Well, here we go again......

My custom dual exhaust is the only picture in my gallery.

I've had it for approximately 18 months and am pleased except for the dratted drone. My exhaust guy suggested a pair of small resonators to eliminate/minimize it........no effect whatsoever.

A friend recently suggested a crossover pipe in front of the resonators. He explained that Ford had OEM crossovers on their high performance cars to reduce drone.

It's not an expensive job, but I'd sure like to have some input from others who may have tried this remedy.

Thanks in advance.

Don

 

So who's resonators did you install? Squirrel Crusher

 

I also have the dreaded drome syndrome. I'm also curious about the crossover piece and to see what others have to say. They do sell resonated Tips that I have been looking into.

 

I have a pair of new FlowMaster HushPower II stainless resonators still in the box. Part 12512400. They are suppose to clear up the drone. If I can ever get away from work,I'm going to loose the rear cats and put these in their place. Squirrel Crusher

 

Drone removal is done by a suitcase muffler, a tuned quarter wave resonator, or a hemholtz resonator.

The exhaust pipe is basically an organ pipe with a fundamental frequency and several harmonics depending on its diameter and length. The "Drone" is a combination of the fundamental and harmonics, depending on the frequency of the sound. As an organ pipe, it also transmits the noise from combustion from the exhaust valves to the tailpipe.

At 3000 rpm with a four stroke engine, there is an exhaust frequency of 100 hz for 4 cylinders, 150 hz for 6 cylinders, and 200 hz for 8 cylinders. The exhaust itself will then exhibit fundamental mode vibration at v/4*L, where v = the velocity of the air and L is the length of the exhaust pipe from tip to exhaust valve. The fundamental mode analysis can be done by essentially treating the entire exhaust system as a straight pipe. Note that since exhaust temperature decreases from around 800 degrees celcius to around 120 degrees celcius, the fundamental mode vibration really is somewhat complex. It can still be approximated. Assume a 10 foot long exhaust system, and an average EGT of 500 degrees C, the fundamental mode of the exhaust pipe is about 52hz. This fundamental frequency is also subject to pipe scaling. This has two effects. One, the fundamental mode frequency decreases slightly as a function of exhaust pipe diameter. Two, the exhaust gas velocity increases as the inverse square of diameter. A 5" cylindrical muffler with perforations around a 2.5" tube will decrease the fundamental frequency of the exhaust by 4 times, typically making the fundamental inaudible. This is otherwise known as an "expansion chamber".

An expansion chamber can also serve another purpose. This is by changing the acoustic impedance of the sound wave between the expansion chamber and the exhaust. Mismatching impedance can result in a drastic reduction of acoustic transmission due to the reflected components at Lower Order Mode frequencies.

Now, is an unsuppressed fundamental really what is causing your exhaust "drone"? If your drone is extremely loud, then probably not. You will need an actual muffler to get rid of a "booming" exhaust, or an otherwise loud droning sound. A muffler will need some kind of baffling to maximize dynamic insertion loss. Generally speaking, the DIL of the muffler will increase as its aspect ratio (length to width) increases, so a longer muffler is more effective than a shorter/fatter one.

What might be best is to start by recording the sound of your exhaust drone and performing a basic FFT on the noise to find the high amplitude frequencies involved. Lower frequency noises may best be canceled by expansion chambers, high power middle frequency (50 hz or higher) noise will require some way of diminishing the power of the exhaust pulse - most likely through reflection and baffling.

There is actually a subset of engineers who work on exhaust theory, most of which is centered around the complex interactions between exhaust pulses, exhaust gas interaction, and exhaust shape. There is, strangely enough, a book definition of "sporty sound". I am not an exhaust engineer so I have no idea what that is. This is just an electrical engineer's view of exhaust theory.

 

I've learned to somewhat live with mine. If it gets to much to handle I do a manual downshift, bring the RPM's up an away we go !

 

I have it with my setup and I decided to install the dynamat extreme in and around the trunk area. It really does reduce the drone and I only did one layer! I am going to add another layer and I think that will be sufficient enough for me. Keep in mind though, it does add a little extra weight to the car. I added about 12.4 pounds of dynamat with one layer.

 

I've had exhaust drone since I had my custom exhaust built in October. Like many of us, I turned a deaf ear to it most of the time. It was 'other half' who prodded me into getting it 'fixed'. Sooooooo a buddy of mine from the local MB dealer referred me to a very good very COOL local shop that does lots of custom work. ("antique" cars, trucks, Off road Vehicles, Dune Buggys, etc) They are the polar opposite of places like Meineke muffler.

Long story short, Joe the owner looks under my car, grimaces at the "quality" of the previous work. He says 'for $250.00 we'll chop everything off from behind the (front) Cats, build in a crossover pipe and re-weld it directly to the Magnaflow mufflers'

I agreed, and they did a superb job! Nice clean SS tubes from my cats, X'ed Over then up close to the body and into the Magnaflow mufflers. A real work of art! No wonder it took them 4 hours! :-) Worth every penny!

Now I just need to find replacement gaskets for where the exhaust headers go into the CATs. I think mine are shot. Leaking badly on the drivers side again. The local parts house guys look at me like I'm from mars when I ask for them.

So in short, yes, the crossover pipes make a world of difference.

:-)

 

I have no muffler and no drone .You may try a silencer in one are both exhaust pipes ,buy 1 silencer and cut in half , and try one in ,1 are both pipes . I inserted 1- 6" silencer in 1 exhaust pipe and now no drone. OH you must have round tail pipes.

 

Great idea to replace the cats with them! Now why didn't I think of that?