Engine/Transmission (1998.5 - 2002) 'New' Exhaust Drone Theory [VERY Long]

[Lsfarm]

Well I think it is a new theory, since I can't remember reading about this idea.



Have you ever wondered why Dodge made the factory exhaust start at 3" going into the resonator then expand to 4" inside the resonator, exit at 4"to the 4" inlet of the muffler, then exit the muffler at 3" then stay 3" over the rear axle to the 4" tailpipe tip??



It aways seemed like a lot of changes in diameters, clamp and hanger sizes with no apparent reason. .



Well I believe that it is done to reduce or eliminate our drone we get most of the time with straight 4" exhaust systems.



My 'theory' is that the exhaust pulses from the engine, even when 'cut-up' by the turbo wheel have a certain frequency, or spacing as they travel down a constant diameter pipe. The intensity of the pulses change as we add more power at a give rpm, but the frequency of the pulses stay 'in tune' with the engine rpm.



So if the pulses go into an expanding chamber then something has to happen to the frequency or the amplitude [intensity] of the pulses. I'm not sure if they speed up or slow down, it really doesn't matter for this theory. What happens [I think] is that once the expansion chamber ends and the diameter if the chamber is reduced again to the original diameter, the pulses can't reorganize to the steady frequency that they had as they entered the expansion chamber. This disrupts the pulses and reduces or eliminates the drone.



When we eliminate the factory muffler and go with 4" from the resonator back the pulses have to change as they go into the larger pipe diameter, but there is no point where they are reduced back to 3". Or with a full 4" system the pulses just travel the whole length of the pipe and set the pipe itself resonating with the exhaust frequency.



Well I have just finished about the 15th modification to my exhaust system since I got rid of the factory muffler in the pursuit of power. I have always [with one exception] had some resonance drone in my exhaust. I could move the rpm of the drone around with various different pipe wall thicknesses, muffler changes, arrangements hangers etc,etc. But I could never eliminate the drone.



The other day I tried putting my Walker bullet muffler at the very beginning of the exhaust system, right at the end of the downpipe. It was previously under the bed prior to the axle My idea was that if I could slightly muffle the exhaust as soon as possible just as it starts to go under the cab floor it could only help with reducing noise and drone. Well it was a COMPLETE failure, in fact I had a LOT more drone from the rear portion of the exhaust. The whole truck bed was droning.



My exhaust system is a 4" downpipe, to which I have added a second layer of pipe to to make it double walled, and then wrapped it with header wrap. Then the [wrapped] Walker bullet, followed by about 20" of 1/8" walled 4" exhaust pipe, [wrapped] then a 24" Allied straight through muffler, [also wrapped] then about 50 or so inches of 1/8" wall exhaust pipe [wrapped] clamped to the 4" tailpipe that goes over the rear axle. The bed of the truck and the cab just hummed with drone between 15-1900 rpm under any load. get up to 2000 rpm, and virtual silence.



In one of my previous systems when I had a lot less power, I had installed as an experiment a chamber I had found on a semi-truck stack. This chamber was about 14" long, 8" in diameter, and had 3. 5" inlet and outlet and a baffle in the middle of the chamber. I cut the baffle out with a long airchisel making it just an open chamber. When I had this in the exhaust just before the tailpipe went over the rear axle, I had no drone in the system. I removed this chamber in the belief that it was restrictive when I added more power.



This morning after making a parts-run, and in agony from the new found drone from the muffler rearrangement the day before, I decided to try the chamber idea again.



I have had a straight through 5" muffler, 20" long, 9" diameter in the shop for a few months, and decided to splice it into the system after the two existing mufflers, before the tailpipe went over the rear axle. The same factory-like expand-then-reduce idea.



Well it works!! Very well in fact. There is NO DRONE from 1500-2000 rpm, even under heavy load. I can hear the engine working as I add more throttle, but the sound is from the engine not the exhaust. It is possible that adding the mass to this portion of the exhaust system helped reduce drone, but I removed 4" 1/8th " walled pipe for only 24" or so to splice in the 5" muffler.



So if anyone has the dreaded drone and hasn't been able to eliminate it with the angle iron clamped on, different resonators, moving mufflers or other methods we have all read and posted about and tried, this might, just maybe be the fix we have been looking for.



Greg L. The Noise Nazi

 

[Gary - K7GLD]

Greg, I'm sure you are referring to AUDIO drone - sound resonance - as opposed to mechanical resonance, or vibration of the exhaust system or surrounding objects. In my case, the drone was mechanical, and quite pronounced from about 1700 RPM to a bit over 2000 RPM. My theory was that the floating, rubber supported exhaust system was vibrating like a quitar string at the power band of the Cummins, and figured that adding mass at the right spot in the system MIGHT eliminate the vibration, or move it outside the power band - to accomplish that, I added about 30 inches of heavy wall angle iron to the long stretch of piping between the resonator and the turbo downpipe, like this:



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It worked perfectly the first try, my vibration is gone - but to add bit more anchoring and stability to the system, also added a conventional exhaust strap from the front muffler hanger ovr to the frame, using a second U-bolt that could be pivoted on the pipe to adjust tension on the pipe:



#ad




Just thought I would toss the above into the mix, so both drone types are covered.

 

[drrhoades]

What size angle iron did you use?



Thanks,

Doug

 

[Lsfarm]

I think one creates the other

Hi Gary, actually I believe the audio pulses create mechanical vibrations in the exhaust pipes, as well as the engine vibrations traveling through the exhaust pipes.



With the angle iron trick, you are one of the lucky ones, many folks, myself included had no relief from the drone with the angle iron. . I also tried a short hanger strap like you show, and it did help, but did not eliminate the drone. I thought one additional strap at 90* to the first one would control or dampen some more of the pipe vibrations. But it didn't work for me when I tried it.



I have tried an amazing number of ideas to get rid of the drone. The most successful [other than the original chamber] prior to today's 5" muffler was to change from the thin, light-weight 16 gauge exhaust pipe to very heavy 1/8" wall [about 10 gauge] pipe. The theory behind this was that the audio resonance and drone was causing the pipes to vibrate in concert. This is a similar theory to adding the angle iron to add mass and rigidity to the pipes.



The heavy wall pipe reduced the drone to a tolerable level until I rearranged the mufflers the other day.



I'm really happy with the current added 5" muffler , the best results yet.



Greg L The Noise Nazi

 

[Bodybomber]

Do you think just using pipe in place of the muffler would have the same effect?

 

[Gary - K7GLD]

"What size angle iron did you use?"



Dunno how they rate "size" to angle iron, but the piece I used was about 2 or 2 1/2 inches on a side, and about 1/4 inches thick - pretty heavy stuff. But as Greg points out these rigs vary, and what works for one may not on another - it just helps to see a few different options - one may work for you.

 

[Lsfarm]

Nope

Hi BodyBomber, there was an approximately 50" long piece of the 1/8" wall pipe wrapped with header wrap in the location where the 5" muffler and a shorter length of 1/8th wall pipe is now.

So there was heavy wall pipe there before, now the big muffler. The same number and type of clamps and hangers too, so the only change is the muffler.



Greg L The Noise Nazi

 

[DLausche]

I think you are on the right track here Lsfarm. Let me start with stating I have never had a drone problem. My muffler is gone, but still have the resonator and small/big/small pipe diameter combination that is like stock. I believe this diameter change helps break down any standing wave frequencies that tend to occur inside the pipe. I think that with a constant diameter pipe the whole length, these waves tend to become very strong and cause resonation. Adding mass (i. e. angle iron or clamps) would help, I think, but probably would not be able to totally overcome a strong resonant standing wave frequency. I don't have anything to back this up, but I think this does make sense.



-Deon

 

[Lsfarm]

Important Point

Gary brought up a very good point, not every truck will rsspond the same to the different anti-drone tricks.



Some of our trucks have drones with stock exhaust, [I had a '99 5spd that did]. And many trucks don't get the drone with a 4" system, others do.

Some trucks eliminate the drone by adding a small bullit resonator.

Some just move the muffler forward.

Some add the mass and stiffness of a length of angle iron.



Then there is my noisy, droning, truck that has been a test of my limited patience. I've tried every idea I see posted. Some helped, but none have eliminated the drone.



I would recommend trying the simpler less expensive ideas first. Gary's angle and stabilizer strap may do the job for most with less expense than an additonal muffler.



Greg L The Noise Nazi

 

[Ncostello]

In steel terms... angle iron is named as such:



L3x3x1/4 for example. 3" each side, 1/4" thick, Lshaped



Could also be L2x3x1/8 and so on.

 

[Hohn]

Interesting.



Gary, I think it confuses the issue to talk about audio vs. mechanical drone since they are one and the same. ALL of this is just energy pulses.



Greg L, your theory about small-large-small diameter pipe is correct. It will break up the standing waves that contribute to resonance.



I bet you would be surprised to learn that the exhaust system flows best when it is resonating. Back to that old compromise of performance vs. comfort.



Personally, I would run a straight 4" with two sections that expanded to 5" ID muffler/resonator (and a 4" pipe connecting them). That should give lots of flow with almost no drone.



Justin

 

[Gary - K7GLD]

"Gary, I think it confuses the issue to talk about audio vs. mechanical drone since they are one and the same. ALL of this is just energy pulses. "



Au Contrair mon ami!



While they MIGHT both exhibit many of the same tendencies, many of the drone complaints across several boards reveal a mix of complaints - some related to the pure SOUND sourced drone that audibly resonates objectionally at specific engine RPM, and MIGHT not exhibit any corresponding objectionable mechanical vibrations that are discernable - on the other hand, MY "drone" was purely related to MECHANICAL vibration that exhibited a physical vibration that resulted in a buzzing resonant drone that was objectionable - but there was NO objectionable exhaust note accompanying it. SURE, both will contain elements of both audible and physical vibration - but the root cause CAN originate from 2 entirely DIFFERENT sources, and create 2 entirely different objectionable end effects...



To elaborate, *my* drone would have been just as obnoxious if the exhaust system could have been purely mechanically shaken at the appropriate frequency, even if NO exhaust gasses were passing thru it, because it was a mechanical function related to the physically generated firing pulses of the engine.



Audible, or sound derived drone, on the other hand, can be present even when NO annoying accompanying nechanical vibration is noticed - much like the peak in exhaust power and volume in "tuned" exhausts used in racing applications - and it can be quite pointless to try to reduce or eliminate sound based drone using methods more appropriate to mechanical drone, as it would to do it the other way around...



But, of course, it would be easily possible to be unfortunate enough to have both types of problem at the same time, each needing specific methods of treatment...

 

[Cooker]

I think Greg brings up a very interesting point that I don't remember ever being talked about before. It certainly seems logical to me.

 

[Snow man]

thanks for the info Greg! I had a terrible drone in my truck ever since i did the 4" exhaust. i tried 3 diff 4" systems,and a bully dog 5" system. they all droned,terrible from 2150rpm to 2500, with 4. 10 2200-2400 is my cruise speed. The only thing that fixed thedrone was putting on twins! Ever since then the drone is gone. Truck is very stealth now!,quiet yet sneaky quick

 

[Lsfarm]

Twins and Drone

HI Snowman, it seems that the second turbo exhaust wheel chops up the exhaust enough to eliminate the pulses, the ressonance and therefore the drone.



Now: if it was engine vibration causing the drone it should still be there even with twins right? But it's gone. !!



The Twins I've done here with 5" exhaust are all very quiet too. And sneaky quick.



Greg L The Noise Nazi

 

[HoleshotHolset]

"... the exhaust system flows best when it is resonating. "



Man, if that's the case... then my old 3" into 5" stack setup must have been the best flowing exhaust ever! It was deafening in the cab. :-laf



Excellent thread!



Yet another reason to run twins... as if I needed another.



Matt

 

[drrhoades]

If one was to bump up from 3" to 4 " then back down to 3" do you guys think that that would stop the resonation?

 

[Gary - K7GLD]

"Now: if it was engine vibration causing the drone it should still be there even with twins right? But it's gone. !!



As I tried to point out, there are *TWO* potential sources for the drone effect:



"Greg, I'm sure you are referring to AUDIO drone - sound resonance - as opposed to mechanical resonance, or vibration of the exhaust system or surrounding objects. In my case, the drone was mechanical, and quite pronounced from about 1700 RPM to a bit over 2000 RPM. "



And my comments here are intended to point out the difference between the two in order to help owners identify which type they are affected by, and then use the proper methods to correct it...



And yeah, I'm sure the installation of twins WOULD have the overall effect of changing both the physical AND audible characteristics of the engine and exhaust system - but it sure is far more extreme than a $5 chunk of angle iron - at least as far as mechanical resonance is concerned!

 

[Lsfarm]

Drone is vibration

A drone is a sound which is a vibration which creates sound and around and around we go.



If the pipes vibrate they create a sound, it might not be at a frequency we can hear, but if anything vibrates it makes a sound.



So Gary if your pipes were vibrating, how did you notice it or care?? Were you silently breaking metal hangers and pipes??



Or did you notice because of the sound?? I'll bet it was the sound. And if it was making noise it was vibrating. So even if you didn't hear an objectionable sound from the outlet of the pipe, it was still making an objectionable sound in the cab of the truck Right??



My truck's exhaust sound or note from the exit of the pipe has Never been an issue for me, it has been the pipes vibrating and beating the air and making an objectionable droning sound in the cab.



Sound impulses make the pipes vibrate in concert with sound pulses, any added mass or hangers or the multitude of other fixes never worked on my numerous different pipe and muffler setups, Except for now.



Greg L The Noise Nazi

 

[Gary - K7GLD]

Whatever floats yer boat!