Resonance--Help-rant
 

I have read a lot of posts here on resonance from mostly chambered mufflers and others but no real solutions,and even when I go locally not one "exhaust Tech" can produce a tried and true resolve.You would think the major manufactures would have an explanation as too why,how and solutions to this issue.I know I sure would like to know how to cure it without cutting up a exspensive exhaust with no guarentee that the resonators will work.
So the question raises its ugly head again.Why after 3 different mufflers,a flowmaster crossflow,a 50 series delta ,and now a edelbrock SDT and still the resonance on two different firebirds and two different engines with different cams.What is the real cause???
1> Some have said if you buy a stainless steel muffler product there is less resonance.REALLY??Why,is it because better quality materials?If thats the case why not just wrap the mufflers I have now with a header wrap??
2>Some of said use a resonator like the factory did on some cars.Problem is on a firebird using a 3.5 inch or 4 inch round resonator,where do you mount it without clearance issues,plus all the ones I have seen appear to be quite restrictive.
3>Some of said if you don.t use good quality hangers it contributes to the problem.I agree so I used factory hanger locations behind the axle and heavy duty hanger brackets into factory rubber isolators.Still have the dreaded resonance
4>Maybe I need more mounting points in front of the rear axle,like a rubber hanger mounted to the floor pan behind the rear seats like a lot of muffler shops do?I really dread putting extra holes in a restored 71 T/A.Maybe a support up at the "X" crossover also.?Is a well supported exhaust system with extra hangers then what would normally be used,going to solve the issue?
5>After all this ranting here is a theory on my part,right or wrong.I have resonance only at minimal part throttle between 2300 rpm and 2600 rpm.Loudest at 2500 rpm.If I push the accelerator down to just into the 4 barrel starting to open it disappears and the same if I back off and just cruise at 2200 t0 2300.My mufflers are mounted in the raised portion of the rear floorboards under the rear seats,but are within a 1/2 inch of the floor.The exhaust system does not touch anywhere.Now at that rpm mentioned lets say the harmonics of the exhaust cause the muffler case to vibrate enough to transfer the harmonics into the floorboards and threww the car.If I am right wraping the muffler to maintain ground clearence or allowing the mufflers to drop to 1 inch below floor should help or get rid of it.No??
Enough rant!Has anybody have any thoughts on what I have said or tried my wrap idea or other solutions short of changing my mufflers.

By the way the idea of the header wrap on the muffler body would give a deadening effect on the sound transfer to the floor area.At least thats my theory yet to be tried.I also have the stock tail pipe tips but bought the 74 ones which are 1 1/2 inch longer to get the sound out from under the car.The system is a Pypes 2.5 inch to the mufflers with an "X" crossover with the pipes down the driveshaft tunnel<then SDT mufflers and the flowmaster tail pipes,all band clamped together.Used the crossflow muffler mounting hangers and fabricated brackets onto the tail pipes to hang the system and used the factory hanger brackets on either side of the gas tank to mount the very end of the tail pipes,just like factory.I did not use a hanger right behind the mufflers like most exhaust shops would do,because the system is very ridgid the way I have it and as said before ,no holes in the floorpan.But if adding those hangers would cure the issue I would do it?The million dollar question?

I had a set of flowmasters on my firebird had a real bad reasonance problem at just above an idle. Bad enough that I got pulled over and ticketed for having defective and or missing muffllers and tailpipes. I went to a magnaflow exhaust system with an X-pipe, no more resonance. I belive the x-pipe has more to do with that than the rest of the exhaust system. It mixes up the exhaust signal from side to side and the cancel each other out. The x pipe is also supposed to help scavenge the exhuast more efficiently, I gained 200 rpm at idle from just switching systems. Flowmaster makes a universal x-pipe and there are probably others out there as well. My brother and built one for his dodge truck out of 4 inch mandrel bent tube.

IMHO any muffler that requires sonic pulses to make over a 45 degree angle will lead to resonance. In short, any chambered or reflective muffler is going to resonate. I would recommend Ultraflows, Magnaflows, Borla, Straight line or Goerelich (sp?)

Well I am running an "x" crossover and formula8 I have tried the super turbo.s and yes next to no resonance but didn,t care for the overall sound.This is a different engine and cam so I may try them again if I find no sutiable solution .

Being selfish about this, I hope you decide to try wrapping the mufflers, so the rest of us can all learn something (for free)! Over the years, I've had a couple vehicles with exhaust resonance, and considered wrapping mufflers- but never did get around to trying it- changed mufflers, if I remember right.

For what it's worth: I once modified the expansion chamber on a 2-stroke motorcycle to quiet it. It worked, but then I realized that a lot of sound was still coming directly off the sheetmetal case of the chamber. I quieted this "radiated" noise a lot by wrapping it with fiberglass cloth and resin.

Everthing in the world has a natural resonate frequncy. Your exhaust system is like an organ pipe and will tend to resonate at certian frequencies. In speakers systems, there is a natual resonance of the speaker, you then design the speaker box to resonate below that of the speaker. Adding the two components together will move the resonate point towards the middle of the two and decrease the amplitude.

For car systems the rpm (pulses) sets the frequency, the length and size of the tubing are like that of the speaker box, but in this case our interest is not in mating the two but designing the system to work with existing frequencies in the engine.

Go to your local library or look on line, there is a book on tuning exhausts systems that can provide you the formulas you need for you specific engine. I'm not at home right now or I could find it (run away from Huricane Rita). If I remember correctly, its called "Designing Exhaust systems" and had a first release in 1960 and recently a reprint.

Pontiac jack,I am going to try dropping the mufflers a 1/2 inch first and see if it has less resonance within the car.I have to drop them anyways to try wrapping.
Dryseals you may be right.

So I took the rear hangers off so the exhaust system was just hanging by the tail pipes and the mufflers dropped 1/2 inch.Went for a drive --no difference.Came back and wrapped the mufflers in header wrap and reinstalled hangers.Went for a drive and it was slightly quieter and a little less resonance but a softer low tone.Very little differance overall.More thought required???

Like Dryseals said, everything has a point of resonance, on alot of newer cars they hang weights on the exhaust to change the point of resonance. Maybe making the exhaust heavier could possibly make it better?

I seem to remember years ago when the whole crossover pipes came on the scene. The exhaust was "tuned for horsepower" by putting the crossover a certain distance from the collectors. I think the exhaust manuf took the idea
and ran to the masses with it, not being concerned with tuning so much as selling the exhaust system with the magic buzzword "CROSSOVER"
If tuning for power was their intention, then they would sell to spec's of YOUR engine.
I am also looking for input to rid my T/A of the "drone" ...it as flowmasters with H pipe w/crossflow muffler.

What ever happened to the Hush Thrush? Had it on a 70 f-bird in 1980, it sounded great

Well guys I learned a valuable lesson to-day.After all my screwing around I pulled the SDT mufflers off and found i had slid the muffler pipes too far into the inlet side of the muffler.It has about a 4 inch deep chamber on the inlet side and I was 2inches into it.So I cut my pipes 2 inches shorter on both sides and remounted the mufflers .Went for a test drive and what a difference.The resonance is 70 percent gone and within a tighter rpm range now.2400 rpm to 2600 rpm but not as bad.Who would of guessed.?At least it is acceptable now.

Way to go! Glad you found an explanation. And thanks for experimenting and sharing the results- saves the rest of us wasted time wrapping mufflers!

Someone asked about the old Hush Thrush mufflers. I remember running one (single-exhaust vehicle) and having the "seat-of-the-pants" impression of being very restrictive- could be wrong.

I had the straight thru, undercar chambered exhaust on my vette. Same thing you ran into plus a lot of popping during deceleration. I ended up adding a set of Pacer resonator tips ( little mini glass packs ) which cleared up 90% of the resonance and about half the popping while still being agressive as heck. I cannot speak to the restrictiveness of the Pacers as chambered exhausts are pretty darn restrictive as it is. The pacers came from Summit if I remmeber correctly. -harry

resonance
 

From my experiance tailpipes and x pipes cut resonance the most without taking away from the sound.

Well my tail pipes either are too long or not long enough.I still have resonance at 2500 rpm at part throttle so i will continue to study the issue and see if i can tune it out with more wild ideas and see what works.What to try next??

While we're just throwing around ideas-
I once had a pair of very-small very-loud glasspacks on a 289 Ranchero, but when I built tailpipes I used bendable tubing (not flex tubing, but the convoluted stuff) going over the axle. The end result was quite mellow, with no noticeable resonance. The 2" ID stuff I used is probably restrictive, but I've often wondered if using something like 3" ID bendable would provide the same benefits without restricting a larger engine. Anybody?

I had the tailpipes on my GTO exiting through the bumper bar. There was a gap between the pipes & the bar, & this was filled with silicon so that there was no metal-to-metal contact. The silicon kept the pipes extremely rigid, but was flexible enough to allow for expansion [ stainless steel ] rearwards as the pipes heated up. I had no vibration & no resonance with this set up. This surprised me somewhat because the pipes were so rigidly fixed that I expected resonance.
I decided to increase pipe diameter, so I cut off the pipes just behind the bumper bar for the drive to the exhaust shop; the ends were help in place with conventional brackets. Now I had resonance with standard type exhaust hangers! Nothing was changed in the exhaust sysyem other than as outlined above??
I should add that while restoring the car, I used TWO layers of underfelt on the trunk floor, TWO layers behind the rear seat squab & TWO layers under the rear seat. I also glued some conveyor belt type rubber to the rear quarters, inside the trunk. My thinking was that these large metal panels would more likely resonate, so I did the above to reduce panel vibration.

Designing your sound!
 

Any restriction of movement on the exhaust system will reduce the vibrations of the system, and dampen the sound coming from the system. Chambered exhausts have little to no material inside to absorb sound, hence the impact of the exhaust pulses will travel to the case and piping. Contrary to popular belief, a very rigid exhaust system will have less noise than the same system held loosly. The frequency/tone of the sound will go up the more rigid the system is. Tuning an exhaust for a mellow tone, while maintaining a performance flow, is a challenge. Most of us don't worry too much about it until that long trip comes up that vibrates the floorboards and tingles your feet, while it makes you deaf for a few hours after.

The trick is to have all piping fully welded, and flanges bolted securely. A good system will hold its' position, even when hanging from just the headers.(not recommended!) You should only use as many hangers as needed to keep the system off the ground, the fewer mounting points, the fewer paths for vibrations to travel to the body and induce other resonances. Once you have a strong set of pipes, the only mounting points you should need will be at the exhaust manifolds, and just aft of the mufflers. If you still have a drone/resonance, then it is time to experiment with different mufflers/resonators/cross-overs/pipe length.

Well ,Pontiac jack your thoughts are well based and I may get rid of the resonance by changing the diameter of the tail pipes to 2 1/4 from 2 1/2.Or it may be the same.Geoff ,i think I will try the carpet trick in my bare trunk and see if it has any effect.It may well be the vibration from the trunk area.And to ontheblocks,it sounds like you have had some exhaust system experiance.I often wondered if adding a couple rubber hangers to and aft of the mufflers would help.Currently I have hangers on the mid point of tailpipes and the tail pipe tip area using factory brackets and locations.I removed the mid ones and test drove with no effect,Remounted them again.