The Drone Zone

[shovel]

I'm starting this thread just because it might help someone in the future figuring out their plan for exhaust. The key point is that the engine makes whatever sound it's going to make & the exhaust system is just how you handle that sound.

When I first got my 94 GT it was stock except that the previous owner had installed a Flowmaster Super 44 catback. It also didn't really run, so at first I got it running with mostly all stock stuff. It sounded the way we all know a small block sounds with flowmasters... and it droned fiercely about 1900-2200 rpm.

Later I installed a 94/95 Cobra intake manifold and it dramatically changed the way the exhaust sounded, though it didn't affect the drone. I expect that it changed the overall exhaust sound because the flow efficiency of the individual intake runners is more consistent among all 8 cylinders on the Cobra manifolds, there's around a 20% difference between the best and worst flowing runners on a stock 5.0HO manifold leading to a very different amount of exhaust gas per cylinder with that manifold.

To attempt to address the drone I swapped out the Flowmaster 44's for some Pypes M80 mufflers, basically just glasspacks. They did not absolutely eliminate the drone but they made it a lot quieter and seemed to move the peak drone down around 1-200 rpm. It was definitely bearable and I could live with that amount of resonance.


The next change I made under the hood was switching to a TBI on a carburetor style dual plane intake manifold, along with aluminum X302 cylinder heads. I didn't expect it to make a huge difference in exhaust but it magnified the drone - now around 1700-2000 rpm the drone is intense! Almost wish I had kept those Flowmasters to swap back in & see if they drone any less.

So.. I'm about to install a set of turbo style mufflers and hope those axe the drone entirely. I know I could also try to stick some helmholz resonator J-pipe doodads in there somewhere but there isn't a huge amount of room under these cars for science experiments.

If you've been dealing with some real bad drone on your SN95 I don't know if this will be helpful or not, just some data points to consider.

 

[dsrtjeeper]

I feel for you. Drone sucks. I've ridden in cars that have it and the drone sucks the fun right out of the hobby.
That being said; I've never had drone in any of my Mustangs. It doesn't matter if the engine is stock or is any sized stroker. A couple of things that have stayed consistent across my builds are the use of headers, X pipes, tailpipes and Jones Max Flow mufflers.
Also; my exhaust has always been 2.5".

Something to take into consideration is the size and shape of the muffler body. The Dynomax Super Turbo's seem to be the way to go to help with drone when nothing else works. I suspect the Super Turbo's tuned frequency helps to cancel out the natural frequency of our V8's.

 

[shovel]

Sorta bad news on the Dynomax Super Turbo mufflers; yes they're a lot quieter though still considerably louder than my 5.7L Ram truck, but they still drone! I also think they're costing enough power I can feel it, not just imagine it because they're quieter. I could do an instrumented 0-60 but I don't have an instrumented, equivalent "before" so it's not worth all the work. Butt dyno's good enough.

There are some common elements here among all these systems, the engine itself and its camshaft profile, the (Ford Racing) headers, the BBK catted midpipe, the overall 2.5" pipe diameter and material and the length of the exhaust system. The turbo mufflers should change the effective length of the system as a whole but they seem to not really change the resonant frequency that's responsible for drone.

I'm beginning to wonder if the culprit here is that BBK x-chamber thing and the pipes between that & the mufflers.




I even tried as an experiment running one Pypes M80 and one Dynomax Super Turbo - still drones and also sounds like garbage in my opinion.


More to come?

 

[dsrtjeeper]

Sorta bad news on the Dynomax Super Turbo mufflers; yes they're a lot quieter though still considerably louder than my 5.7L Ram truck, but they still drone! I also think they're costing enough power I can feel it, not just imagine it because they're quieter. I could do an instrumented 0-60 but it's not worth it.

There are some common elements here among all these systems, the engine itself and its camshaft profile, the (Ford Racing) headers, the BBK catted midpipe, the overall 2.5" pipe diameter and material and the length of the exhaust system. The turbo mufflers should change the effective length of the system as a whole but they seem to not really change the resonant frequency that's responsible for drone.

I'm beginning to wonder if the culprit here is that BBK x-chamber thing and the pipes between that & the mufflers.


View attachment 51166

I even tried as an experiment running one Pypes M80 and one Dynomax Super Turbo - still drones and also sounds like garbage in my opinion.


More to come?

The BBK piece is not a true X pipe and by design causes turbulence in the exhaust flow. The X transition should be longer and smoother.
https://static.summitracing.com/global/images/prod/large/big-12115yflt_w.jpg

 

[shovel]

 

[dsrtjeeper]

Nice demonstration. It's very deceiving. Keep in mind that we're dealing with exhaust pulses.
Google "Dr. gas X Pipe" which has been used in NASCAR for decades. Also "H.A.R.M. exhaust x pipe" which has been dyno and track proven to provide more power than the others. See how the two pipes are siamesed? That's where the magic happens.

 

[GTamas]

You could try experimenting with different tailpipe lengths.
How are your exhaust hangers? Maybe they are transferring enough vibration to the chassis that it amplifies the resonance?

 

[badass98svt]

Sorta bad news on the Dynomax Super Turbo mufflers; yes they're a lot quieter though still considerably louder than my 5.7L Ram truck, but they still drone! I also think they're costing enough power I can feel it, not just imagine it because they're quieter. I could do an instrumented 0-60 but I don't have an instrumented, equivalent "before" so it's not worth all the work. Butt dyno's good enough.

There are some common elements here among all these systems, the engine itself and its camshaft profile, the (Ford Racing) headers, the BBK catted midpipe, the overall 2.5" pipe diameter and material and the length of the exhaust system. The turbo mufflers should change the effective length of the system as a whole but they seem to not really change the resonant frequency that's responsible for drone.

I'm beginning to wonder if the culprit here is that BBK x-chamber thing and the pipes between that & the mufflers.


View attachment 51166

I even tried as an experiment running one Pypes M80 and one Dynomax Super Turbo - still drones and also sounds like garbage in my opinion.


More to come?

Dynomax Ultraflo mufflers are great for no drone, and they dont have any baffles to rob hp

 

[ttocs]

I ran their side exhaust with no problems.

 

[GTamas]

Dynomax Ultraflo mufflers are great for no drone, and they dont have any baffles to rob hp

I have those on my otherwise stock exhaust. There is a very moderate drone around 1900rpm. It isn't annoying at all, I would recommend them.

 

[shovel]

I haven't yet decided what I'm going to do here - I could experiment but the trouble with any homebrew acoustic tuning is that it's basically impossible to get it right the first time - sure, we can estimate exhaust temperature and calculate the speed of sound at that temperature and then calculate what a 1/4 wavelength is at a given frequency, that part is easy.

What's not easy is that it assumes the 1/4 wave helmholz pipe has a continuous cross section and intersects with the exhaust stream at a 90 degree angle at full size (in other words, a 2.5" perfect "T" ) and that the exhaust flow at that part of the pipe is laminar so that the exhaust velocity and pressure at that moment is actually anywhere near what can be calculated from a static value. And also fit under the car. That's why CFD is still complex and expensive, it's outside of my area of expertise and I don't want to trial-and-error 50 iterations to get it right I just want to drive the car.

It would be pretty cool if an exhaust manufacturer would make something like this:


Excuse my cheesy MSPaint drawing but hopefully you get the idea - a standard 14" muffler body with a tunable helmholz resonator so the installer can just slide the "bottle" like a trombone until drone goes bye-bye, clamp it and enjoy the absence of drone forever.

Surely there's already a patent for something like that, maybe someone's even tried to make one but I've never seen it. I know BBK makes the "varitune" but that's just a manual cut out valve and doesn't address drone in any direct fashion.
I don't have a problem with the exhaust volume just the resonant drone that happens exactly at highway cruise RPM & I have no interest in changing the axle ratio and no interest in swapping out the BBK midpipe or spending thousands of dollars on heroic efforts over this.

What might make me somewhat more sensitive to drone is that this isn't a drag strip or "ice cream" car for me, I bought this car to tour. It's a 9 hour round trip to the nearest actual big city (over 500k population) from where I am so really any kind of drone anywhere between 60 and 80 mph is unacceptable.

Anyway I'm not asking a question here for anyone to answer, every car is going to get "excited" slightly different and it's safe to say the recipe under my hood is one of a kind so what seems to work for other people's drone doesn't for me - plenty of people on forums (for various platforms) have asserted that turbo style mufflers don't drone. Well they drone on my car.

More to come later - maybe in a few months, whatever. Hopefully this discussion also helps somebody else in the future.

 

[dsrtjeeper]

It's hard to say if any of these will address your resonant frequency but...

https://www.goapr.com/products/exha...ponents/mufflers_and_resonators/parts/MUH0015

https://jackspaniaracing.shop/produ...z-style-ultra-quiet-quarter-wavelength-304-ss

Interesting app:
https://wilhelmraceworks.com/blog/drone-reduction

Calculator
https://strikeengine.com/helmholtz-resonator-calculator/

 

[badass98svt]

I have those on my otherwise stock exhaust. There is a very moderate drone around 1900rpm. It isn't annoying at all, I would recommend them.

I get zero drone with them, but mine is on a turbo setup. That could be the difference.

 

[RAU03MACH]

You also forgot cam notes

 

[kas_lar]

Interesting thread, Shovel. I’m following this one closely because my experience has been somewhat similar.

My ’95 GT originally had Flowmasters and the drone was absolutely terrible. I eventually switched to a Pypes system with the M80-style glasspack mufflers, and that made a huge improvement. The car was still plenty loud, but the really annoying highway drone was almost gone and I was very happy with it.

Unfortunately my car has been going through a rather unhappy period lately (missing compression, a badly done GT40 intake installation, vacuum leaks, etc. – some of you may have seen my other threads).

During all of that, both catalytic converters in my Davico catted midpipe were damaged and the catalyst material had to be removed. So at the moment I’m effectively running the same midpipe gutted, with no catalyst material in it.

And the interesting part is: the drone is now MUCH worse again, even though I’m still running the Pypes setup that previously worked very well for me.

So I’m becoming increasingly convinced that mufflers alone are only part of the equation. Removing the cats clearly changed something in the acoustic behavior of my exhaust system. Whether that is simply because the cats were acting as effective resonators/dampers, or because removing them changed the frequencies being excited further downstream, I don’t know.
Whatever the explanation, I’m very interested in where this thread goes because I have exactly the same goal as Shovel. I don’t necessarily want the car quieter when I’m accelerating. I just want to get rid of that horrible steady-state resonance at cruising RPM.
Mine is a convertible and I actually use it for relaxed cruising, so drone matters a lot more to me than another few horsepower. At the moment it’s bad enough that my wife barely wants to ride in the car because after a while the drone can genuinely give you a headache.
Once I have the engine running properly again, fixing the exhaust drone is definitely going to be on my list. I’d be very interested to see what combination of cats/resonators/mufflers or possibly a quarter-wave solution actually works on these cars without just replacing one set of mufflers after another.

 

[dsrtjeeper]

My son had Dynomax bullets dumped before the diff using long tube headers, 2.5" exhaust and X pipe. This was on an '86 GT which is nothing more than a big tin can. If I tried that, I guarantee that my exhaust would drone. There seems to be no rhyme or reason.

 

[shovel]

Thinking out loud here a minute while waiting for a meeting to start @ work there's a common element on my Mustang that I had not factored into the drone equation - the body and cab itself.

My car is a 94 coupe.

In the passenger compartment the carpet has not been out so it's stock carpet and underlayment.

Added about 2 square feet of 80mil butyl deadener to each door skin while I had those apart to replace window motor & speakers.

Replaced the stock front seats with LMR Fox Body reproductions, they are considerably lighter in weight which probably has some effect on acoustic damping.

Replaced the rear seat with a ScottRod aluminum rear seat delete. Under the rear seat I put 2 layers of full coverage 80mil butyl deadener and I also lined the underside of the seat bottom, and I lined the trunk side of the seat back with 1 layer of 80mil butyl. The seat back feels pretty "dead" when knocked with knuckles, but it also doesn't absorb any sound like padding and fabric would.

Non-"Mach 460" rear deck lid, so it has those two 4"x6" rectangular holes in the center of the deck and the perforation above it in the deck cover so trunk sound presumably can pass through with nothing to stop it. I have applied 80mil butyl to the top of the metal deck panel, but not covered any of the holes.

Speakers mounted to deck (so the back of the speakers are "open" to the trunk airspace, i.e. sound inside the trunk can directly act on the speaker cones)

Underside of trunk lid has some 80mil deadener installed judiciously

Spare tire removed. Entire exposed* trunk & spare tire well floor surface laid with 1 layer of 80mil butyl deadener. Low profile AGM battery mounted in front half of spare tire well. Back half of spare tire well carpeted (to carry tools, inflator, etc). 3/8" plywood floor cut to size and shape of trunk floor and laid over all that, with OEM trunk carpet on top of that.

*clarification that the exposed trunk and spare tire well have received deadener, but not the areas covered by the molded fuzzy trunk liner panels

Exhaust tips do extend past the bumper but just barely.

So here is what got me thinking about the drone -

I still got almost exactly the same drone (quieter but same flavor) even after switching to "turbo style" mufflers which really should be just about drone free. If nothing else they significantly change the apparent tuned length of the exhaust system and when I experimented by running one of each kind of muffler that should have disrupted any standing waves sitting in the area between the BBK x-chamber thing and the mufflers, at least moved the center frequency of the drone somewhere entirely different. But instead the drone is kinda always there about in the same place. So what if the resonating element here isn't the exhaust at all?

I think it's time to revisit the trunk a little bit. I haven't had the fuzzy side panels out of the trunk since I replaced my rear shock absorbers, is there a "well" behind the fender, below the tail light that could be picking up exhaust vibrations and blasting them into the trunk?

The trunk itself doesn't really have anything to absorb sound waves other than the thin carpet on the floor, the butyl deadener can keep the metal from ringing but it doesn't absorb any sound in the air volume. The fuzzy side panels are hard plastic with just the thinnest flocking so they aren't going to absorb any sound and since I have no big cushy back seat there's not really anything to absorb cabin resonance there either. Again, even if the metal has enough mass and butyl to avoid participating in the party it's also doing nothing to stop the air cavity from becoming resonant.

As a free experiment I can just completely stuff the trunk and back seat with every pillow, towel and blanket in my house then go for a cruise. If that works out favorably I guess it's time to work on some sound barriers and absorbers to isolate the trunk from the exhaust tips.

 

[dsrtjeeper]

There are wells behind the in the trunk behind the fender wells.
I have a friend with a SN that has done everything to make his 1200hp beast quiet. He is OCD about having a somewhat quiet beast. He added sound deadener and foil backed jute throughout the car. He had terrible drone and noted that the butyl sheets and jute did nothing for the drone. He ended up building his own J tubes for the exhaust and that cured it. I think he added another 500lbs to that car with all of the butyl sheets and steel tubing. Lol.

I've seen muffler blankets with a dimpled aluminum outer skin. Maybe that could change the frequency?

 

[GTamas]

Have you checked exhaust hangers? Those could be transferring vibrations to the chassis if they are not flexible enough, maybe got stiffer over the years?

 

[shovel]

Exhaust hangers are new(ish), were new at the time I installed the exhaust and they're still nice shiny fresh looking rubber. Exhaust isn't physically contacting the body, axle, control arms, etc.

I took out the trunk liners to put eyes on what's in there, completely forgot about these guys:



presumably those flaps are there to release pressure when slamming doors, slamming the trunk and if the airbags deploy. With 30+ years of wear on the weatherstripping around the doors and trunk maybe that's not absolutely as critical as it was new, but I still don't want to block those off entirely. I did put some dense fiber batting down into the "wells" behind the wheel arches, and draped loosely over those flap vents on both sides so presumably high volumes of air can still blast out but at least there's not a direct conduit of wide open airspace from there to the rear seat area - in fact there was an unbroken air channel from there into the cabin because I removed the rear seat belt assemblies entirely when I did the rear seat delete.. nothing at all but air from there into those holes.

The little slits that the seat belt passes through is kinda weird looking inside the cabin but I don't want to try doing any DIY stuffing or covering for it from the front.. maybe if I ever get a 3d printer I can print a pair of plugs that snap in.

Haven't gone for a drive yet since reassembling, because I have nowhere to go at the moment & gas is too expensive to just put on miles on a hunch that it might be quieter. Will report back if I learn anything later.


I've gone for a drive and noticed it's slightly quieter while driving at not-drone-resonant RPM but the character of the drone itself hasn't changed at all.