Harmonic Resonance Help

[deven2015GT]

Good morning, S550 brothers and sisters!

Anyone who has seen me posting like a giddy child knows that I just had a Ford Racing by Borla Cat-Back Touring Exhaust installed on my Mustang. The first guy I took it to to install it said it didn't fit right and was 1.5" off. He ended up "having" to cut 1.5" off to make it "sit properly in the hangers". The following weekend it took it to the local muffler shop back home (originally took it a shop up here where I'm working at out of town) and he tightened what needed to be tightened and moved a clamp or two and fixed the tiny leak the first guy said would seal by driving (which was bs). So no leak, startup sounds great, and the exhaust sounds fantastic, B U T...
There is a harmonic resonance that comes around 2500-3500 rpms. It is not drone, I know this, but it is a musical type of sound. It is similar to rubbing your finger dipped in water around the lip of a wine glass.

I guess my questions are:
1. Has anyone else experienced a similar issue with aftermarket exhaust?
2. Could the 1.5" cut off have affected the way the exhaust system piping was tuned to cancel out harmonic resonance?
-AND finally...
3. Does anyone know of a solution to this or is it something I just have to live with?

I know I've read about adding weights to the muffler to help with the resonance, but I've also heard that could affect throttle response among other things. I'm just researching solutions and there isn't a whole lot of information on this that doesn't have to do with just the science of it (which honestly just seems logical to me - i.e. exhaust being pushed through the pipe creates some harmonic frequencies so in turn the exhaust pipes are shaped and tuned to send out the same frequencies created to cancel out the noise).

Thank you in advance for the input and God bless!

EDIT: I was told by a co-worker who claims to know a good bit about cars that that type of harmonic resonance is not a bad thing. It is merely the exhaust tuned to peak performance and thus the musical slight resonance at that certain rpm range (he says). Can anyone confirm this who has experience with this?

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[deven2015GT]

UPDATE:
Talked to a Borla rep over the phone and this type of harmonic resonance is completely normal. It is built to Ford spec and this is just a byproduct of the system. I am relieved to say the least.

 

[Cascadia_302]

UPDATE:
Talked to a Borla rep over the phone and this type of harmonic resonance is completely normal. It is built to Ford spec and this is just a byproduct of the system. I am relieved to say the least.

There's also a good chance it will diminish or resolve itself completely over time as the the system breaks-in.

 

[Vile_the_Bastage]

There is a harmonic resonance that comes around 2500-3500 rpms. It is not drone, I know this, but it is a musical type of sound. It is similar to rubbing your finger dipped in water around the lip of a wine glass.

THIS

THIS exact thing is happening to me right now. Pairing Stainless Works Headers with a FRPP Sport exhaust as produced this musical note. I've tried to record it to no avail, but it's there. So it's normal?

 

[Vile_the_Bastage]

[ame]

This is the sound I'm getting, but just a bit lower at 2500RPMs. Are we the only ones experiencing this?

 

[Cascadia_302]

Could the exhaust be acting as a type of tuning fork? If you don't like it you could try some header wrap at various places and see if it stops.

 

[dwaleke]

We discussed this a bit in another Ford Racing Exhaust thread.

Mine did that at first a bit (nowhere near the glass video), but was a lot less noticeable after the exhaust breaks in.

Maybe carbon builds up inside over time and changes it. I don't know.

 

[Chameleon]

This is the sound I'm getting, but just a bit lower at 2500RPMs. Are we the only ones experiencing this?

Yeah that would drive me nuts. :headbonk:

 

[Varekai]

It's just your mustang singing the song of her people.

 

[IvanCRF]

Add some mass to it. Like the poster above who noted better results after it being broken in with more carbon.


http://www.ge-mcs.com/download/orbit-archives/understanding_km.pdf

 

[IvanCRF]

Thinking of ideas to add mass... Perhaps adding a spare clamp or two at a straight section of pipe and seeing the results. It will of course change the where the natural frequency occurs and hopefully it is somewhere way past the redline instead of where it is currently.

Mass is the easiest factor to change in the equation for natural frequency!

 

[deven2015GT]

THIS

THIS exact thing is happening to me right now. Pairing Stainless Works Headers with a FRPP Sport exhaust as produced this musical note. I've tried to record it to no avail, but it's there. So it's normal?

This is the sound I'm getting, but just a bit lower at 2500RPMs. Are we the only ones experiencing this?

No we are definitely not the only ones! This is 100% normal!! I talked to a Borla engineer. I asked them, "I get a harmonic resonance that is similar to rubbing a wet finger around the lip of a wine glass. Is this normal? It's not constant, but comes and goes with acceleration or driving in certain rpm ranges."
His IMMEDIATELY responded with, "Oh yeah definitely is normal. That cat-back system is built to Ford spec and that is one of the characteristics of it. It's a tuned system... *yadee yadee da*"
You get the picture. It's normal, and it may or may not resolve itself over time. I have probably around 700-900 miles on mine already and I can tell the exhaust note is getting a tid bit deeper and more aggressive when I lay on it, and the resonance is diminishing slightly. I assume it will continue to diminish, or at least settle into an amount that is reasonable and less irritating.

 

[5starr0]

man that sucks to hear, i was pretty set on the FRPP sport catback.

 

[deven2015GT]

man that sucks to hear, i was pretty set on the FRPP sport catback.

I promise it's not as bad as it sounds. It only bugged me because I didn't know it was normal. It's to the point where it's become much quieter and hardly even noticeable. The exhaust note is perfect and aggressive. I would highly recommend it. The more it's getting broken in, the better it sounds and the less the harmonics are. My girlfriend has never noticed it even when I pointed it out to her.

 

[deven2015GT]

Yeah that would drive me nuts. :headbonk:

It's no where near that obnoxious for me. It's a deeper note and not constant at all. It's a slight swell here and there between 2500-3500 rpms. Not bad at all.

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