[meatybtz]

06 GTO, no power mods, except JBA LT headers and the 3" cat-backs. Other stuff is the car has the peders suspension all round, monster clutch and a one piece drive shaft.
I have the Magnaflow 16734s, the stainless 3" cat-backs. I love the sound and they are actually quite quiet at certain RPMs, but the DRONE is physically painful accelerating, under load, about 1500-1800rpms (sometimes at 2500rpm too, but not quite as bad).

Idle, interior sound levels are 57db, which is fine. All except drone RPMs are 69-71db. The DRONE? 78-79db. I've added sound deadening and sound absorbing materials in the trunk (two different kind of materials). That quieted it down. Previously it was 81-83db which is just below hearing damage over extended periods. Using my sound meter I was able to say, yes, the resonance is coming from the mufflers themselves. Sticking your head in the trunk will rattle your brain.

Searches came up with just general commentaries that it was a "quiet" exhaust. It is, except when it drones.

Figured I would pick your brains. Even though it's the mufflers producing the sound and it's pretty much all coming up through the trunk, was still going to attempt a "mute" using wrap to check if we were getting some kind of tubular bell resonance which a mute would alter the harmonics. But it's just grasping at straws really.

I actually think my meter is on the conservative side because the Drone activates my hearing protection.

 

[Rich-Tripower]

Are you catless? Adding cats and/or resonators up front toward the header collectors would help.

 

[meatybtz]

I have cats. Been trying to read up on the actual nature of the drone. I suppose I don't quite have a grasp on it's mechanisms. Some articles consider it an entirely internal resonance that then sets up certain resonances in the muffler itself (pulse pressure waves) and the others indicate resonance back along the exhaust system (which would be tubular bell). It's probably both and a resonator will help with both forms. I can still quickly mute the tubular bell resonance with some strategically places wrap that will cause the pipes themselves to cease to resonate until the resonators come in and I weld them up.

Only real question is.. before X pipe or after X pipe for the resonators. Got a lot of straight pipe post X, pre-X is a little crowded.

 

[LS2-GTO]

Those are the tiny magnaflow mufflers. Not surprised they are loud af.
I switch back and forth between really loud spintechs when I want to be obnoxious and the larger magnaflow mufflers when I can’t stand the noise anymore.

 

[SkiPopeye36]

Yeah, no shock here. What you have is the competition catback with the much smaller and louder mufflers.

Your search was probably yielding reviews of the standard 2.5” catback, which is significantly quieter than the 3” competition catback.

 

[WickedGoat]

06 GTO, no power mods, except JBA LT headers and the 3" cat-backs. Other stuff is the car has the peders suspension all round, monster clutch and a one piece drive shaft.
I have the Magnaflow 16734s, the stainless 3" cat-backs. I love the sound and they are actually quite quiet at certain RPMs, but the DRONE is physically painful accelerating, under load, about 1500-1800rpms (sometimes at 2500rpm too, but not quite as bad).

Idle, interior sound levels are 57db, which is fine. All except drone RPMs are 69-71db. The DRONE? 78-79db. I've added sound deadening and sound absorbing materials in the trunk (two different kind of materials). That quieted it down. Previously it was 81-83db which is just below hearing damage over extended periods. Using my sound meter I was able to say, yes, the resonance is coming from the mufflers themselves. Sticking your head in the trunk will rattle your brain.

Searches came up with just general commentaries that it was a "quiet" exhaust. It is, except when it drones.

Figured I would pick your brains. Even though it's the mufflers producing the sound and it's pretty much all coming up through the trunk, was still going to attempt a "mute" using wrap to check if we were getting some kind of tubular bell resonance which a mute would alter the harmonics. But it's just grasping at straws really.

I actually think my meter is on the conservative side because the Drone activates my hearing protection.

I have a JBA catback that I was going to use on my 2004 GTO that split the system like the 05-06 GTOs. I decided to go with the 05-06 Corsa Sport system instead. I will sell you the left and right mufflers and piping fairly cheap if you are interested in trying a different setup to lessen the drone. I have pics of the JBA catback and will post them if you are interested. When I say cheap, I mean cheap. It is brand new, never fired up and sitting packaged up in my garage.

Drone is tough to eliminate. I had a Corsa Touring on my 04 GTO and it didn’t drone at all, even with long tubes and cats. This is why I decided to pony up for the Corsa Sport. Yep, expensive as hell but worth it for no headaches. Countless thread reading and video watching leads me to believe the JBA catback will have a little drone, depending on H or X pipe setup and cats and resonator usage, but not the headache inducing resonance of the louder systems.

 

[meatybtz]

Yeah, no shock here. What you have is the competition catback with the much smaller and louder mufflers.

Your search was probably yielding reviews of the standard 2.5” catback, which is significantly quieter than the 3” competition catback.

Would a set of resonators after the X cool them down a bit? Or would I be better trying to cram them between the cat and the X. I don't need it "quiet". It's actually quite "quiet" for competition level mufflers. About 74 db at idle right next to the exits. At steady throttle above the drone level the interior is below 66 db.

Only complaint is the damned drone. At 78+db, the standing wave pressure of the drone is painful. Rolling down the windows doesn't even help as that pressure wave is just.. It's like standing inside a ringing bell. Oof.

Figure I should get this all sorted before I start cutting and welding the nice stainless.

 

[Rich-Tripower]

I'm no expert, but I'd think that as long as they are ahead of the mufflers they would help. You could also swap out the tiny mufflers for larger oval Magnaflow mufflers in the same location. The larger (longer, wider) the better.

 

[Nothubertjfarnsworth]

the answer is swapping mufflers, not redesigning your whole exhaust.

 

[IBM06GTO]

You can’t have 3” completely free flowing exhaust and be surprised you have drone.
Only 3” I’ve seen with real mufflers I know of is stainless works. I have it. It’s loud but it doesn’t drone.

 

[Squag27]

I have the same exhaust with Kooks LTs and cats and was surprised when I went to this cat back just how bad the drone was. Previously I had a pair of different Magnaflow mufflers with some resonators in a 3" cat back made by yours truly and that wasn't anywhere near as bad. Swapping in different mufflers are probably the way to go honestly. The biggest issue though is the fact that the location of these mufflers under the trunk on the GTOs is known to be an issue with drone.

 

[meatybtz]

I have the same exhaust with Kooks LTs and cats and was surprised when I went to this cat back just how bad the drone was. Previously I had a pair of different Magnaflow mufflers with some resonators in a 3" cat back made by yours truly and that wasn't anywhere near as bad. Swapping in different mufflers are probably the way to go honestly. The biggest issue though is the fact that the location of these mufflers under the trunk on the GTOs is known to be an issue with drone.

I noted a lot of stuff that just makes the muffler location bad. There is the plastic pan under the trunk as well as the fact that the trunk had zero sound control and the back seats didn't either. All great sound chambers for low frequency drone.

 

[Rich-Tripower]

Well, the stock mufflers are in the same location. The standard Magnaflow catback mufflers are in the same location. Point being, neither of those drone at all. So it would seem that the location itself is not the problem, the problem is the mufflers/system used that create the resonance in the first place. I can't imagine that the same mufflers placed in the 04 location would drone any less. When I had an SLP LM1 system on the 04 without the factory mids, and therefore no resonators in the mids, the sound was stupid loud in the cabin and the drone was horrible through a very wide RPM range. That system puts the "mufflers' in the 04 location right below the back seats. The drone was oppressive.

 

[SkiPopeye36]

Location and construction of the mufflers makes a difference, and as such, there is no one right answer.

Spintech mufflers, for example, sound completely different from 04 to 05-06 location. The drone is unbearable in the 05-06 location, but move them to the 04 position and 90% of the drone is eliminated.

 

[meatybtz]

Mind you, this isn't my first "loud" vehicle. I used to have an old 70s Ford Truck. That sucker was LOUD AF, to the point that on the highway you had to wear ear-pro, it would actually deafen you temporarily. Drone is just a complex series of harmonics and cavities.

What really still blows my mind is how whisper quiet it is at low throttle or cruise. It's only under load that it get's mega drone. It's kind of why I want to find a solution that doesn't need muffler swapping. I like the tone, sound levels, and how it sounds outside the car, and under WOT.. it's beastly fun.

My work on sound control in the interior and the back seating area have brought it to only problematic. I took care of the air-gaps between the cabin and the trunk, sealed it off, added sound absorbing 1/2" batting behind the seats and under them. Put on rubber harmonic matting on the bare steel.

I've got one more trick to try before I swap to a different muffler set. Which, as suggested above, is probably the "cheaper" of the options when one considers the need for the resonators and the cost of a pair of good ones.

Though, it was a bit of an eye opener how poorly sound controlled our cars are. I kind of expected better. Our stock sound control in the car is about on par with something from the 70s. I will also share that studying the problem showed me that the two real drone problem areas for the cabin are the speaker deck and the back-seat area. All the drone comes through those to spots with the rear deck being the BIG elephant in the room for drone. There is some 1/2" batting there stock, kind of cheaper version of what I am using. But the plastic base material is what is transmitting the sound. Like a big ole 16" subwoofer. The frequency it vibrates at is the drone frequency. It's basically a giant plastic speaker... because it's "loose fit" and the stock batting does little good. It has to be "tight fit" so it can't vibrate and some rubber damp on the underside and some extra batting so that it fits damn tight took a large chunk out of my drone RPM range. Now it is limited to a more narrow band. Still loud, still drone, but now the RPM band of it is about half as wide.Don't believe me, climb in the back seat and whack the deck cover with a bare hand right in the middle. It will hit your ears with the exact drone frequency and pressure.

I appreciate all the help so far as well. It was probably a mistake to get the 3" but I recently had trouble going back and finding the 2.5" exhaust system. Put a lot of work into the car recently. All the work is done by me, as usual. Pulling trans, doing rear-main seal, new Monster clutch, which I love, the new driveshaft, the headers, new steering rack, whole suspension work over.

It's been a fun bunch of car work after not building cars like I did when younger.

 

[WickedGoat]

You aren’t wrong on the lack of sound deadening material but I think the GTO was quieter than most when new. Full exhaust systems change that quickly. Corsa has been one system that advertises no drone and I can say first hand that that is an accurate statement. The 2004 system goes full 3” and places the muffler in the 05-06 location and there was no drone, even with long tube headers with cats and no resonators. I switched to the 05-06 system because I wanted the split system rear bumper exit. I even went to the Sport from Touring and still, there is no drone. The only other change was the 04 system used a Y pipe and the 05-06 system uses a X pipe. I have no added sound deadening in my car but I plan to add it when I start the stereo system install in a few weeks.

I’m not here to advocate for Corsa. Their systems are expensive and I wanted to avoid the expense but went there anyway. Should have just bought it first and sucked it up.

 

[Squag27]

For funsies I emailed Magnaflow to see if they had a recommendation on a different muffler from their catalog for the 3" exhaust and they got back to me with this:

"After reviewing the kit you have installed it appears you already have the 5" round body mufflers and there are only two sizes wider the 6" round (#12619) and the 7" round (#14719). There isn't enough clearance to add mufflers at the front of the kit, so the only point of modification would be at the rear with those mufflers."

Frankly I'd think you could probably add some resonators in the straight section despite what they say but that's what they had to say about it.

 

[Sacrifice]

Had no drone on the kooks full exhaust with catted mids

 

[meatybtz]

You aren’t wrong on the lack of sound deadening material but I think the GTO was quieter than most when new. Full exhaust systems change that quickly. Corsa has been one system that advertises no drone and I can say first hand that that is an accurate statement. The 2004 system goes full 3” and places the muffler in the 05-06 location and there was no drone, even with long tube headers with cats and no resonators. I switched to the 05-06 system because I wanted the split system rear bumper exit. I even went to the Sport from Touring and still, there is no drone. The only other change was the 04 system used a Y pipe and the 05-06 system uses a X pipe. I have no added sound deadening in my car but I plan to add it when I start the stereo system install in a few weeks.

I’m not here to advocate for Corsa. Their systems are expensive and I wanted to avoid the expense but went there anyway. Should have just bought it first and sucked it up.

It wasn't really expense. I mean the Magnaflow cat backs were not exactly "cheap". Only about 400 or so less than the Corsa. I don't mind spending. I actually was using samples of audio to help me chose and I liked the "cleaner" wave-form from the Magnas .. it was more on the lower end of the spectrogram.. which.. really translates as.. DRONE (yeah, hindsight is 20/20). The Corsa Sport does sound nice. But I don't think they sell that muffler separately as they only seem to list oval mufflers in the no-drone, unless they are using resonators in the 14189 system instead of the mufflers. Would say touring mufflers or the round resonators they are selling?
CP300SS2 Isn't cheap, at about 650 for the pair. But that's better than going back to stock and allows me to keep the 3" setup.

For funsies I emailed Magnaflow to see if they had a recommendation on a different muffler from their catalog for the 3" exhaust and they got back to me with this:

"After reviewing the kit you have installed it appears you already have the 5" round body mufflers and there are only two sizes wider the 6" round (#12619) and the 7" round (#14719). There isn't enough clearance to add mufflers at the front of the kit, so the only point of modification would be at the rear with those mufflers."

Frankly I'd think you could probably add some resonators in the straight section despite what they say but that's what they had to say about it.

It wouldn't kill me to change mufflers. I would just pull the rear pipes and bandsaw off the mufflers and chock it up to sunk costs. I would then weld on the new mufflers. Welding stainless isn't hard and a little solar flux goes a long way to helping that. Would be easier, I think, than making more cuts and welds up front for resonators. Ideally thought I would try to ensure.. no repeat performance of land of the Drone People.

 

[whippersnapper02]

Magnaflow 2.5" system here and it drones like crazy at the times you mentioned. Nothing else done to the car.