exhaust drone

91BLOWNBIRD

Registered User
I thought this was interesting I found this guy's thoughts on a wrangler forum

"bubba_zenetti11-17-2010, 11:47 PM
All exhaust systems (and intakes) have a drone to them. That drone is the resonant frequency of the pipe length. It is called Helmholtz resonance. Even the stock system does, You just need to be outside the vehicle to hear it.

You can demonstrate Helmholtz resonance by blowing your lips across the top of a bottle. vary the height of the bottle or the width and you will vary the frequency of the resonance.

This resonance plays a very important factor in power tuning. Since gas flow is never a one way thing out of an exhaust because of resonance, it is important that the resonant frequency be timed just right to help suck the air fuel mixture into the combustion chamber as the valve are in their overlap period.

Generally speaking, a well tuned exhaust system will produce some sort of resonating note at some point in the RPM range.

This same principle applies to intake runners as well. Longer ones tend to make more low end torque while shorter ones make more torque at peak rpms. There is still a point where the intake resonates and if it is timed right with the valves, it will push the air fuel mixture into the combustion chamber before the valve closes."

thoughts?
 
Exhaust tuning and intake tuning are pretty well documented techniques. The goal is to leverage the reflection of pressure waves in a tube to improve gas velocities so that more combustible gas will get into the combustion chamber than would otherwise occur. Note that on our cars, we have an air pump (positive displacement blower) raising the pressure of air charge well above ambient in front of the intake valve. This in and of itself imparts much more pressure differential than a reflected pressure wave in a tube will, once the valve opens and during valve overlap.

http://www.epi-eng.com/piston_engine_technology/exhaust_system_technology.htm

The drone heard in the car occurs when the frequency of the exhaust note resonates with other objects or spaces in the car amplifying the sound to a level that normally would not be so annoying.
 
so is there really such a thing as a low drone muffler? or is it simply the larger the muffler the less likely the drone? or is the muffler unrelated?
 
What you could do is build a Helmhotz resonator tuned to the frequency of your drone which should eliminate it.

They work quite well but by their nature are very frequency specific. So if your drone is at 143hz a Helmoltz tuned to that would absorb that frequency.

Sean
 
more mufflers help, i.e. converters with a resonator style muffler year the manifolds help. Directing the outlet of the mufflers to the rear of the car rather than at the pavement help. Changing tube size in the exhaust pipes will change the frequency of the note, and often the rpm where it occurs. Thicker wall exhaust tubing would help.

all sorts of things to try.
 
You could install a valve in the exhaust that activates by vacuum from the engine. This will close up the system and make it act like a smaller pipe while you are at idle or just cruising. When going WOT it will open. That should help some, although very complex, it works on high end vehicles and even vettes have the system
 
Thanks for the ideas I have the exhaust out the rear but muffler is in place of resonator and it is very small...exhaust sounds great but drone is pretty intense at cruising mainly in 5th gear

Was thinking installing a resonator back in and mOving the muffler back could help a bit
 
There is a thread on this very thing on corral.net. They add what I call boom tubes to the exhaust to get rid of the resonance. It allows the expansion of the exhaust down a tube and then it discipates (sp) the drone. There are quite a few people there who have added them and had great results.

SWS
 
There is a thread on this very thing on corral.net. It allows the expansion of the exhaust down a tube and then it discipates (sp) the drone. There are quite a few people there who have added them and had great results.

SWS

What this guy did was build a helmholtz resonator at 134hz and attach it to his exhaust.

You should get the same result by building one and putting it somewhere in the car. Would be a lot easier too. (the building).

Wouldn't be expensive so maybe a good experiment for a Saturday afternoon.

Sean
 
You could install a valve in the exhaust that activates by vacuum from the engine. This will close up the system and make it act like a smaller pipe while you are at idle or just cruising. When going WOT it will open. That should help some, although very complex, it works on high end vehicles and even vettes have the system

I believe the Corvette's uses vacuum to open and close the bypass but a computer tells it when to do it.
 
What this guy did was build a helmholtz resonator at 134hz and attach it to his exhaust.

You should get the same result by building one and putting it somewhere in the car. Would be a lot easier too. (the building).

Wouldn't be expensive so maybe a good experiment for a Saturday afternoon.

Sean

Let us know the results!
 
Nope. I just call them that for some reason. It may be something I saw in the thread, but the pic of the exhaust on the gto in the last few pages of this. It always comes to mind when I think about this thread.

http://forums.corral.net/forums/general-mustang-tech/496793-exhaust-drone-completely-gone.html

SWS

After reading the ridiculously long yet informative thread mentioned above ^, it appears that one just needs to know what frequency our exhuast resonates at then you could figure out what length quarter wave tubes need to be made. So does anybody know or have the capability to figure out what frequency that is?

Chris
 
After reading the ridiculously long yet informative thread mentioned above ^, it appears that one just needs to know what frequency our exhuast resonates at then you could figure out what length quarter wave tubes need to be made. So does anybody know or have the capability to figure out what frequency that is?

Chris

A couple of methods are mentioned in that thread.

One is to record the in-car sound while the drone is present. Then take the audio file and put it into an audio analysis program on the computer. Not a bad idea.

Another idea is to play bass test tones through the car's audio system until a cancellation effect is noticed. (Although I suppose this could end up being an enhancement effect, depending on the exact timing!) This sounds pretty good as well. Here is a web page that has test tones you can download and burn to CD. http://www.realtraps.com/test-cd.htm

The thread in question is pretty interesting. Good job by that guy to apply something that has been staring everyone in the face, but is rarely used. "Why didn't we think of that?"
 
Back
Top