do nothing for acoustic standing waves in the pipes.

 

[D.Hearne]

Thicker insulators between the body and hangers would also be a good idea. I had straight pipes on a 80 Western Star truck that sounded good, in and out of the truck. Tried the same thing on a 85 Pete that had a stiffer insulator between the pipe and cab, kept em on one whole week before I went back to the mufflers. The cab resonated like a Sum-Bitch with the insulators it had.

 

[BornInAFord]

Why not add short resonators like the 65-66 GT's with dual exhaust (as well as a host of other performance cars with duals) have?
Daniel

 

[D.Hearne]

Resonators cost(more) money

 

[S-Car-Go]

Mo money and the resonators I've seen are for 2" tailpipes.

I just need to spend the time to figure out if it's coming from the tail pipes or a vibration exciting floors and panels. With the tailpipes out the back I wouldn't think it could drone that badly in the car (with the top down).

I was thinking.. Hypothetically.. If I suspended a coke bottle off the bumper with a string it would resonate at a certain speed when the wind blew across the opening, but it should not resonate in the cab. If it was under the center of the car it should set up a standing wave or reflection off the pavement to shake the floorboards.

I'm working 12 hr days this week then taking the family on vacation, but when I get back the plan is to:

1) Rig up tailpipe extensions and go around the block to see what happens
2) Add temporary mass to the mufflers. Bricks and long hose clamps.
3) Take a closer look at the isolators if more should be added or what could be done to decouple the exhaust from the chassis.

It kinda goes back to the "Just what is it?" question. Vibrations exciting panels or coming up through isolators, acoustic energy that's a design flaw (or designed to drone at 2,000 rpms)? Why will the same muffler drone more on one car than another?

 

[dodgestang]

I've never really heard what it is. Just muffler X drones more than muffler Y. How does pipe size, loading, compression, cam or anything else come into play and why is it always around 2,000 rpms?

The primary cause of muffler drone is unspirited driving. I've found that through practice and hard work I can completely eliminate muffler drone above 4,500 RPM

 

[Great68]

The primary cause of muffler drone is unspirited driving. I've found that through practice and hard work I can completely eliminate muffler drone above 4,500 RPM