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355 Exhaust Booming sound

Discussion in '348/355' started by j34, Jun 10, 2020.

  1. j34

    j34 Karting
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    Hi,

    I have fabspeed headers, fabspeed 200 cell sport cats and capristo stage 2 exhaust. We also have the capristo bypass valve. In low torque situations there’s a pretty annoying booming sound. It doesn’t happen at all in 1st or 2nd gear. 3rd gear it’s most noticeable around 4K rpm and later gears noticeable 2500-3000. In later gears it stays booming all the way to 4k or higher. 3rd gear probably only 1000 rpm of boom. We tried temporarily plugging vacuum lines and it made it 10x worse.

    Wondering if anyone has any ideas. Thx
     
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  3. yelcab

    yelcab F1 World Champ
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    what year 355?
    What model Capristo?
     
  4. j34

    j34 Karting
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    1998 355. I think it’s the 5.2
     
  5. yelcab

    yelcab F1 World Champ
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    Try putting back the stock bypass valve and see if that changes things.
     
  6. The Outcast

    The Outcast Formula 3
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    If the model year is 1998, you have a 5.2.

    Since you're running the Cappy bypass valve, did you reverse the vacuum connection, or was it reversed by the installer if that wasn't you? The Cappy is "normally open" without vacuum, so when you plugged your vacuum lines it got "worse" and was likely open 100% of the time. Sounds like it's correctly connected, but the sound you're getting doesn't sound right to you. Is this a new Cappy stage 2? If it isn't, and the "open bypass" sound isn't good, there may be a crack in one of the baffle welds within the muffler; I had this problem with an original Challenge muffler because one of the internal welds cracked. One way to check for this is to remove the muffler and tap on it with a rubber mallet, with enough force to cause the suspect weld joint to vibrate.

    While you're at it, maybe you can post a video of a drive-by or some situation where the bypass opens? That would allow the community to hear what you're hearing.

    Also, yelcab's suggestion to use the original bypass valve, if you have it, is a good one. You'll have to "de-reverse" the vacuum plumbing if it's already set up for the Cappy.
     
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  8. j34

    j34 Karting
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    Thanks. It’s a new exhaust and the stock bypass valve was broken. I’m pretty sure the setup is correct. I will try and make a video so you can hear it. It’s more audible from inside the car so I might try an interior and exterior video.

    We are thinking of adding a new resonator as an attempt to reduce the boom. Can consider buying stock bypass valve as well.

    I’ll try and get some videos ASAP and post.

    thanks guys! This is driving me nuts. Otherwise sounds really great.
     
  9. The Outcast

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    If you can shoot some video from inside the car while it's being driven; I will play it back over my hi-fi system so that I can hear the lower end. It may be the the Cappy installation is creating resonance. The Cappy also has resonating diaphragms built-in (at least that's the case with Stage 3), but these are high-frequency and aren't likely to be the source of your issue.

    Make sure you check ALL of your exhaust system joints; if one of the two sets is loose, you'll definitely get a resonant boom.

    Intake resonators will not help.
     
  10. j34

    j34 Karting
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    Please see videos here:


     
  11. Drock28

    Drock28 Formula 3

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    if what you are describing as a booming sound.. that's the bypass valve opening, changing the exhaust tone..which can be heard in first clip..
    which is completely normal.
     
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  13. j34

    j34 Karting
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    thanks. And it’s supposed to last quite a long time when you’re in higher gears? For example if you’re in 5th or 6th at 60 mph whenever you hit gas you get the boom from 2k to 4k rpm. You have to downshift to 4th to avoid. Does this really happen with stock cats and stock exhaust on 355’s?
     
  14. Drock28

    Drock28 Formula 3

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    hi.. yes that's correct.. the bypass valve opens based on throttle position and rpm's..
    so if you are cruising in 3rd or 4th.. and then you push it.. as the rpms rise to 3.5-4k the bypass valve opens and you hear it in the exhaust tone..
    and is stays open as long as you are maintaining that throttle..

    once you let go or down shift.. the valve closes..

    having your capristo exhaust obviously heightens the tone, but the mechanics is the exact same even with a complete stock exhaust setup..
     
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  15. j34

    j34 Karting
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    If that’s the case have others tried different exhaust system designs? Seems pretty rough compared to my 360 CS for example. Maybe the 200 cell cats and capristo exhaust heighten booming sound in addition to normal, great exhaust sounds. Seems like quicker vacuum process would solve the booming?
     
  16. bcar1

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    I have a similar noise in my setup- it is a result of putting too much load on the engine in too high a gear. My solutions are either downshift to a gear where rev range is still below bypass opening but more within the power band or accelerate slower. I think the secondary cat bypass amplified this.
     
  17. j34

    j34 Karting
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    Yep, exactly what I end up having to do. I’m really trying to figure out of this is caused by sport cats, back box, etc. I think the fact that it gets so much worse when vacuum lines plugged makes me feel like we need more vacuum when the valve goes from closed to open.
     
  18. The Outcast

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    I listened to your samples numerous times and the condition you’re referring to is a low-frequency resonance; it appears to be propagating BETWEEN the two exhaust paths due to the Bypass being only partially open, which really underscores your statemwnt regarding “not enough vacuum”...I think you may have nailed it with this insight. The system is not designed for “partial” bypass positions...it’s supposed to either open or closed, utilizing only one exhaust path at a time.

    in a nutshell, the Bypass valve being partially open is temporarily creating a nice big resonator, where long low-frequency sound waves can easily develop.

    Since your new bypass is “normally open”, you will need full vacuum for it to seal in the closed position, or the system will continue to produce this “dual path” resonance.

    My hunch is that your vacuum system is compromised. This is usually very cheap and easy to fix.

    Start at the rearmost vacuum connections to the manifold first. Remove the pair of check valves and check their operation, then continue back to the valve block (just under and inboard the RH tail lamps), and pay special attention to the accumulator...it’s function is vital to Bypass operation. Sounds as if your valving solenoids are working just fine. If you don’t find leakage after these steps, you’ll need to examine the vacuum links between each throttle body; they’re a pain to get to because they’re under the intake chambers, but they’re in a spot that makes them prime candidates for getting “crispy”.

    Hope this helps!
     
  19. j34

    j34 Karting
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    awesome. Very helpful. Will dig into this and let you know the outcome.
     
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