383 intake manifold change AFR big time

Big Block A body Tech

  1. Zwap

    Zwap Well-Known Member

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    Well, to make a long story short...
    This winter I change from Mopar M1 dual plane intake mainifold to the manifold that all recommended for my mild 383 .030 over, decked block, Lunati 256/262 @050 213/220, Speed Pro flat top pistons @about 0.0 in the hole, 915 heads, 650 AVS2 and Mopar HP exhaust manifold with wideband AFR meetering about 40" from the closest exhaust valve.
    All recommendations where the Edelbrock RPM intake manifold.
    So far so good.
    With the M1 IM it ran pretty good and AFR was very satified, about 14@cruise and 12-13@WOT.
    With the RPM I need to rod and jet up quite a bit to get a decent AFR@cruise.
    Well, so far so good.
    But then I floor it (and all of the information below is when I floor it), a major bog and totaly lean out then reading 16-17 AFR@WOT
    I was running .098 as main jet with M1.
    As it was way to lean my first step was to jet up to .104, not much difference in AFR.
    I then tighten the AVS one full turn, not much difference.
    I then jet up to .110, still not much difference in AFR, first bog then about 14 AFR area but stil too lean.
    Is there really that much difference in flow between the RPM IM and a "standard" dual plane IM?
     
    Last edited: Mar 23, 2020
  2. rumblefish360

    rumblefish360 So close, yet so far away FABO Gold Member

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    “IM?” Intake Manifold? LOL. Never seen it abbreviated.

    Yea, there is a huge difference.
    Small carb, big-ish cam.
     
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    • dano

      dano Evil Handy Man

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      Have you checked and double checked for vacuum leaks? Have you tried changing the accelerator pump linkage?
       
    • Zwap

      Zwap Well-Known Member

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      Well, yes "IM" is intake manifold, too tired to write it but you get it right :)
      "dano" I suspect vacuum leak too, but could not find any, tried the old trick with starting gas but no sucess, maybe underneath the "IM" @the cam side...
       
    • PRH

      PRH Well-Known Member

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      I’ve never done that particular swap, but I really wouldn’t expect there to be much of a jet change requirement.

      On the dyno, my 383 didn’t require any jet changes going from the factory intake to an RPM.
      And multiple carbs were tested with both intakes.

      I’d be looking at a possible float height or fuel pressure problem.
      Try a new fuel filter, make sure the timing is correct.

      Maybe see if you can borrow a different carb for testing.

      Another thought, is it possible the added height is causing an issue with the air filter and/or air intake into the motor?
       
    • Zwap

      Zwap Well-Known Member

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      I run with Spectre Cold Air intake Spectre Performance Musclecar Cold Air Intakes 733 thats why I dont do the jettings when it's too cold and the height is no problem. Today it's about 35F here...

      The way I use to tune a carb is that I'm finding an cruising (eco) area that works, then finding an Pwr area that works, then finding a jet/rod combination that matches that areas. Then I go for WOT afr settings and it use to work.
      The reason I start this thread is that I was amazed that an intake manifold made that HUGE difference in afr and I'm not yet sure that it isn't a hidden vacuum leak that I cant find, the jetting is like a top performer 440 but as you can see it's a fairly mild 383...
       
    • AndyF

      AndyF Well-Known Member

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      I bet you have a different problem than what you think. I don't know what it is but I'd carefully look at everything you touched or changed during the swap.
       
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