Quarter Wave Resonator to Kill Exhaust Drone

[niles]

I installed my header-back exhaust about a week ago, it sounds and performs amazing! Except for an insanely annoying drone in the 1500 to 2500 RPM range. It annoys my ears and pretty much ruins any music I want to listen to. I am automatic BTW. So I was reading around the interwebs about solutions and there really only seems to be two:

A) Install an resonator (inline) after the cat like in a stock system
B) Add a quarter-wave resonator (perpendicular) to the exhaust pipe

I am leaning towards B because I love the way my exhaust sounds, except for the drone. A 1/4 wave resonator is designed to cancel out certain resonant frequencies in the pipe, and can be tuned to any frequency you want. I found a lot of hot rod and truck forums discussing this, as I am sure that the drone on a big block would be even more insane. I want to design mine with a trombone style tube so I can tune it to perfectly cancel out my drone. I am thinking I will have room for it in the back next to my muffler.

If I went with A: I think a standard resonator (probably Vibrant brand) it will kill a lot of the tone, which I want to keep. Plus, a standard resonator costs around $60 and I think that's stupid. It's basically an expansion chamber and to me it's not worth the money. I could always build one, but I do not think it will fit my need.

My current setup:
Pacesetter Header
2.25" pipe all the way
Vibrant high-flow cat
Borla straight-through muffler

Here are links to the best threads I found:
I did it. Eliminated the 2000 rpm exhaust drone - Hot Rod Forum : Hotrodders Bulletin Board
How to build an exhaust resonator tube to eliminate drone - PerformanceTrucks.net Forums

Anybody here ever have experience with this idea? It's news to me and I am glad I found the option before I threw away money on something I didn't need.

[RAZR]

Where is your exhaust tip in relation to the back bumper? If it too short it couldbe resonating the exhaust sound back into the cabin

[niles]

Where is your exhaust tip in relation to the back bumper? If it too short it could be resonating the exhaust sound back into the cabin

Right now it is just a piece of pipe clamped on to the muffler and it sticks out about a half inch from the bumper.

I am considering redoing the angle at which the pipe goes into the muffler to try to relax it a little. The piping in the rear wheel area is a little tricky. I am thinking the angle at which the exhaust is entering the muffler could be part of the problem too.

I have also discovered that the sheet metal in the trunk, right above the muffler, rings like a drum When I do whatever I end up doing I will take off the heat shield and try to reinforce that section of paneling. I like the sound to stay on the outside of the cabin lol

[niles]

Here is a video of the drone. It might not seem like much but when on a road trip going up long hills it is very annoying. Also the engine was cold, its louder when warm.

[2oodoor]

I would be upset if I had your system (awesome) and had a drone @ cruise rpm. Are you using factory style hangar set up at the muffler? Isolation of the exhaust parts from tbe suspension and body is key usually. Then you can still have frequency vibrations from differences in density of assembled parts by the exhaust turbulance. Kinda like a reed effect.
i thought the original Borla systems came with tip resonators, some dual out tips.

[niles]

Borla does still offer mufflers with tips, I bought a generic one because I was unsure if the tips on their mufflers would extend beyond my body kit, also because I found the model I bought on eBay for $60. I was actually planning on installing a dual tip once my system is solidified.

Ya know 2oodoor, my brother noted how worn out the rubber hanger gromits looked, but none were cracked or broken so I reused them. Do you think old rubber could be a problem? I'm not very exhaust experienced.

Also the pipe does rub on the heat shield in one spot, which will be addressed when I install the 1/4 wave resonator. It doesn't sound metallic like I would imagine a rubbing noise though, just a very dominant tone that overpowers all other sounds. I am going to get the extra pipe this week and I hope to do the install soon.

[Tdurr]

okay, so I didn't read the link u posted, but from my experience with these motors(and 4bangeres if you will) any time you have a free flowing exhaust you will have a drone at some rpm. yes it blows, yes you want to get rid of it, esp going uphill; but a resonator is the best bet. get the longest one you can fit on the straight part of the cars exhaust and it will cut the drone. I personally have never dealt with the drone from an auto but I can picture where it can make things uncomfortable for the ear/body. When I had my 4door I put in a 10yr old resonator from a junk car we had. Pipe size was the same it made a world of difference. it made the exhaust note less raspy overall and took the db levels down a good bit at cruise and WOT.

if you want performance you HAVE to give up the luxury of having a exhaust you cant hear in the car. put a super long resonator in place of your cat and/or get rid of your muffler and replace it with one of a different brand. I know for a fact that vibrant has a very good note but no "drone" that most na cars have from 2-3grand.

[2oodoor]

okay, so I didn't read the link u posted, but from my experience with these motors(and 4bangeres if you will) any time you have a free flowing exhaust you will have a drone at some rpm. yes it blows, yes you want to get rid of it, esp going uphill; but a resonator is the best bet. get the longest one you can fit on the straight part of the cars exhaust and it will cut the drone. I personally have never dealt with the drone from an auto but I can picture where it can make things uncomfortable for the ear/body. When I had my 4door I put in a 10yr old resonator from a junk car we had. Pipe size was the same it made a world of difference. it made the exhaust note less raspy overall and took the db levels down a good bit at cruise and WOT.

if you want performance you HAVE to give up the luxury of having a exhaust you cant hear in the car. put a super long resonator in place of your cat and/or get rid of your muffler and replace it with one of a different brand. I know for a fact that vibrant has a very good note but no "drone" that most na cars have from 2-3grand.

The links are excellant reading but I read after posting and it breaks out science to describe what I said.
This subject interests me because my car didn't drone before now it does. Gotta fix it, Borla mufflers for 60 bucks wow thats good.
I like the those new dodge darts sound 4 cyl with ss catback oem dual output muffler parallel w/ bumper and outlets about 2.5 ft apart. No drone ever, just a guff guff crazy banshee sound...rice or not I like it. Disadvantage: extra weight, redesign lower bumper cover, possible departure angle dragging.

[niles]

Thanks for the advice Tdurr and 2oodoor, I know it is possible to get a silent exhaust and retian the high flow. (or maybe I'm irrational)

Oh, and the Borla muffler was being sold by a company that is in the town next to me (I wish I knew where so I could just go to their warehouse, but I'm sure they'd hate that) so I received it NEXT DAY! It blew my mind. US PARTS OUTLET Highly Reccommended!

I like the science behind the quarter wave resonator and the small amount of techical schooling I have agrees with the concept. I believe an in-line resonator is essentially doing the same thing, except killing the drone with turbulance rather than an 180* out of phase pulse. The math will say that a chamber approximately 36" will eliminate the lower freq drone (longer for lower tones) whether it is in-line or perpendicular.

The thing is, a decent resonator will cost me at least 3X as much as a mandrel piece of pipe and will have less attenuation to boot. Here is where everyone will think I am crazy - I believe it is possible to have high flow and zero bass note. I want to see how close to that I can get. It's the foundation of science after all

Once I saw pictures of really high performance exhausts using this method, it really made the idea real for me, for example Corsa uses this method inside of their mufflers.


And here is an external design:

[cygnus x-1]

Neat idea! Here's something even wilder. How about making the resonator pipe with a telescoping tube for tuning, but then connect that sliding tube to a linear actuator so it can change length on the fly as the engine RPM changes. Too much?

C|

[Tdurr]

Lol that would be the most creative and interesting thing id ever seen.

[niles]

I was thinking of making a trombone style tuner. But I could add electronics to the mix. I read about how some people rigged a switch to open/close the resonator on the fly. BMW has one that closes automatically after 3k RPM since it is not needed at higher RPM.

But I'm not sure if I am ready to work that hard most of my projects are limited to what I can achieve in one day, damn daily driver...

[niles]

Odered the pipe today. Can't wait to install it, but we just got snowed on so it might take a while - weather permitting.

I found the perfect cavity in front of the gas tank right before the muffler. I measured the resonant frequency to peak at around 100Hz - annoying is an understatement. I have to crank my tunes to hear any bass from my subwoofer.

To cancel 100Hz, I calculated that the resonator needs to be about 36" long. Good thing the pipe is a U-bend! I am also going to try to angle it so that condensation doesn't collect in the pipe and cause problems.

[niles]

Mini update - I have all the supplies and should be able to install this coming weekend. Can't wait to compare the before and after videos.

[niles]

VICTORY! The installation and function is a complete success. Here is the after video:

[niles]

I bought the J-bend and added 4" of pipe, the total length on the outside of the bend is exactly 36" total. Capped the end with a giant washer from the hardware store. I opted for a non-adjustable design just because I didn't want to take it all back apart for final welding once I had it tuned correctly. But it ended up working perfectly anyway.

About the before and after video: the effect is actually less noticeable in the after video. But when riding in the car the exhaust tone no longer completely dominates all other sounds. I can once again enjoy my music at a low volume without interruption, aaaand most importantly my wife and I can once again talk to each other without shouting!

Additional Notes: With this design the resonator is nearly at the physical limitations of the cavity. There is this piece of the frame that hangs down in the middle of the chassis right in front of the gas tank. If the resonator was any longer it would hit that piece.

Just to clarify: this mod only notches a small bandwidth of frequency. The exhaust still is full volume. I eliminated the loudest bass tone only. I still have some very low frequency drone <70Hz. This does not change the actual sound of the exhaust. I still have rasp, and it is still loud due to the high flow, high performance nature of the design.

Now for the pics::::

Finished product


Rear LCA clearance


Wheel Clearance


Ground/Vehicle profile

[Tdurr]

glad it worked out for ya man! that thing looks crazy.

[2oodoor]

Wow, congrats on the objective accomplished !

[gp02a0083]

I got an exhaust drone as well once I had my 2.25 stainless steel system made. I have a magnaflow Cat, Jones resonator and a Flowmaster 50 series muffler and its still kinda loud. I believe that the closer the resonator is to the cat the less the drone will be

[niles]

I wanted to put it closer to the cat, but these cars are like skateboards... no clearence anywhere until the rear. I might still add a typical cherrybomb resonator after the cat if I want to go back to stealth exhaust volume

[Hazwan]

Thats some crazy piece of engineering haha.

Exhaust sounds great tho!

[niles]

Mini update: I am going to be adding some type of resonator after the cat. Still deciding on what to throw in there though. Open to ideas.

Right now I am really interested in Powersticks from classicchambered.com. It's like a glasspack and a muffler had a baby. I've had them on big-blocks before and they sound amazing.

Anyways, I am doing this because I don't like the rasp. Here is a video of my custom rasp:

[gp02a0083]

I wanted to put it closer to the cat, but these cars are like skateboards... no clearence anywhere until the rear. I might still add a typical cherrybomb resonator after the cat if I want to go back to stealth exhaust volume

Niles , there is plenty of room under the whole car, plenty to even tuck a 2.5" system nice up until the rear section

[niles]

Niles , there is plenty of room under the whole car, plenty to even tuck a 2.5" system nice up until the rear section

I was referring to the quarter-wave resonator in post # 16, nowhere under the car is there room for a pipe sticking out from the exhaust like that.

But yes, I have plenty of room for traditional resonators after the cat.

[niles]

An interesting observation about the quarter-wave resonator; depending on the weather conditions, exhaust system temp, and exhaust gas temp the resonator has different frequency response and dB reduction. Careful calculation must go into incorporating on into an exhaust system.

I ordered a powerstick from classic chambered.com today so in a week or two I will have an update with a vid. There is no packing in a powerstick, just louvers and crimped chambers. So I am able to maintain a 100% straight-through exhaust!