Killing Drone With 1/4-wave Tubes

Discussion in 'Fox 5.0 Mustang Tech' started by MFE92, Sep 26, 2012.


  1. MFE92

    MFE92 Active Member

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    Some of you may be familiar with this really long thread on the corral and others like it, which detail the science between adding capped resonator pipes branched off the main exhaust that are exactly 1/4 of the length of the soundwaves of the "drone zone". The idea being the sound wave enters the capped pipe and bounces back so that when it gets back out, it's exactly 1/2 a wave out of phase, so it meets the next pulse in the system head-on, totally cancelling it out. I can't do this on my mustang because the aftermarket suspension components leave me no room to add teh extra pipes. I've added Dynomax VT mufflers to it, which work, but I just bought a Pontiac G8 GT with a Magnaflow exhaust that droned like crazy at 1900 RPM, 125 Hz, but otherwise sounded great. I didn't want to change it, I just wanted to kill the drone.

    The G8 folks are hip to the phenomenon, and a company called SOLO makes a catback that incorporates adjustable-length capped J-shaped pipes that mount back by the mufflers. It's important to note that the phenomenon doesn't care where the pipes are mounted, or how they're shaped, just how long they are. It just so happens that there's a perfect spot for them on the G8's platform back by the mufflers near the bumper. These ones are adjustable kind of like a trombone, but they come pre-assembled at the perfect length. I had them installed yesterday, and the difference is incredible. And measurable ;)

    As seen by this screen shot from my handy-dandy sound meter app on my iphone, the big hump in the 125hz range is the drone:

    [​IMG]



    I don't have any sound files, but the meter tells the story: Same RPM, same stretch of road, same time of day. Look how much smaller that hump is, and look how many fewer decibels it's reading. That 10-decibel difference is huge. The way the decibel system works, every 10 decibels higher means twice as loud. That means the old setup was twice as loud as the new one, or put another way, the new setup is half as loud as before. :

    [​IMG]

    The drone has been virtually eliminated, and I'm a happy camper. This **** works, and it works on any car with enough clearance to add pipes of the appropriate length.
     
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  2. Noobz347

    Noobz347 Stangnet Facilities Maint Tech... Er... Janitor
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    You got a pic of these things? I don't mean installed on the car but a general product pic?
     
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  3. 7991LXnSHO

    7991LXnSHO Well-Known Member

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    My 05 G. Marquis has goofy looking factory growths on the intake tube. Thanks for explaining what they have to be for. Now if I could do the same to my Mustang's exhaust!

    I remember stories of flat heads getting exhaust cutouts with a removeable gas cap end plug for track use (actually made from gas tank filler necks). Combine both the drone tube and EZ open cutout on the end and you'd be set for street/track use. The new electric opening cut outs would make it even easier.
     
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  4. MFE92

    MFE92 Active Member

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    Here's what the J-pipes for the G8 look like. There's also tons of pics in that corral thread. [​IMG]
     
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  5. NIKwoaC

    NIKwoaC 中國製造

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    I wonder if there is any measurable effect on performance...?

    Interesting concept for sure.
     
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  6. David Pepiton

    David Pepiton Active Member

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    hmm i might need to try this on my wifes car the drone is killing me in that thing
     
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  7. MFE92

    MFE92 Active Member

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    Here's a chart showing the length of pipe for various drone RPM's, but this only applies to 8-cylinder engines. The math is different for other configurations.

    pipe
    length @
    1075
    ft/sec
    RPM HZ (8-cyl) 113 deg
    1500 100 32
    1600 107 30
    1700 113 28
    1800 120 27
    1900 127 25
    2000 133 24
    2100 140 23
    2200 147 22
    2300 153 21
    2400 160 20
    2500 167 19
    2600 173 19
    2700 180 18
    2800 187 17
    2900 193 17
    3000 200 16
     
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  8. David Pepiton

    David Pepiton Active Member

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    hmm do you have a link to figure out all this length stuff. its for a v6 car
     
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  9. 85_SS_302_Coupe

    85_SS_302_Coupe it sucks (I know) to be on the receiving end

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    Or just get Magnaflows and not have drone to start with :nice:

    Cool idea though. If this eliminated the drone from Flows i *might* consider thinking about maybe contemplating entertaining the idea of potentially someday running Flows again. Maybe.
     
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  10. David Pepiton

    David Pepiton Active Member

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    her car has 2 magnaflows on it and drones. ill see if i can find a pic
     
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  11. 85_SS_302_Coupe

    85_SS_302_Coupe it sucks (I know) to be on the receiving end

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    Strange. I must have a special set of Magnaflows because everyone tells me they sound different, and i have no drone at all at cruising speeds, just a mellow growl. It could also be the X pipe i guess.
     
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  12. Noobz347

    Noobz347 Stangnet Facilities Maint Tech... Er... Janitor
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    I'm not even running flows... I run what is supposed to be (and was before a major motor upgrade) a quiet exhaust... UNTIL... That magic portion of RPM where the wife has to put her fingers into her ears. o_O

    So this may be just what I need.
     
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  13. MFE92

    MFE92 Active Member

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    A lot of mustang guys are T-ing them off the tailpipes and running them laterally, above the axle/in front of the gas tanke. I can't, because that's where my panhard bar lives.
     
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  14. David Pepiton

    David Pepiton Active Member

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    well i cant find any pics of her exhaust but its on a 05 kia sportage using magnaflow 10426 mufflers

    i dont have any drone in my stang with magnaflows but she has a ton.
     
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  15. Bullitt347

    Bullitt347 man bewbs please...

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    You should hear me drone on and on when I have to work on a Kia:rolleyes:
     
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  16. Noobz347

    Noobz347 Stangnet Facilities Maint Tech... Er... Janitor
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    That requires a different cure.

    [​IMG]
     
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  17. NIKwoaC

    NIKwoaC 中國製造

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    Magnaflows on a Kia? Wait... Aftermarket exhaust of any kind on a Kia?
     
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  18. jrichker

    jrichker StangNet's favorite TOOL
    SN Certified Technician Founding Member

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    From a PM conversation Jerry Beach & I had on July 30, 2006. Sorry no pictures, but if you read carefully, you'll get the idea. Maybe he will show up with some pictures. jerry beach

     
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  19. Boosted92LX

    Boosted92LX Mustang Master

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    You've never heard of it?

    rice.jpg


    David Pepiton :stick:
     
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  20. gearheadboy

    Mod Dude

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    Working at a dealer for 17 yrs I got to talk to a lot of engineers and read a lot of tech bulletins. We hung exhaust weights on stuff that they would design and changed PCM calibrations all the time to bump the RPM's one way or the other up or down to get rid of drones, converter shudder, etc. Pretty neat the stuff you learn. We applied the same stuff to drive line vibrations, manual transmission gear rattle, etc.
     
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