Droning exhaust fix ideas?

improbcat

Well-known member
I'm hoping you all can help me with this problem. I had a custom exhaust put in my '62 Comet (stock '71 Maverick 200 drivetrain). It is 2" stainless that comes down from the manifold in one pipe, splits into duals, passes through Smithys glass packs and goes to the rear (ignore the megaphone tips, those were and error and have been removed). It looks great, but drones unbelievably at highway speeds. Your whole head vibrates and you can't carry on a conversation.

Basically, does anyone have any suggestions oh how to fix this? I've had all sorts of suggestions from replacing the mufflers to adding resonators, to reversing one muffler to cutting it down to single exhaust, etc. I'm looking to narrow it down before I throw more money (I really don't have) at this.

Shots of the system:



 
I had a y into 2 hooker mufflers on my falcon and it had a good drone, knocking it back into a single got rid of it.
 
I had a couple different mufflers and all of them droned pretty bad. I finally went with a resonator after the muffler due in part to suggestions here and the old timer who was doing the exhaust work. It worked wonderfully. I recently drove from Detroit to Pittsburgh (300 miles) and no drone.
 
I did a few low budget experiments on a two litre cat-back system.

Being a high flow converter, it didn't really change the "note" of the engine, just damped it overall very slightly.

Straight out 2" gave MASSIVE drone. You got a headache and it was way loud.

Adding a midlength 2'×8" long resonator (straight through centre with perforated pipe) was bearable at lower speeds and sounded excellent (in the import sense of sounding good) when you got on the fast pedal. It would be fair to say the resonator here, cut drone a good amount.

I finished with a muffler near the end, keeping the resonator as it had negligible impact on flow at normal speeds (say, below 120MPH). Muffler was a 2' offset in/out with perforated pipes and fibreglass packing. Had a turndown put on the tip and it is fairly quiet.

Another car in the fleet has a smaller centre resonator and it does drone more - it also has a square cut tip. This (oddly) seems to affect the sound perceptibly.
 
I ordered a set of 17" resonators, which I'm going to try and install alongside the gas tank. I'll let you know if it helped once I get them in.
 
2ec599298673__1288612477000.jpeg

(sorry for the quality, it is a cell phone pic)

I did a trial run wit hte resonators, attached them to the ends of the exhaust like you see above and went for a drive. There was little to no difference is drone or sound level. Looks like I'm back to the drawing board.
 
I've read before that a crossover pipe will do it too. Can you fit that in? Also, make sure the exhaust isn't grounded somewhere, meaning, not hung by a rubber mount. Looks like a professional job so I doubt that's it.

In the OEM car world this problem is called "booming". It's a very common concern for new cars too. Sometimes it's fixed with a dynamic dampener aka a hanging weight. They're subtle and easily missed on new cars, but down there.

Search google for booming and see what you find.
 
I suspect that on your system the way you have it set up, the two halves start "singing in two part harmony" at certain rpm's. On duals, either V8 or six, the pulses come out at intervals and not at the same time. On your setup, both halves are pulsing at the same time and you hit a harmonic at certain rpms. Basically, you built a big, exhaust powered tuning fork!

To cure that, you can change either the diameter or the length of one side to shift it to a different "vocal range". Also, you can experiment with changing the backpressure on one half by baffling one side. Stick a motorcycle type baffle in one pipe and see what happens. I think you'll get a noticeable change in results.

Another remedy I've seen is to install a "resonator stand pipe" in the system. A capped off section of exhaust pipe is teed into the system and by adjusting its length you can tune the system to eliminate the drone. Works like a lot of musical instruments do. Never tried it, but I've seen it on upscale cars like Bimmers.

Or you could use a different length glasspack on one side.

The object, though, is to get the two halves of your system to "sing" differently. I think you made them a little too symmetrical.
 
My Pontiac had a bad drone with the Flowmaster 40's. When I rebuilt the engine I installed the Dynomax Ultra-flow mufflers. But I also instlled an overdrive transmission. My drone is way down, but I don't know how much if any is due to the muffler swap since the change in the rpm's is probably the majority of the reduction in drone.
Doug
 
Moving the mufflers and/or resonators as far forward in the system as possible helps to keep resonance down too.
 
xzerokidx said:
Moving the mufflers and/or resonators as far forward in the system as possible helps to keep resonance down too.

The muffler on my car is a "normal" one: "in one end out the other", like later Mustangs, and the resonator is in the tail pipe section next to the gas tank. It sounds normal now.
 
@ improb cat

had same problem with my new maverick. dual flowmasters arcing around the rear end. i told dad im cutting the pipe right past the muffler because the drivability was terrible. ever since then it runs alot smoother. never had that problem with my 4 door as i ran dual glasspacks out the side of the back doors, so exhaust had a smooth path. the arc around the rearend in my opinion catches too much of the exhaust vibrations.
just my 2 cents
 
mugsy":2km4d2og said:
xzerokidx":2km4d2og said:
Moving the mufflers and/or resonators as far forward in the system as possible helps to keep resonance down too.

The muffler on my car is a "normal" one: "in one end out the other", like later Mustangs, and the resonator is in the tail pipe section next to the gas tank. It sounds normal now.


I may be misunderstanding you, but what I was trying to explain would be to get the longest resonators/glass packs that you can fit (say starting right after the Y section or as close to it as possible, entering into mufflers underneath the car BEFORE the rear-end) than continue over the axle and out the back or through another set of glass packs than tips.

Hope that clears it up.
 
xzerokidx said:
mugsy said:
xzerokidx said:
Moving the mufflers and/or resonators as far forward in the system as possible helps to keep resonance down too.

The muffler on my car is a "normal" one: "in one end out the other", like later Mustangs, and the resonator is in the tail pipe section next to the gas tank. It sounds normal now.


I may be misunderstanding you, but what I was trying to explain would be to get the longest resonators/glass packs that you can fit (say starting right after the Y section or as close to it as possible, entering into mufflers underneath the car BEFORE the rear-end) than continue over the axle and out the back or through another set of glass packs than tips.

Hope that clears it up.

I'm just saying putting them at the back works too. I've near heard about putting them closer to the engine. I just figured anywhere in the exhaust system should work, but if putting them forward is better sound attenuation, then by all means do so.
 
MustangSix":159h5njk said:
I suspect that on your system the way you have it set up, the two halves start "singing in two part harmony" at certain rpm's. On duals, either V8 or six, the pulses come out at intervals and not at the same time. On your setup, both halves are pulsing at the same time and you hit a harmonic at certain rpms. Basically, you built a big, exhaust powered tuning fork!

To cure that, you can change either the diameter or the length of one side to shift it to a different "vocal range". Also, you can experiment with changing the backpressure on one half by baffling one side. Stick a motorcycle type baffle in one pipe and see what happens. I think you'll get a noticeable change in results.

Another remedy I've seen is to install a "resonator stand pipe" in the system. A capped off section of exhaust pipe is teed into the system and by adjusting its length you can tune the system to eliminate the drone. Works like a lot of musical instruments do. Never tried it, but I've seen it on upscale cars like Bimmers.

Or you could use a different length glasspack on one side.

The object, though, is to get the two halves of your system to "sing" differently. I think you made them a little too symmetrical.

This makes a lot of sense. I suggest moving one of the glass packs, so they're not side by side. A more expenisve suggestion is smaller pipes.

http://www.classicinlines.com/ExhaustKits.asp see under "Sizing up your exhaust system". OR a cross over pipe might help.

Good luck,
 
A lot too symmetrical. My 300 with 6 into 2 clifford headers and matching flomaster clones has about 6 or 8 inches difference in tailpipe length before exiting sideways and it only drones off idle. At speed it beats more like a bent motor. I'd like to try quieting one side just to lower things an octave, like some 3 cyl
Cat diesel. Gotta confess I've set off some car alarms with it as is. Oops!
 
Interesting problem. A crossover would be of no help at all since it's a single outlet manifold already. I believe that a resonator before the exhaust split would work well, but would unfortunately require shop work. Resonators should be as close to the front of the exhaust as possible, to dampen the sound before it causes the whole exhaust to sing.

Make sure that the hangers aren't tightened excessively, because it can transmit the drone more. Old-timers used to clamp lead weights on the exhaust to change the resonant frequency of the exhaust, to move the drone outside the cruising speed. I suspect that the use of stainless has exacerbated the resonance problem.
 
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