Converter slip vs "blowing through" the converter

Discussion in 'The "Juice Box"' started by sporty-to-GN, Mar 12, 2018.

  1. sporty-to-GN

    sporty-to-GN Well-Known Member

    Took the 69 Skylark to the track yesterday. Affectionately referred to as the Rust Duck, it's an all steel, rust, and Bondo body, full interior, AC deleted, PS, with manual drums.
    455/TH400 with an unknown blue painted Tc, 8.5/3.55
    Edelbrock heads, SP1, QJ or Demon 850, unknown cam with 1.6 rollers at .507/.512 lift, duration @ .050 around 230/245.
    MSD Digital-6, billet no vac distributor, 35 total all in by 2500.
    275-60-15 EAT Streets @18psi
    Shifted at 5200.
    Ran 12.3s/106 with 1.555-1.666 60's with traction. 13.s/106 with no traction.
    It felt like it was max'd out at 105-6 / 5300 rpm range (my guess) 100+ feet from the trap. Just wouldn't rev past 52-5300.
    Can the converter inhibit RPM at the top end? Driving normally it feels way more loose than the 3200 L/U I had in the GN.

    Attached Files:

  2. TrunkMonkey

    TrunkMonkey Well-Known Member

    If you shift at 5200, and your hitting the trap at 5300, "Aye!, maybe that's all she kinne do, Captain Kirk!"

    If your engine can run safely above 5300, you could try a second gear pull to 5300+ and see if it pulls or flattens out.

    But there are quite a few variables that can give you the same results that you are experiencing.
  3. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    336/tire height X rear gear X MPH = RPM

    336/28 X 3.55 X 106 = 4516 RPM with no converter slip.

    You have roughly 800 RPM slip which is a lot, about 18% slip.

    What kind of Tachometer are you using? Accuracy? Is it flattening out or nosing over? What's your fuel system like?
    Last edited: Mar 12, 2018
  4. sporty-to-GN

    sporty-to-GN Well-Known Member

    Yeah, that's the figure I had.
    Flattens out. Stock tank and lines. 3/8 steel and rubber from the Stage 1 pump to carb.

    Bosch FST 7900 that was in the car when I got it. I'll compare it to a Snap-On timing light.
    I believe fuel is ok. First pass with the Holley was way lean, 13.2-5 on the AFR. Bumped jets up from 77/83 to 80/88 which brought AFR to 12.5/8. That tells me the supply is there.
  5. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    Were you running with or without an air cleaner? If without, did you remove the center hood insulation? It can suck down into the carburetor. Just suggesting that since it happened to me.
  6. sporty-to-GN

    sporty-to-GN Well-Known Member

    One of these.

    Attached Files:

  7. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    OK. At the track, I run without the air cleaner. Once, I forgot to remove the center hood insulation, and it got sucked over the top of the carburetor. Engine flattened out, which is why I asked.
  8. ap1672

    ap1672 Silver Level contributor

    What Trunkmonkey said, Will it pull strong past 5300 in second gear?
  9. sporty-to-GN

    sporty-to-GN Well-Known Member

    I'll need to check that sometime. No tags on it yet.
    Bosch tach is right on the money up to 4k,

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