Exhaust X-Pipe location and Resonance Question??

[NasaRacer]

I am acutally an Audio Engineer but like playing with cars and have run into an exhaust design issue.

Me and many other owners of Chrysler LX cars(300C,Charger,Magnum) are having major exhaust resonance issues when attempting exhaust modifications and would like some technical advice if possible.

My example: A 2007 Dodge Magnum SRT8 ...stock except for the problem exhaust mod I just did.

I didn't really care for the aftermarket systems. So on the advice of others I removed the rear resonators/mufflers and replaced them with a nice set of 3" mandrel bent tailpipes. Most have said this is a non objectional mod. Well I guess there assessments were based on 5.7 Hemi with MDS(multi displacemnet system) which have a whole other set of concerns for noise control. The 6.1 Hemi has no MDS and even though it appears similar on the outside the exhaust is designed differently.

I have included an artist illustration of an underside view of the stock exhaust system.

So what I did on my foul up is where that single stock tailpipe unbolts from the what we call the stock suitcase muffler since it is one big unit. The suitcase inside seem to be a straight through design. But I have never seen it cut apart to see how it deadens sound. Reflective Baffles of some sort I assume.

I did transitions from the stock 2 5/8" exits of the suitcase pipes up to 3" with a flange on each side so now both tailpipes are removable.

Then the tailpipes are nice mandrel bent 3" to the rear exit with a 3.5" tip slipped over the last about 6" of the pipe but the 3" internal pipe extend all the way to the end. That double pipe look. The tailpipes (not including what ever length the resonators add) are most likely a bit shorter than the stock tailpipes which will effect where they resonate at. Ofcourse.

So now except for the resonance the car sounds great ... overall it gets louder when you open it up but not too loud. Sounds like a performance car should inside and out. The stock sound was kinda of tame.

The bad resonace is from about 1200-2200 RPM. With the Throttle closed or at light cruise in the 1200-2200 RPM range it is actually not very loud.

But just pulling away from a dead stop and going through that 1200-2200rpm range with any throttle angle and it really sings in a bad way. My Personal Pipe Organ!!

But by 2500RPM it quiets completely.

So I guess I kinda of scewed up here. The rear resonators I am assuming added basic length to change where the tailpipe resonate or did some sort of 180degree out of phase reflective cancelation of sound and the bad freqs. A Helmholtz Resonator design.

So for simple solutions I was considering two things:

1) Puting some Dynatech Vortex Inserts after the suitcase muffler at the start of my New removable tailpipes. I guess they break up the sound pulses with a Vortex Effect? But not sure they could effect such low sound freq sound waves??

2) After reading some about "Balance Pipes" I am considering adding another closer to the engine in the exhaust system.

As you can see from the stock illustration the X-pipe is near the middle of the car infront of the suitcase. What I was considering is near the rear of the tranny right before the pipes expand out toward the sides of the car to meet with the Cats was to add another balance tube.(See Pic Below: Additional Balance tube in Blue with Arrow)

My hope is to lower the exhaust pulse Freq below what the tailpipes are resonating at.

So can we do that math backwards here??
Since we clearly know the RPM we get a harmonic in the tailpipes. Based on the current exhaust pulse freq which is a factor of the distance from the current rear X-pipe to the engine's exhaust valves? Right?

Can we figure a distance forward from the current X-pipe to add the small H-connection. There is about 18-20" forward from the current X location before the pipe spread to goto the cats.

Thanks in advance for the help.

Chris Teasdale
cmcracerguy@netscape.net

 

[oab]

I think the resonance is being produced in the rear pipes, hence the reason for the resonators fitted as standard. Fitting a crossover pipe up closer to the exhaust valves, is not the way to fix the rear resonate problem, it can hurt performance by allowing high pressure gas pulses to reach the open exhaust valves on the other side of the engine. It also reduces the length of the pipe that produces inertia scavaging.
The fix is to work on the rear pipes, A crossover pipe back there would split the sound wave between the two, to reduce the resonance intensity.

Harvey.

 

[NickE]

Hows this sound.

1) Setup a good realtime audio analysis system like Smaart Live.
2) get some data on what is actually happening.
3) Engineer a helmholz resonator that notch filters the annoying resonance.

I do materials for a day job and audio/cars for a hobby. I used a cheapo mic and my laptop w/ some free FFT software to get a spectrum of my car once. I didn't do anything real with it, but seems to me you should have the background to solve the problem.

And Yes I do realize that this could be taken to the nth power, much like a test lab for exhaust systems I saw once.

 

[GregLocock]

firing order at 1700 rpm is 110 hz or so, so it's about a 4m wavelength, ie roughly the length of the full system. Nobody can really tell you what to do,I suspect, as modern exhausts are complex beasties.

The rear muffler is typically the most important, but that may be because it is the largest in most cars. Cut the old one open, and make a copy with half the attenuation - you'll have to read up on the ways that exhausts attenuate. Chances ar that it is an expansion chamber for 110 Hz, so just reduce the area ratio by half.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Rob45]

Based on the current exhaust pulse freq which is a factor of the distance from the current rear X-pipe to the engine's exhaust valves? Right?

Not hardly; the exhaust pulse frequency is related only to engine rpm and architecture.
In your case, firing frequency is RPM divided by 15.

Your problem is however due to the overall system pipe length AND also due to cabin dimensions: you no doubt have a cabin internal standing wave excited by an exhaust acoustic mode, as Mr. Locock suggested.

That's why those resonators were located where they were; they undoubtedly contain Helmholtz tuners set to the frequency you find objectionable. You could try making your own tuners to replace them, as has been suggested, but you'll have to keep in mind the minimum volume for them to be effective would be around 180 cubic inches (each).

You could also try putting a cross-over pipe to the rear of the main muffler, at the point where the RH and LH pipes are closest. I have had some success in the past eliminating low-frequency exhaust drone with a pipe like this (e.g., C4 Corvette).

Or you could just put your mufflers back on.

PS: Ah, there is one other possibility: you didn't by any chance also get creative with the exhaust pipe hangers, did you? You know they're supposed to be rubber isolated, and attached to the original locations?
Chrysler has made some colossal screw-ups in the past with their exhaust hanger locations (e.g., Grand Cherokee), and they have made some improvements in their hanger locations recently, so second-guessing them is not likely to make your car sound better.