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To bring up an old thread, this is on an entirely different car however I have a bunch of rasp when I rev the crap outa my 04 bmw 330ci performance pkg. If I have read correctly shorter pipes welded onto my exhaust would cancel out the higher notes? or is this simply an rpm related fix? After aftermarket headers and deleting the cats my car sounds like crap when it revs, need to get the high frequency notes out of there. Thanks!
What's up Tyler! It's Cameron.

Look into the "rasp pipes" for the bimmers. I think using an inline resonator or inline perforated core muffler will help. It's not feasible to use Helmholtz resonators for higher frequencies on exhaust due to the miniscule size the tubes would have to be.
 

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ok so I have a 97 5.0 explorer. I decided to cut the stock exhaust system off, removed the rear cats and installed an H-pipe. Then branched the exhaust immediately right and left, the driver side goes right under the end of the transmission and then they both run down in between the frame rails and sidesteps. I have a magnaflow 5" in daimeter 14" long round muffler on each side and the tail pipes dump right in front of the rear tires.

I'm getting the 2k drone but my tailpipes are about 14" long.....So does that mean I "T" in a 7" pipe into the tail pipe? I don't have alot of room to play with....I was thinking maybe installing 3" tips compared to the 2.5" in there now....
 

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ok so I have a 97 5.0 explorer. I decided to cut the stock exhaust system off, removed the rear cats and installed an H-pipe. Then branched the exhaust immediately right and left, the driver side goes right under the end of the transmission and then they both run down in between the frame rails and sidesteps. I have a magnaflow 5" in daimeter 14" long round muffler on each side and the tail pipes dump right in front of the rear tires.

I'm getting the 2k drone but my tailpipes are about 14" long.....So does that mean I "T" in a 7" pipe into the tail pipe? I don't have alot of room to play with....I was thinking maybe installing 3" tips compared to the 2.5" in there now....
You can loop the resonator pipes back and run them in parallel to the exhaust too if packaging is a problem.
 

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I did this on my GTO...



Are those other mufflers that have the ends welded up? Also, why did you put them in the middle of your exhaust?

I think that putting the pipes in FRONT of the mufflers will actually take care of more of the resonance. If you think about it, by killing the pulse waves before the muffling process, you may kill the waves even more with the muffler right? Or...it could be like a math problem and be just like an addition problem where it does not matter where it occurs in the system.

SWS
 

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Measuring drone in-car

I posted a message a while ago about my rusted out factory muffler on my 90 LX (5.0), and how I was considering stock (type) mufflers to keep drone away. As one person noted, this is not really a good option. The 2005-2008 GT mufflers were too wide to fit under my Fox body, the stock muffler for my car is no longer available from Ford and would cost a small fortune if it was, and I don't trust aftermarket stock replacement mufflers to have flow even as good as the supposedly restrictive stocker.

So, I am going to try the Dynomax Super Turbos. A lot of people say they are about as quiet as stock and don't drone... some say they drone badly. I had one on a 90 Ranger 2.9L I had years ago, and it didn't make an objectionable drone. The same truck with a 3 inch Dynomax droned HORRIBLY, though. I had gone to the shop for the Dynomax catback, but the exhaust guy decided to be an artiste and create one from scratch using mandrel bent pieces. It's a shame it was the wrong size; it was nice work. It was just too big for the little 140 hp engine. I went back and they put in the actual catback and it was great.

Perception of drone varies so much from person to person, not to mention what mods have been done to the car, it's nearly useless to try to find a "drone free" muffler from the posts. There does not seem to be one. I'll have to take a leap of faith and hope for the best with regard to drone on these.

From what I have gathered, mufflers themselves don't usually drone, so trying to find a drone-free muffler sort of misses the point. Graham's successful experiment with quarter wave resonators was based on the idea that it is the length of the exhaust system that creates the resonance. There is always going to be a resonant point in any exhaust system; it is the frequency of that resonance and how it is dealt with (such as with resonators designed to attenuate that one specific frequency) that determines whether a particular car has a drone problem or not.

The cutaways of the SN95 stock mufflers posted on Corral show that they are of a dogleg design with a rather large Helmholtz resonator taking up most of the interior volume of the muffler. I would bet anything that this resonator is tuned to create destructive interference at 133 hz or so, the frequency of the drone at 2000 RPM. I would assume the Fox mufflers are of similar design-- I will cut open the rusted one after the new mufflers are put on and find out.

It does not seem that the drone-free nature of the stock mufflers is related to their differing sizes. SN95 and later cars have same size mufflers and don't drone. The Fox-body cars appear to have different sized mufflers because there is more room on the driver's side, between the subframe member and the driveshaft, than there is on the other side. The pinion shaft is off center on the rear axle. This means that the driveshaft is not centered between the subframe members. It's a little toward the passenger's side. The left muffler is larger simply because there is more room on that side.

Aftermarket mufflers (other than Corsa) do not, apparently, have a specific frequency they are designed to attenuate like the stockers. So, when a drone is heard, it is probably true that the muffler is attenuating the 133 hz frequency as much as many other frequencies that don't result in a drone... but the 133 hz output at 2000 rpm is still higher than the specifically attenuated 133 hz output of the stocker, so it causes the entire exhaust to resonate more than stock.

So... all that said, I have obtained a sound level meter so that I can get some actual numbers to go with all of the subjective reports.

My engine has a K&N filter (conical in a custom made cool air box under the hood), but other than that, it's stock all the way to the tailpipe. The left stock muffler has a few rust holes in it, so it is louder than stock. There's no way to know how much louder, as I never measured the spl before getting the holes in the muffler.

I went on the freeway and drove at constant speed at 2000 rpm, which is about 68 mph on my car (3.55 gears, T5Z .63 fifth gear). dB(A) was 77, with the dB meter sitting on the front left side of the passenger seat with the mic pointing toward the dash at an angle (the lcd display of the meter situated so I can read it easily). It may not be the ideal position, but I will use the same one when I test with the Super Turbos for an apples to apples test.

I will also continue to test the Super Turbos to see if they get louder over time.

Idle reading at 0 mph was 59 DB(A), and cruise at 2000 RPM at 43 mph in 4th gear was also (which surpises me) 77 DB(A). I would have thought that the higher engine load and wind noise would have made it louder at freeway speed, but the meter said no.
 

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I just put some muffler cement on the holes a few hours ago so I could test it. It has dried now and seems to have fixed the problem for the time being... 2000 rpm freeway reading is now down to 75 db(a).

Should be getting the Super Turbos tomorrow.
 

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Subscribing....curious how it turns out as I'm considering the super turbos as well. BTW, measuring noise in db isn't necessarily the best way as the human ear reacts not only to volume, but also to frequency and the quality of the sound. Fans are rated in a unit called 'sones' -- and it is a better rating of how the noise appears to our ears. Unfortunately I've never seen mufflers rated that way. The other HUGE issue with regard to the noise we perceive in the car is the way the interior and trunk/hatch cavity of the car resonate with the muffler noise. You can have VERY loud drone in the car without much noise outside, and vice-versa. So - it's not all about the muffler....

I'm looking to replace my single OEM Boss 429 transverse set up with something else. Sounds good and isn't too loud (although getting louder with time) - but doesn't flow worth a crap. http://forums.corral.net/photopost/showphoto.php?photo=716&ppuser=30684
 

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I have seen the sone rating on some of the cooling fans for my PC, and I agree that dB may not be the best way to go.

With regard to PC cooling fans, I see a lot of the same subjectivity that we do here about drone. One person will say a given fan is quiet, the next will say that it sounds like a Ford Trimotor taking off. It would appear that sone is a unit that attemts to quantify this.

That said, though, what I have is a dB meter, not a sone meter, and certainly the numbers from this are a step up from the "it doesn't drone" or "it drones like crazy" subjective qualitative reports on exhausts here.

I am thinking mainly of two anecdotes with regard to the Super Turbos. In my search for info on the web regarding mufflers (I have spent a lot of time reading), I saw one muffler "shoot out" type article where they put on a Super Turbo and it did not drone...then they tried an old Super Turbo with a lot of miles on it and they said it sounded the same except that it droned on the highway.

The other one is MFE, saying that it was quiet for 3 days and then got super loud.

I want to establish a baseline dB reading and see how it changes over time. I will run it on the same section of freeway at the same speed and see what I come up with. So far the tests have been at night (light traffic) in about 50 degrees ambient temp, fwiw.
 

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If the weather was better, I'd go out and do some recording of mine. Here's the thing: When I took the old, too-loud super turbo's off, I cut one of them open, expecting all the fiberglass packing to be gone. It was still there. Now, I've never cut open a new one and it's been over 15 years since I saw a demo of one from Dynomax, so I have nothing to compare it too, but there was still a good measure of fiber matting wrapped around the trio of pipes inside.

The drone at 1900-2100 rpm is straight-up ridiculous. I installed these last January and they were nice and quiet, made me giggle they were so mellow. Then within a week or two they were loud again. I didn't take it to a track event until April, so it wasn't track usage that caused it.

The only things that make my car any different than any other street driven 5.0 is the factory carpet has been replaced with aftermarket stuff without as much sound deadening, and some of the rest of the factory deadening is missing, but mostly replaced with "peel & seal" poor-man's dynamat. It's got full-length tailpipes 2.5 inches, an offroad h-pipe with hi-flow cats welded back in (didn't change the sound much), and the exhaust isn't contacting anything on the car to transmit noise into the cabin.

I'm going Borla XS next time, when I get the combination of time, motivation and money.
 

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Well, Super Turbos are in, and... they are horrible. Drone is insane. I would rather have my rusted out old stockers on. I didn't even get the 2 weeks of quiet!

The dB meter doesn't tell the whole story using the methodology I did with the stockers. Cruising at 2000 on a flat stretch of freeway, it's 79 to 80 dB(a)... subjectively not that bad. But give it just a *little* bit of throttle to climb a very slight hill, and it jumps to 85 dB(a). And it gets worse if the RPM is a little lower; my drone seems to be centered at 1800.

The stock muffler did not exhibit this tendency to get louder with slight changes in throttle position at freeway speed (2000 rpm in 5th).

I guess if you want anything but stock mufflers, you have to put in the quarter wave tubes. More money.

I don't really know why I would even want any kind of high flow muffler; I probably haven't even seen full throttle in the last few years. I guess I got old.

Maybe I will get lucky and find stock ones that are not rusted beyond belief in a junkyard. Probably will be cheaper than having someone fabricate the tubes.
 

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Well - sorry you had to find out the hard way - But I suppose I'll avoid those. Perhaps it's back to my original plan. I'm going to search out a totaled large German/Japanese sedan that makes similar or more power than mine (newer Lexus, BMW 7 series, big Merc, etc.) -- and see if I can make the stock mufflers on those work. If they'll support 350-400 HP on those cars, they shouldn't hold mine back too bad......in theory. Maybe I'll make my own...hmmm.

Quarter wave tubes?
 

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Just like an intake resonance chamber - got it. Do you have an algorithm for telling you how long/what diameter/where to place it? Also seems like it would need some support out on the end to prevent a fatique failure at some point (edit - just saw the hanger). What would happen if you tied the tailpipes together with a same-diameter tube flanged (run it across the car just before they exit) so you could unbolt it? Would that have the same effect? Is there room there to do it?

I can't make out what's going on in the pic - is that the front of the car or the rear? edit again - just found D/S and rearend. This one's like mine - tailpipes go under the axle instead of over.
 

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Wow, I posted that last reply before that pic showed up (I just saw the url)... that looks like it would be a Helmholtz resonator rather than a quarter wave. I found a page online that tells how to determine the resonant frequency of a Helmholtz, but I don't know how to make sure it would be out of phase with the exhaust pulses going by. Is that one in the image a custom made one or is it taken from some other application (generator etc?)
 

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Just like an intake resonance chamber - got it. Do you have an algorithm for telling you how long/what diameter/where to place it? Also seems like it would need some support out on the end to prevent a fatique failure at some point (edit - just saw the hanger). What would happen if you tied the tailpipes together with a same-diameter tube flanged (run it across the car just before they exit) so you could unbolt it? Would that have the same effect? Is there room there to do it?

I can't make out what's going on in the pic - is that the front of the car or the rear? edit again - just found D/S and rearend. This one's like mine - tailpipes go under the axle instead of over.
Read through the whole thread linked in post # 2. There is a long discussion of this, several pictures, and links to more pictures.
 

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Thanks Dan - Graham's explanation makes sense - and also explains why my 12" long tailpipes don't drone. :) And why changing the muffler made no difference to the OP. If this is right, drone should be absent with turn downs.....
 

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Another success

One thousand times thank you, Graham, for this idea and the research behind it.

Forgive me for repeating myself in posts... but a while ago, my stock muffler developed a rust hole (initially I thought the flow pipe rusted too, but it turns out that what I saw from my bad angle was the residue of a plastic shopping bag or something similar that cemented itself to the pipe). I had resisted replacing the stockers for fear of the dreaded drone.

Finally I decided to take a leap of faith and I got the Dynomax Super Turbos. Some said they were quiet, some said they droned... same story as all mufflers if you search long enough. So I took the leap and... drone!! Lots of it.

I pondered whether to put stock muffs from a junkyard in, if I could find non-rusted-to-death ones, or getting the quarter wave resonators made. Ultimately, obviously (since I am posting thanks on this thread), I had the resonators made. It took three shops before I found one willing to make something so seemingly pointless as a pipe that goes nowhere, but I found one, and it looks great. I will try to get some pics of it when I can... I did not have my camera at the shop when the car was on the lift, and I can't get under there easily to get the picture. You cannot see them with the car on the ground... they are hidden well by the gas tank.

The exhaust guy that finally did them said he had seen similar things on some Euro cars... pipes that go nowhere and are capped. So he made the set, 28 inches long in my case (my resonance was centered at about 1800, so I added 2 inches to Graham's measurement).

When he drove the car out of the shop, he was amazed at how different it sounded. He said it sounded stock-- and it does. It may even be quieter than it was with the stockers at cruise (keep in mind that at least one of my stockers was rusted pretty badly... I patched it for the dB test in the other thread, but the internals were rusted too). There is NO trace of the drone.

Every exhaust shop should know about this. How many people (with various cars, not just Mustangs) are out there tearing their hair out over drone, and this works so well? Two of the shops didn't even want to try it, even though I told them I would accept full responsibility if it failed to work. It was the educating of the professionals that made me consider going back to stock (or trying to)... it was easier than I thought to get it done, though.
 

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More info/pics/soon....inquiring minds want to know more!!!

Next you can jump into this to really quiet it down....http://forums.corral.net/forums/showthread.php?t=432018

And it now sounds like the super turbo muffler MIGHT quiet mine down since I shouldn't have the drone issue to contend with.
 

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With the quarter wave resonators and Super Turbos, I saw a dB(A) reading of 75 on the freeway at 2000 RPM (68 mph). This is only 1 dB louder than the patched stock muffler at the same speed and on the same stretch of freeway. I can hear the exhaust tone at this speed... but it is not drone. It's the same subdued exhaust sound it had with the stock mufflers at that speed.

Other than the drone, the Super Turbos are very quiet on my car, Michael. It has a slightly deeper idle tone with the Super Turbos, and they have a low growl just above idle, not loud or obnoxious in any way... just above that the resonators start working and the exhaust tone is subdued, just like stock. If not for the deeper sound at idle and just above, I would think the car has a stock exhaust from its sound.

Keep in mind that these Super Turbos are brand new... there have been reports of their sound changing as they age a bit. I will run some more dB tests as the miles add up. I am not worried about it with the drone buster pipes on there.

As for dumps not droning because of the short tailpipe length-- I think that it might be the entire exhaust system length that determines the resonant frequency, not just the tailpipe. The counter guy at the exhaust shop where I had the pipes done earlier was a (Fox) Mustang fan, and he told of how he had a 1990 notchback 5.0 that droned with dumps... but it was at a much higher frequency than the full tailpipe Mustangs. A shorter exhaust system would raise the resonant frequency. There is a basic calculator for this which suggests that it is the entire exhaust system as well.

As long as I am on the topic, there are a few other calculators I ran into as I rread up on drone:

Quarter Wave Tubes- DiracDelta

Cavity Resonance (Helmholtz resonators)- HyperPhysics
 
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