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Exhaust problem on 72 with 350.

Discussion in '1969-1972 K5 Blazer | Truck | Suburban' started by PGL4VT, Nov 29, 2002.

  1. PGL4VT

    PGL4VT Registered Member

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    I added headers and 2.5" duals turned out in front of the rear wheels with super turbo mufflers. Not too loud. Not too quiet. All seems fine except when I get up to higher RPM's. I have no tach so I'm guessing pushing 4000 RPM when I get this bad popping sound like a rattling (bad) in the exhaust. Is this a classic case of needing a cross over pipe or H pipe? Ideas? Suggestions? Thanks.
     
  2. ntaj*ep

    ntaj*ep 1/2 ton status

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    What do you mean by "rattling"? Like a popping noise? Or a metallic type noise? There could be a few diffrent things going on with it. As you are running half an exhaust system and I'm ignorant as to what cam your running (i will just assume its one step over stock) A "H-pipe" would do you no benifit.
    At the RPM you describe I will assume the noise is a metallic type noise and not a popping type noise, and that you didn't have it before this system was installed. If all that is true then I will guess that under load the engine is torquing to the point of tweaking the exhaust just enough to contact the body/frame or something and makeing the noise. But that is all just shear speculation. /forums/images/graemlins/thumb.gif
     
  3. PGL4VT

    PGL4VT Registered Member

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    The engine is pretty stock overall except manifold and a Holley 4 b.b. carb.
    The sound is not metal to metal. I've checked and there's no rub or hitting any where. I'm pretty sure it's coming out of the exhaust.
    Not sure what you mean by "half" and exhaust system. Do you mean by dumping it in front of the rear tires?
     
  4. Stroked72Blazer

    Stroked72Blazer 1/2 ton status

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    H-Pipe will improve any exhaust system except an x pipe. The popping sound as though it might be bad plug wires or plugs. I asume the sound you are describing is almost a backfire. Is it present mustly on accell, deceleration, or just an idle/cruise.
     
  5. ntaj*ep

    ntaj*ep 1/2 ton status

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    OK then with what you described a few things could be happening. First check that you have no leaks in the new system. What I think the problem is however is at the higher RPMs your Holley is leaning out the mixture. /forums/images/graemlins/thumb.gif With the exhaust cut in front of the axle, or in your case in front of the rear wheels the A/F mixture needs to be modified to compensate.
    The fix, re jet your carb.
     
  6. ntaj*ep

    ntaj*ep 1/2 ton status

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    Explane to me how that is possible? /forums/images/graemlins/rolleyes.gif
    I'm obvously ignorant to how the crossover in the exhaust would benifit a rock stock motor. /forums/images/graemlins/1zhelp.gif
     
  7. Stroked72Blazer

    Stroked72Blazer 1/2 ton status

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    A properly placed H-pipe will allow a scavanging effect. This is a very complex proscess so rather than spend the time to type it out I will refer you to http://www.hotrod.com search their archives for exhaust articles mainly ones refering to scavenging, H-pipe, and X-pipe. If you can not find it there then let me know and I will try to explain it the best I can. I only have a basic understanding of how it works so I will have to do my best.

    Another benifit of an H or X-pipe is it changes the harmonics which will aid in quiting you exhaust. I like quit because I never let the enemy know what I realy have so why broadcast.
     
  8. chulisohombre

    chulisohombre 1/2 ton status

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    from what ive heard a h pipe equalizes the pressures in the dual exhaust so that it has the same back pressure on both sides.
     
  9. PGL4VT

    PGL4VT Registered Member

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    Problem is there only on hard acceleration. Plugs and wires are good (Bosche Platinum and it's all new). It could be some sort of back fire in the exhaust. That's an idea.
     
  10. Steve_Chin

    Steve_Chin 1/2 ton status

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    THe purpose of any balance pipe or crossover is not to equalize pressure. It is to share the reversion pulses from one bank to the other and create an environment where the pulses reach the exhaust valves at the correct time for the RPM range of interest (i.e. enhances the Helmholz effect on the exhaust system). Cross-sectional area, angle of incidence, and downstream locaton are all very important to the harmonic tuning. The biggest problem in the case of most vehicles (and 4WDs in particular) is that the placement of the crossover is dictated by the gear train and is usually not ideal for a given vehicle.

    FWIW, the problem that was posted about in this thread indeed sounds like a carburetion problem or an ignition misfire. Remember that an off-the-shelf Holley is likely less well-tuned for your stock engine than the original Quadrajet was so you're probably wasting fuel, creating a lot of emissions, and giving up some power.
     
  11. '72brett

    '72brett 1/2 ton status

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    YES, I have had the same problems with rich-running carbs. Personally I would scrap the Holley amd go back to the original if you've got it. I have used holleys in a friends race car before, and they perfrom the same or worse than a slightly modified quadrajet. I use the original carb on mine and its working great.
     
  12. R72K5

    R72K5 Banned

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    i second that, rochester/carter/edelbrock are lots better especially in street use
     

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