I just installed the spacers and went for a 25 mile cruise. Overall, I think the biggest improvement is that it seems like there are less vibrations in the car and because of that it seems like the overall volume inside the cab has gone down and that there's a little less drone; however, the drone is still there. It was also hard to judge as it took awhile for the engine to warm up (it was about 50F today). Most of the ride I was under 180F water temp but I did notice that the hotter the engine got, the more the car droned. I think the next step will be resonators before the mufflers.

Good cheap progress!

Ok, looking under the car, resonators may not work as most I see are at least 4" in diameter. If anything, maybe a glasspack muffler, but it's going to be super tight to the floor. I've got about 2" between the centerline of the tube and the floor. The smallest diameter glass pack I'm seeing is 3.5". However, I do have 20" between the X-pipe and my mufflers.

Maybe something like this?
https://www.summitracing.com/int/parts/jex-a3516b-8/overview/

It's either this or possibly putting tiny mufflers out back, but those are usually chambered mufflers. There is a tiny glasspack, but I'm not a huge fan of the red.

https://www.summitracing.com/int/parts/sum-630853/overview/

https://www.summitracing.com/int/parts/jex-fb426/overview/

https://www.summitracing.com/int/parts/sum-630142/overview/

what ever you choose, make sure the internal core is the same size as the inlet/outlet nipples. the cheapies just use a 2" or so core with the bigger size nipples added to the "basic size" body.

I had Full length TTI headers with Dynomax Ultra Flows and full length 2-1/2" tail pipes on my 440 Cuda, and I did not have any drone at all. maybe tail pipes will help.

A little off topic but I'm curious how close your headers actually are to your steering box and whether you have any issues with power steering fluid temps.

- EM

Well, it's not touching anymore so it probably got cooler.

I talked to my dad and he said as a Christmas present he'd pay for an exhaust shop to throw resonators on the car, so I'm going to take it to a shop sometime this week to check it out and get a professional opinion.

Our exhaust systems are fairly different as I have shorty (unequal length) headers, a shorter wheelbase car, and different length exhaust tubing with a different engine. I don't think it can be fairly comparable car-to-car.

You said earlier that you were going to put stock style tips on it. You might want to cut off those turn downs that end 3" before the bumper and install the tips out past the bumper like stock before going the resonator route. You might be surprised with the results.

I just called the exhaust shop that had great reviews on Yelp. It was a one-man shop, but basically he told me that my car is going to drone and there isn't much he can do about it. He said I'd need a muffler that has both packing material and chambers to break up the exhaust while quieting it. He said he's worked on a couple of Dodge Darts and there's no room in front of the mufflers for him to put anything, as I suspected with my 2" between the centerline of the tube and the floor pan. I asked him if there was anything we could do on the tailpipe end like adding a glasspack, chambered muffler, or resonator and he said that odds are all it will do is quiet the car down.

I asked him about the turndowns too and his response was, "Resonance is resonance." So, I'm not sure if I should get another opinion or take all of the knowedge I've gathered from this thread and try something.

I think my next step may be to grab all of my blankets, like mentioned before, shoving them in the trunk, and seeing if maybe that will help as he hasn't had any experience with sound deadening.

The blankets in the trunk is a worthwhile experiment.
So is silicone caulking cracks that might vibrate, or filling voids made of flimsy sheet metal.

Another worthwhile experiment is to use Vice-Grips to clamp different sized sockets inside the exhaust pipes at the outlet. This slightly increases the back-pressure and changes the speed of sound in the exhaust gas.

You already mentioned you can hear a difference in the drone was the engine coolant temp went up. That is also a speed of sound in hotter gas effect.

The pictures you have posted seem to show you have room to add closed end T- stubs. These are typically 26 to 18 inches long. Think of these as “blind alleys” the sound wave charges down, hits its head on the closed end, and dies. Start long and trim off length until the sound cancellation is greatest at your cruise RPM. The stub tubes can be bent, even coiled. They can also be the next smaller diameter pipe with just a small decrease in effectiveness.

You could also extend the length of your exhaust pipe past the critical 70 inch length somehow., like have the driver’s side exhaust exit on the passenger side, and vice versa.

I just stuffed a blanket, a comforter, a pillow, a large sleeping bag and two foldable mattresses in the trunk and it didn't do much of anything. Granted, there was still a void between the battery and the quarter panel and a little over the rear end but it was 90% filled. I just didn't want to mess up the sheets or anything. I tried to film myself driving the car to capture the drone and ironically, my phone cannot pick up the drone frequency. So, everything sounds fine on the video when it is obviously annoying in real life.

I can call the exhaust guy to see if he's heard of or would be interested in a Helmoltz-style resonator, but it didn't seem like he had any idea of what it was or he probably would have brought it up.

Is that 70" past the muffler? Rear End? Does it have to be a straight line of tubing?

Tailpipes from the muffler exit out to the open end that measure 55 to 70 inches length on V8s tend to have an annoying drone somewhere at an RPM and throttle opening typical of highway cruise.

The stub pipe does not have to straight.
Think of all the curves in headers.
The science phenomenon that causes drone is the same one that gives added engine power with headers.

Alright, I'll measure under my car. But on the other hand, I've been trying to see how some OEM's do it. I found the underside of an LX Charger and it has a muffler in the center followed by two resonators by the tips; however, like mentioned, there is plenty of exhaust length (> 55"-70") between the muffler and tailpipes.

I also found some dual outlet Borla intercooled tips on Amazon prime that I really liked and are marked down like 65% compared to Borla's website. Or I can get standard round ones for $20 cheaper per tip. I might pull the trigger on those more for looks over trying to help the drone.

The Dart has a longer wheelbase than any Cuda. Yours is 111" and the 67-69 Barracuda/Cuda is 108". The E body Barracuda/Cuda is also 108".

Fix... "Different wheelbase*"

Yeah...
It is odd that subtle changes can make huge differences, right?
You could duplicate a winning formula from another car with only one small change and it can end up sounding different.

So Sunroofcuda's offer to build you a set of resonators that are 3.5" body was not sufficient? He gave sound advice, and your YELP dude muffler shop is an idiot because the factory did install resonators to get rid of the drone!

You keep asking questions for answers you are seeming to ignore. Just cut the friggin tail pipes off, move the mufflers forward, install the resonators and extend the tail pipes past the bumper.

That LX Charger underside picture shows many “tricks” to get rid of drone while keeping back-pressure low. There are also aerodynamic tricks that older cars did not have.

It would be even better if we could see inside
the big center muffler AND
inside the smaller two “resonator” type units near the rear bumper.

There might be T-stub pipes hidden inside.

I also would not be surprised if the distance between the two exhaust ends is critical too.
It might partially cancel a particular drone noise at a particular RPM.

Is there any Sales Literature that states that the LX Charger factory Radio/CD/ MP3 player has Bose-like noise cancellation electronic circuits?