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Resonator or Muffler?

Discussion in '6.9L IH & 7.3L IDI Diesels' started by renjaminfrankln, Jan 19, 2021.

  1. renjaminfrankln

    renjaminfrankln Full Access Member

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    89’ F250 7.3 with turbo. Straight 3” exhaust, in good shape (new).

    The exhaust is not loud at all but I have an annoying resonance in the cab. I have dynomat on the floor and roof of the cab, and the truck is pretty quiet for an idi.

    between about 65-75mph I get a loud drone in the cab. This is right where I would like to cruise on the highway of course. It is definitely a resonance thing because if I move my head forward over the steering wheel, the noise disappears. Also if I get up close to 80mph it disappears.

    Anyone have experience with adding a resonator?
     
  2. miles1400

    miles1400 Full Access Member

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  3. renjaminfrankln

    renjaminfrankln Full Access Member

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    Behind the right rear tire
     
  4. miles1400

    miles1400 Full Access Member

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    @renjaminfrankln hmmm its odd that you have that resonance, i have a 4 inch straight pipe with a 6 inch tip exiting behind the rear tire and i dont seem to have a resonance but next time i am on the highway i will try and listen for it

    https://www.amazon.com/Exhaust-Muff...+inch+diesel+,aps,157&sr=8-19#customerReviews

    i think this is the muffler i got when i did my NA 4 inch exhaust. its either the same one or very similar but most importantly it is budget friendly if you want to do a muffler.
     
  5. franklin2

    franklin2 Full Access Member

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    I have the same drone. I thought it was the exhaust also, because I could get going 65-75mph and then suddenly push in on the clutch and let the engine idle down, and it would go away. But I just changed the muffler and it did not help, does the same thing.

    I am now thinking my front pass side cab mount is rusted (I know it's rusted) and has let the cab sit down directly on the frame mount. I think that is causing the noise, since the body mount no longer does it's job. That's the next theory I am going with anyway.
     
  6. renjaminfrankln

    renjaminfrankln Full Access Member

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    I was thinking the same thing today... I no longer think it is an exhaust resonance, and more of a resonance that gets transmitted to the cab. Seems to have more to do with RPM than throttle position. I plan to replace all of my body mounts soon anyway, I won't be messing with the exhaust until that is done.

    Another issue for me, I have a westin step bar on the side of my truck that connects directly (metal to metal) with the frame, and then in one spot connects directly to the cab right by the driver and passenger rear corners.. not through any rubber mounts that I can tell. That can't be helping.
     
  7. renjaminfrankln

    renjaminfrankln Full Access Member

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    Here’s what I’m talking about w the step bars.

    they bolt straight to the frame and then to the cab, looks like they are using the rear seatbelt bolt. No isolators

    0FD22E19-0286-447B-A5DE-86A73C9C9A24.jpeg
     
  8. renjaminfrankln

    renjaminfrankln Full Access Member

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    Well after lots of driving and thinking about it I determined it was an exhaust drone. I drive this truck on the interstate almost every day for about an hour (commuting). Its pretty wild how it resonates just right in the back of the cab, like I said if I lean my head forward over the steering wheel, the drone is gone.

    I researched a bunch of options, just googling "diesel exhaust drone" and got a lot of potential solutions, from large glasspack resonators, hemholtz (J pipe) resonators, to just cutting the exhaust in front of the axle and doing a side exit in front of the wheel or a turndown (removing a significant amount of exhaust length can change the resonance to where its above or below normal operating RPM range)

    I called up the local muffler shop the other day and described what was going on, they threw on a big 3" glasspack today. It definitely took the edge off of the drone and narrowed it significantly, but it's not gone entirely. It starts around 67mph now and tapers off entirely by 75mph, whereas prior to the muffler I had to set the cruise at 77-78mph to get above the drone (the IDI is not super happy at those speeds).

    So, still have some work to do but for now i'm good, got about a 500mi round trip coming up this weekend.
     
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  9. renjaminfrankln

    renjaminfrankln Full Access Member

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    4" should entirely change the volume and resonance compared to my 3" so that could explain it.
     
  10. IDIBRONCO

    IDIBRONCO IDIBRONCO

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    Wear ear plugs for the trip?:joker:
     
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  11. miles1400

    miles1400 Full Access Member

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    @renjaminfrankln i bought my pipe from summit. it was $21 for a 4 foot length of aluminized steel pipe.

    my exhaust setup is the banks downpipe to a 30 degree 3 inch bend to 3-4 adapter to the straight pipe to the tailpipe. i believe you would have a 30 degree 3 inch bend after your down pipe. i hung the 4 inch pipe level with that 3 inch pipe. my exhaust hangs a bit lower than it probably should. but it cost a 1/3 of what a hypermax kit would cost including shipping and it preforms better too
     
  12. renjaminfrankln

    renjaminfrankln Full Access Member

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    Eh it will be fine as long as im above 75! Seriously the truck is much, much quieter at 80mph than it is at 70.

    Not exactly sure what I am going to do with it yet. Maybe just swap the glasspack out for a turbo muffler. For now the plan is to just drive real fast
     
    Last edited: Feb 16, 2021
  13. The_Josh_Bear

    The_Josh_Bear Full Access Member

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    It's funny how things turn out, I have a 3" dp to 4" straight to (I think) 5" straight through muffler off a dodge, turned into the ground just before the axle. And after a bunch of snow play the other day the exhaust sound changed and got louder...

    Then yesterday on the way to work my exhaust snapped before the muffler and I was laying some sparks till I could pull over!

    The exhaust is louder without the muffler but the point is that I leaned over my steering wheel for giggles and sure enough there is a bad resonance there! In my seat it's fine and the wind noise on all my old seals is easily the loudest thing cruising. But lean over the steering wheel and it's like being too deep in the pool where your ears start to feel pressure.
     
  14. franklin2

    franklin2 Full Access Member

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    I would never exit the exhaust between the truck and the road. That makes it even louder. I always exit the exhaust beyond the truck, whether out the side or out the back.

    I too changed my exhaust muffler from stock to a glasspack, everything else being stock walker exhaust. It did change it a little, but it's still there.

    I figured out like a previous poster did, it's rpm related, my problem just came about because I took the 35 inch tires off and went with a smaller 31 inch tire. I got all my pulling power back, but the rpms are higher at cruise in overdrive, and that brought the drone into play.
     

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