Resonance sound

[Belgium R/T -68]

I will try to explain a problem I have with "resonance" sound as I call it. My GTX -69 vert gives at certain RPM/highwayspeeds a sound wich makes it really uncomfortable to drive the car.
It's not vibrations, not the sound of the "engine". It's like the rearend at certain speeds gives this phenomen. It's really terrible for your ears. My Chrysler 300 vert -69 have the same
engine,transmission and rearaxle + gears and is a dream to ride. You don't even hear the engine in a matter of speaking. The only difference is that the 300 have a single exhaust system.
Any possible explanations? 

[Just 6T9 CHGR]

By chance do you have Flowmaster mufflers on there?  Most chambered style mufflers will give you cruise RPM resonance

[74Rallye]

I have the same problem with my SpinTech mufflers around 1900 to 2300 RPM. From what I've read, if the tail pipe is 1/2" smaller in diameter it can help a little. I'm considering drilling through the muffler and using a long bolt to "dampen" the vibration of the muffler at resonance. Another thing I've been thinkling about is adding a pluged pipe to one end of the muffler to tune out the resonance.

This link will help to understand the forces of resonance:  http://en.wikipedia.org/wiki/Helmholtz_resonator 

[andy74]

i have thrush turbo mufflers on mine,with dual 2 1/2 inch pipes,and at some rpms i get a resonance through the car,just a little off idle,i think every type and brand has some

[Belgium R/T -68]

Mine is definetly coming from the exhaust system. 3" H-pipe system with one dynaflow muffler, no headers. Is this to big and causing the resonance? 

[Tilar]

Probably. The cheapest out, and to test your theory would be to put a resonator in line. Basically it's just a second muffler.

[Belgium R/T -68]

Probably. The cheapest out, and to test your theory would be to put a resonator in line. Basically it's just a second muffler.

Can you explain that a little bit moore. 

[Tilar]

Back in the mid to late 70's, Just after the muscle car era, They started adding what they called a resonator to exhaust systems. These were put on the higher horsepower systems to quiet them down. They were generally added behind the rear differential with the downspout of the tailpipe coming out of them. They were really nothing more than a second muffler. It's the same for single or dual exhaust systems.

Basically it looked like this. Dont pay any mind to the terrible paint drawing 

[Charger-Bodie]

Do you have good exhoust hangers? If they are hard or too ridgid you can get a sound vibration from that too.

[Belgium R/T -68]

Will work on your ideas and come back with results, thanks so far. 

[andy74]

make sure that no part of the exhaust is touching the body or frame,i have seen a lot of cars that have had vibration/resonation issues and that s all it was,a little adjustment!

[1969chargerrtse]

Here's a picture with the resonator mufflers.

http://www.accurateltd.com/6870%20Hemi%20System.htm

[FJMG]

According to a thread on Dodgeforum.com about mileage in a new Ram, the Resonance theory is bang on. For example, the stock mufflers on the 94-01 Rams have what is called a Humholz resonator and if you replace your factory unit with one that does not have this you will get a drone at approx. 1900 rpm (I'll have to reread the thread to make sure). A fellow going by the name of Hankl on that forum indicated that if you install those mufflers then make sure that your tailpipe length after the muffler is nowhere near 66 inches (for the ram anyway).  This would seem to indicate that the resonance is a function of tailpipe length aft of the muffler. Maybe someone knows if the old dodge resonator is actually A Humholz resonator?

[Belgium R/T -68]

Now I have tried everything without getting anywhere with my problem. I am starting to think moore and moore that the newbuild engine is serious unbalanced
or wrongly balanced. It would fit with the fact that it's very shaky on idle although it's should have been build as a -69 standard 440. You also feel when you drive that
the engine has to work a lot when you push the pedal. Anyone with experience? 

///Per

[Brock Samson]

it's a sound or a vibration? couse what comes to my mind (what little is left of it)  is a driveshaft vibration which is a cycle kind of drone vibration, i used to have it in the small block and it bugged the crap out of me, but after i got the drive shaft balanced, it completely went away..
just my 

[Belgium R/T -68]

it's a sound or a vibration? couse what comes to my mind (what little is left of it)  is a driveshaft vibration which is a cycle kind of drone vibration, i used to have it in the small block and it bugged the crap out of me, but after i got the drive shaft balanced, it completely went away..
just my 

Thanks for the input but I forgot to write that you can also hear it sometimes if the car is standing still on idle.

[Belgium R/T -68]

Got some answers back from the previous owner concerning the engine build, it was done by Huntsville engine and should be very good balanced. One thing he did mension
was that he didn't took out the heat-riser flapper valve on the right side and it may be stucked during powdercoating the manifold, could that be an issue although my
problem is according to me on the left side.

//Per

[Mike DC]

 
This is probably not a popular idea, but I would consider avoiding the secondary mufflers and just switching to a smaller exhaust system entirely. 

A 440-inch motor can run a 2.5" dual system pretty well (there will be a bit of power loss, but nothing catastrophic) and the whole exhaust noise will be less.  The larger pipes basically increase the deep bass noise.  Run a smaller system, and you'll keep a lot of the 3" system's characteristic sound but at a lower volume setting.   Or even just neck down the system from 3" to 2.5" after the mufllers before it gets to the tailpipes.



An H-pipe (or X-pipe) crossover is a benefit for a few pounds of torque, but it's also going to decrease the raspy sound too.  Eliminating that is another way to get a bit of a cooler sound from the tailpipes.