Listen to this...what is causing that resonation?

My old engine did this, now the new one does it as well. Is it normal? If not, what is causing it?

Short video with a couple seconds of the harmonic sound.

 

And, to answer the questions before they start rolling in...

original engine was 302 with old firing order

new engine is 5.0 with 351 firing order

Transmission is recently rebuilt, with new bushings, seals, and mounts.

Engine mounts are new.

Driveshaft recently balanced and straigtened and new u-joints.

I did all of this with the old engine trying to quiet it down, but nothing worked.

 

Is your exhaust turned down at the axle??? Mine makes the exact same resonance around 3 grand. So did my first mav. Same gears,same exhaust turn down at axle. It'll rattle your brain till you get used to it. Mine gets tail pipes next year.
Mine is definitely not a mechanical vibration. Its just the music from flowmasters blowing the highway clean.

 

Are you referring to the surging of the engine (accelerating/decelerating) or the actual sound of the exhaust system?

 

It isn't surging.

It is sound and vibration only. The engine isn't losing and gaining power, the sound is just changing. It sounds like an old prop-driven airplane.

It is vibrating enough that the rearview mirror goes fuzzy every second on the down-tone.

And, yes, dual flowmaster super44s turned down at the axle.

And now that you mention it, I don't recall this sound with the sidepipes.

Is this just muffler resonance?

I have spent big money over the past 2 years trying to resolve this. No money "wasted", since it was all spent on new bushings, bearings, seals, and balanced parts. All improvements to the car, overall.

 

Trust me its just the exhaust resonance. When I listened to your video I said to my wife does that sound familiar. She questioned it as a vibration in my car when she first heard it. I could record my car tomorrow and you wont be able to tell the 2 apart. I expected it with mine though since it used to drive me crazy on my first Mav. I swore I'd never run without tail pipes again and here I am 20 years later with the same brain rattling resonance. It actually bothers my eye sight sometimes at a certain point. Dont leave loose articles especially metal on the lower dash. I have a rag on the lower dash I throw my house keys on so they dont vibrate across the shelf. And yes my mirror does the same thing on the low note growl as the resonance drops.

Think of those exhaust sound waves hitting the road then bouncing back up to the bottom of the car. Its like a big boom box under there. Completely gone on my first Mav after calling in a favor and getting some pipes.
We used to fight a similar resonance in the early Cherokees. Guys would spend hours trying to fix it and I'm sure it cost huge warranty dollars. I worked with Windsor on it and finally convinced them it was an exhaust resonance. ( Maverick experience) They came up with a fix for the current models and the later Jeeps got different exhaust and some dampner weights.

 

What make of rear end gears are you running?

 

Gears are Richmond 3.80, with Richmond rebuild kit. Auburn mini-spool.

I also took the rear out last year and took it to "experts" to make sure my assembly was done correctly. So it is tight and within specs.

 

the sound is quite soothing, but just worries me that it may be a balance issue somewhere. But since I am pretty much convinced that it is exhuast resonance, I can live easy with that.

I hope it doesn't drone me to sleep on the highway like my truck does. 5.7 hemi with 40 series flowmasters....makes everyone in the truck fall asleep as soon as I put the cruise control on.

 

Easiest way to convince yourself is to put tail pipes on it. They sound so damn nasty at idle though with the turn downs.

 

That's so weird. I had 40 series Flows turned down at the axle on a Bravada with an SBC swap and I never got that sound. Maybe because it was higher off the ground? That would drive me nuts on the highway.

 

fox mustangs had a bad resonance when they got aftermarket mufflers. the stock mufflers were different sizes and the first flowmaster american thunder kits for them had two different sizes of mufflers to reduce the resonance.

the other thing ive heard of to try to reduce both resonance and the desible level is to have the dumps coming off the mufflers pointed donw and towards the center of the car so the exhast streams will hit and cancel each other out.

 

scott, mine does that also and i have tailpipes. mine didnt do it until i had the mini spool put in.

 

Is it typical for spools to add a resonance? I have never heard of it.

Ed, I want to ride in yours at the Grace show, and I will take you for a spin in mine and we can compare.

Maybe it is a combo of muffler and spool noise. I am anxious to see if mine is louder than yours...

 

would putting in a X-pipe or H-pipe perhaps solve the problem? just an idea.