Exhaust Drone
Exhaust drone & exhaust noise while both are different by definition, both are created as a result of impactors/promoters. Exhaust drone is that which is caused by harmonics- a soundwave generated which in essence takes the exhaust sound and sends it back up the exhaust system (towards the engine) and is released or transferred into the passenger compartment. This transfer typically happens where large contact areas exist that have little to no sound dampening qualities and/or materials- as a result of the muffler construction or vehicle manufacturer.

Exhaust Noise (in passenger compartment) is what most people actually complain about. While harmonics do play a contributive role, it is not the primary cause.

Impactors/Promoters

I would love to tell everyone that “x” is the cause 99% of the time for exhaust noise complaints, but it is typically a series or combination of numerous factors which leads or develops the level of noise which is uncomfortable to the ear. The first “rule” of an exhaust system is that the noise should be generated at the point of discharge (ie tailpipe tip). Obviously, if the tip terminates at your door or immediately under the passenger compartment, the sound level will be much greater at the exhaust discharge point than if the tip exits the rear of the vehicle, but this is common sense.

Assuming that there are no exhaust system leaks and isolators are correctly installed at all mounting points, what is not acceptable by industry standards, is when an exhaust system has been installed correctly, exiting well past the passenger compartment to have substantial exhaust sound transfer into the passenger compartment at a level which does not allow a person to talk without raising their voice to well above ambient noise levels. Whether you love to hear your engine roar (which I do), stereo blast or have a conversation, this interfers with your ability to enjoy what you like.

Now for the Causes

The first problem is most muffler shop staff today have little to no knowledge of exhaust systems, the “tricks” to installing a system that will “please” the customer, and simply verbally repeat the marketing materials provided to them by the system manufacturer. As previously noted by several others, there are 3 basic muffler designs, Absorption, Diffusion, and Chamber (Resonator), no matter which one suits your ear best, the criteria for promoting unnecessary in-cabin noise is the same.

Impactors:

Exhaust system material thickness should be no less than 16 gauge- double-walled pipe was commonly available through the 1970’s until manufacturers began seeking ways to reduce costs. Many of the “Muffler Chains” who marketed lifetime warranties always used double-walled pipe before “stainless steel” became the “in-crowd”. If a shop had it available, I would choose double-walled aluminized coated steel or 14 gauge stainless steel (depending on environmental conditions or personal tastes) - it is very quiet, solid and in terms of costs, pipe is pretty cheap, depending on length, 16 gauge aluminum coated runs about $11 to $20 wholesale (depending on diameter), stainless about double for the same gauge.

The muffler should be located as close to the axle (far away from the passenger compartment) as possible, allowing at least 3” of straight run pipe before a bend occurring (if installed to the forward position of the axle), with the overall length as long as possible. In the oem location, 24" will fit fine, located at the head pipe, 30" will fit fine as well and eliminate almost any possibility of drone in this particular location due to the insulation characteristics and length of the muffler (and yes, Allied makes units this long).

With regards to the 5.4/4.6 engines, The head-pipe (the pipe installed in between the exhaust manifold and the muffler) should be no greater in inside diameter the 2 7/8”, at 3” it now becomes a contributor.

Duel Tailpipes should be limited to no greater than 2.25” inside diameter, single tailpipes 3” maximum inside diameter- tips which are larger can certainly be installed but should be limited to 12” in length and have a discharge point past any material that may “catch” a soundwave or exhaust. When you exceed these diameters, it now becomes a contributor.

Tailpipe length in general should be as short as possible, minimal bends, etc, terminating past the axle, 1.5" past the body or bumper. Lengths greater than 4 feet promote “drone” (ie harmonics), typically about 3 ½ feet past the muffler.

Pipe diameter reduction should occur before the muffler. While many muffler installers like to use mufflers which reduce pipe diameter or split the exhaust at the point of discharge at the muffler, or install reducers to connect the tailpipes after the muffler induces “noise”.

Ultimately, it is the quality of the material and the installer which 90% of the time leads to a successful installation. I would like to also emphasize that it is not necessary to spend a fortune on a muffler ($100+) or system to have an excellent sounding, performing and long lasting result.

 

Too much to read.

 

Good writing, beechkid. It is easier to control volume than drone.

This may be of interest?

http://forums.corral.net/forums/show...ighlight=drone

 

If you don't like drone then you don't want loud pipes!

 

Great accurate explanations. AV put it simple

 

My point was that you can have volume and good exhaust tone without drone.

 

Well I have some serious drone. Cat back SISO behind wheel (MBRP). So if I got this right I should run the factory pipe as close to the 3 1/2 inch pipe of the muffler? As it is right now I have about 3 feet of 3 1/2 inch pipe between the stock pipe and the muffler. I know you'll get some drone but the amount I'm getting is driving me crazy.

 

which is what I've experienced with this Borla SI/DO 140291 system.

A Nice..... BUT not rude!!.... volume increase, with a really sweet tone and best of all....no drone!!

 

Before you go whole hog, take a look at this. ( not my drawing )


This works on the same principle as a factory intake where there is an adjacent volume of air which disrupts the sound waves.

 

Yes, and that can work in many cases...what IMHo can also suffice is placing a Y at the head pipe and run separate mufflers/resonators...to a certain degree I think you can accomplish the same thing (partially) since the sound waves would be reverberating against themselves at the "Y".

 

It's called get a bigger quieter muffler. Borla Pro XS SI/DO for example, which has already been mentioned...

 

I can see how a Y would disrupt the sound waves, but with di/do it wouldn't work. I added two 14" resonators before my di/do X muffler and H'ed the pipes there, but only a 5-10% decrease in cabin drone resulted.

 

^ The resonators is where the reduction in drone came from, not the H pipe. An H pipe will add drone to a true dual set up.

 

same problem here after i removed cats it was nerve racking I added a 18 inch glasspack in front of the stock muffler it took all the drone away still left it with a nice deep sound cost me 10 bucks it was a used pack

 

Modified the tech to reflect questions and clarifications that have been sent