mbgator

I recently re-worked an OBX exhaust system to eliminate a very annoying exhaust drone or resonance inside the cabin at part-throttle, cruise speeds. The method used was the design, assembly and installation of what is called a 1/4 wave resonator. While the name may sound a bit funky, the engineering is sound and the results are amazing.

Here's the problem; sound travels down the length of an exhaust pipe as the direct result of exhaust pulse. As the sound wave rolls down the pipe, the entire car essentially becomes a speaker, amplifying that sound to the point of irritation for the driver and passenger.

The quarter wave resonators act as a blocking device, forcing the sound wave or pulse back against itself, thus cancelling most, if not all of the noise that was resonating back into the cabin.

In my case ('89 Vette Coupe, stock with 2 1/2" OBX stainless pipes, x-pipe, no cats and OBX cans with quad tips), the drone began at 1150 RPM and built to ear bleeding proportions by 1400 RPM, tapered-off at about 1600 RPM.

So here is the math behind the project. Pictures are attached of the result.

A V8 engine pulses (fires) four times per revolution - that's a given. So, using 1900 RPM as an example.
1900 RPM times 4 pusles per revolution = 7,600 pulses per minute. Divide that by 60 seconds per minute will give 125 pulses per second @ 1900 RPM

The speed of sound depends on temperature. I used 1075 feet per second which equates to 113 degree temperature. That's about what I was reading at the ends of my pipes just before they entered the muffler. Remember these pipes are only attached to the exhaust system, will only receive pulses but no through-flow because they are capped.

To determine the wavelength of each sound pulse:

1075 feet per second divided by 125 pulses per second = 8.5 feet or 102 inches. You want the quarter-wave tube to be 1/4 of the wavelength. So - 8.5 feet = 102 inches / 4 = 25.5 inches. Your tubes would be 25.5 inches each.

These can be straight (given room) or bent, doesn't really matter. The size of the tube needs to be close to the pipe size. In my case I had 3" pipes, but had the ability to cut/weld 2 1/2" tubes.

The difference is amazing. At 1,000 RPM I was reading 72 db, 1300 RPM was 89 db, and at 1500 RPM I was at 91 db.
Now the 'noise' peaks at 65 db at 1200 RPM, and maxes-out at 72 db at 1400 RPM. Essentially cutting the 'volume' of sound in half.

Hope this helps and isn't too confusing. If anyone needs more specifics, I will be happy to assist.

WVZR-1

Resonance has been corrected/reduced in this fashion for years. Yesterday it was posted to your OBX thread and this is only "republished" of the same information. resonance is a "car to car" issue and depends on many aspects of the components a person starts with and how those components are utilized in a system.

The method you chose to use doesn't generally work in a very confined area, there's "systems" that do functionally when installed with proper components make exhaust system resonance "tolerable".

It's good that it works for you but certainly might not be for everyone.

Raysur

I love this approach. Can you spiral the tube so it takes up less space?

rocco16

Could you achieve the same results with a 25.5" crossover/connector pipe, or do the pipes have to have closed ends?

ch@0s

I saw a thread than only required a cross over tube with a 1/2 in hole like an H pipe with a small hole.

ghoastrider1

is it a horse power killer?

Tommycourt

I still have stock mufflers, I think they are Walkers on my 88 vert. and they are a little more quiet than I would like. I have asked on a former thread before and discussed this matter and I was told that the 50 series Flow Masters would give me the little throaty sound that I desire. Don't care if it increases power, I just like the sound. Besides with the TPI, I ain't out racing from stop light to stop light, I just put it in D and go about my way.
Tommy

mbgator

We put almost a 180 degree bend in the pipes and angled them up, toward the undercarriage of the car to decrease the visual.
From what I've read, the angle doesn't matter. It is simply the sound waves bouncing back against the originating source that makes this work.

mbgator

I understand this is a duplicate post. I re-posted for anyone who had not seen the original, or were simply searching for exhaust resonance. And yes, this approach has been around for a while. I've used it in music for a while. Other auto enthusiasts have used this successfully, but I could not find a relevant post on our particular cars.

mbgator

Not a power killer at all. In fact, with the original OBX install, I didn't have any back pressure at all (or very little), so my midrange seemed a bit weak.
Car 'feels' like it runs much better, and seat of the pants is all that matters to me.
This isn't a track car, garage queen, or show vehicle. This is a toy and nothing more. I wanted a nice sound, good response and to continue to enjoy the ride.

DJmaya

I am confused about your math.
You said Why are you using 1900 rpm as a base instead of using 1400 or 1500 which were the RPMs you pointed as the loudest?

JimLentz

An almost 20 dB reduction is around 1/4 the volume, cool idea. Similar concept is used in radio frequency transmission line theory.

DJmaya

The other question i got was where did you get the 1075 ft/s number?
That's the speed of sound at 17degree F. wouldn't the speed of sound be more in the around 1400ft/s given the hot exhaust temperature?

v8vette84

Even though I'm not a fan of reviving long dead threads....


The 5th gen Camaro mufflers use this idea on a very small scale and so do the Corsa mufflers.

v8vette84

It also says that he is using it as an EXAMPLE. Its right in there...

Pwnage1337

Interested in this concept and have been researching the topic for about half a year. I too have the ear rupturing drone at 1300-1400 RPM.

Question I have is: I do most of my cruising around 2000 RPM. Would building a resonator to slay drone at 1400 RPM help cut drone at the 2000 RPM frequency too? Is the drone at 2000 RPM a result of "second wave harmonics" (not sure if that's the right term) caused by the 1400 RPM frequency