Technical Exhaust Drone/Resonance

I am wanting to quiet exhaust noise, preferably without replacing mufflers. It is what I would call a resonance, or maybe the proper term is drone. Part of the noise problem is turn-downs about 18" forward of the rearend with only about 18" of tube length behind Megaflow mufflers. I do want to extend the exit point to angle down slightly just ahead of the rear axle tubes. Just wondering, would a cross-over pipe ahead of the mufflers help to eliminate the noise level and drone at highway speeds?

Lynn

 

Move the noise to the rear of the car, and drive fast so it won’t catch you.

 

I'm sure it won't do 700+ MPH.

Lynn

 

Pretty much any exhaust that exits under the car is going to do that, especially behind "mufflers" that really don't attenuate the sound a lot. Move the exit point of the pipes out the side or rear.

 

Add a Helmholtz resonator to each exhaust

 

Somewhat, but not enough would be my guess. I'd replace the mufflers. Megaflow? Like who drives that fast to justify the noise? I'm a fan of quiet. Value my hearing maybe? But to each his own. If you keep them you might try a wrap. That header wrap should dull the drone. I did an asbestos wrap over the tail end of duals I made and secured it down with half shells and clamps and while it was more for preventing heat transfer, it also had the effect of dulling down the vibes.




Maybe take a page from the gun crowds and add home made exhaust silencers in that last 18" ?
The other thing to ask is what sound dampening and sound proofing is there in the car? If your doors are vibrating because the're hollow core, you might need a different fix? Well that what I got to offer.

 

Change mufflers and run the exhaust out the back of the car. I had an OT Falcon that I ran 3” exhaust to a set of 2 chamber Flowmasters with dumps before the rear end and it was almost unbearable at times. I ran the exhaust all the way out back (still 3”) and all it did was move the rumble out back but the resonance was still unbearable.

 

Never heard of this before, but reminds me of those copper pieces of pipe next to faucets used to eliminate water hammer. lol!

 

Interesting. I built a dual exhaust (2.5”) using a Flowmaster 10 series (2 in/2 out) with side exists on PS before rear axle for daughter’s OT pickup with a 5.7. It was super loud. NASCAR like. It’s now my DD, and was thinking of moving it to the rear. Guess not now…

 

I don’t know if you see it but when I did the exhaust on my 53 I dumped it out just in front of the rear wheels.

It has a very healthy 261 Chevy 6 straight pipes I built the exhaust of heavy wall 2” Stainless Steel milk line.

The loud does not bother me but it drones, which bother me on long trips.
I plan to solve the problem by running the exhaust all the way out the back

 

This was a 12:1, solid roller, gear driven beast of a small block. It snap crackled and popped when it was running and even with a gear (4:30) it still was camming pretty hard cruising in rush hour traffic. It was fun to listen to but after a while it would give you a headache

 

An OT car I had used a Borla muffler near the end of the exhaust system. I later added a round Magnaflow muffler upstream to quiet it down and mellow the tone.

 

Some Chrysler cars from the factory had a weight hung on the exhaust system to absorb resonance,sometimes a hunk of cast iron on the end of a rod.

 

Exhaust exiting under the floorboards is like being inside a drum.

 

A crossover or H pipe would help, but I've been told a X pipe was better by a guy that still had a bad drone with an H pipe that got less with the X pipe.
Tail pipes get the noise out of the p***enger area

 

Thanks for the replies. The comments that the mufflers need to be changed is what I expected but sometime I need someone else to validate it. I'll get the end of the pipes as far back as practical. I would love to dump the exhaust out to the sides just behind the rear wheels but the configuration of leaf springs and a gas tank in the way would make the one pipe look like something Rube Goldberg designed.

I've heard it said, "If it's too loud you're too old" and I guess this confirms that I'm just really old. When I was in my early twenties open headers would have been my exhaust system of choice but that's certainly no longer the case for a daily driver.

Lynn

BTW.... In my own defense I'll mention that I didn't install the mufflers, they were on the car when I bought it.

 

Walker FX or quietflow works nicely, have had 'em on two of mine and love 'em. Can thank straights, headers, straight thru muffs, and just screwing around with loud **** on old cars --oh, and i forgot the 8 years of USAF airplanes-for my 2K hearing aids that I now need to replace because my hearing is getting worse--if that's possible!!

 

I've always liked that look, and did exactly this on an O/T Nova with an evil, wicked, mean and nasty 350 under the hood, through what were then known as "turbo" mufflers. This was AFTER running it for about two weeks with header mufflers and short dumps underneath the car, which was so loud and annoying that I couldn't stand it. It made it a whole different car.

 

My car and truck have similar engines, same pipe size and same gl*** packs. The truck has rams horn manifolds and drone. The car has fender well headers and no drone. Both have turndowns under the car, near the rear axle. The only explanation I can come up with is the car exits outside the frame rails and the truck are inside the rails.

Gary

 

It’s been my experience over the years that the exhaust systems that have more drone is usually in the muffler. I’ve even had a OT stock late model Dodge pu that had quite a bit of drone and it had the 4.7 v8 not the Hemi. I’ve had full duals installed on several cars and had problems with drone and what I’ve found in all but one was the hangers mater. To short without flexibility or too many hangers can cause drone, often one flexible hanger on the muffler and one the tailpipe is all you need. On only one vehicle a 69 Pontiac With a 400 I ended up with resonators on the tailpipes. But even this was done only when the original owner wanted to buy the car back but didn’t like the sound of the exhaust I had put on it. I’ve had good results with even gl*** packs bolted to the headers with 3” cores and turndowns as long as the turndowns end level with the body so the exhaust is pushed away from the body at speed. It might make the ashtray lids dance at idle but it’s reasonable at any road speed.

 

I've got a pair of Porters on my Model A (401 nailhead), no drone or really loud exhaust inside the car at cruise speeds.

 

I agree with both of your suggestions plus moving the mufflers as far back in the system as possible / reasonable.

 

Dual 2-1/2" Vibrant Performance Street Power stainless, headers, no crossover, plumbed out the back. Mufflers mounted forward of the hump pipes at the rear end. Nice rumble until hammered, then wicked roar...and no drone. Flowmasters sounded like **** to me. These VP's are nice.

 

Been around since the 1850's in various forms.
You see them a lot on plastic intake ducts/snorkels


They work on the same principle as a whistle [blowing across the top of a piece of tubing creating a resonance]
Basically a crossover [H-pipe] is a Helmholtz resonator .
I've seen a 3rd gl***-pack muffler used as a crossover H-pipe very successfully
But sometimes there is no room for a crossover.

 

When it's droning can your finger tip feel the floor or door panels buzzing faintly?
That can be too stiff hangers OR loud noises coming out the pipe exciting flat panels with no sticky gunk for damping/deadening.

Is it a particular engine speed, or a wide range of engine speeds.

There are apps for Smart phones that can ****yze sound frequencies.
Helmholtz resonators need to be tuned to the frequency that bugs you. https://www.omnicalculator.com/physics/helmholtz-resonator

 

They don't actually work as intended for water hammer. They eventually just fill up with water.

 

You need to get the exhaust pipes to the rear and out from under the car, and you need to pick the right mufflers.

 

I use stainless baffles similar to this. Insert & secure in end of each tailpipe.

 

I am also running Vibrant SS mufflers ( oval shaped) 2 1/2” SS tubing with all mandrel bends. I have big tube headers, no crossover, the mufflers are located under the rear seat area just before the tubing goes over the rear end housing. I installed small Vibrant resonators just behind the mufflers, the exhaust tubing exits behind the rear wheels. Absolutely no drone or “school bus” rapping, just a deep muscle car sound. Perfect.

 

^^^^^Beautiful!!

Lynn