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,
  10. sporty-to-GN

    sporty-to-GN Well-Known Member

    Just thought I'd update the discovery made after tagging and putting some miles on the car. Amazingly, it still pulled hard and fried the 275s on demand in 1st or 2nd. The blow by got ridiculous so it was time to for tear down.
    Turns out the 1970 block had been decked, bored .040 with Speed-Pro hypereutectics, rod beams polished, TA cover, drilled oil galley with 3/8 crossover tapped into the rear galley ports. Cam is a TA 413.
    Not sure why #5 gave up but it was on the verge of catastrophic failure. I was able to get the top ring out, 2nd one is brazed in with aluminum transfer. Top ring may not be true but squaring it in the cylinder showed end gap of .016
  11. john.schaefer77

    john.schaefer77 Well-Known Member

    Oh. Well at least she held together! I ran a 413 cam and loved it.
    Last edited: Jul 22, 2018
  12. TrunkMonkey

    TrunkMonkey Well-Known Member

    Oh that?

    It's a mod to provide pressure for positive crankcase ventilation.

    Nobody wants stale blowby hanging around and gumming up the internal spinny choppy smashy things.

    Gotta get that crankcase to breath!

    Seriously. Glad you found that. I've got a borescope (now that they are so cheap) and poke it in the spark plug holes anytime I have them out, just because I can.

    I won't say I ever "borrowed" one while working swings back in the USAF days, after seeing what we found in the hot sections on jets 40 years ago...
    (We did it in the parking lot, we did not take it from the shop.)
    GranSportSedan likes this.
  13. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    Lucky it didn't shatter completely. A little detonation can do that to Hypers.
    john.schaefer77 likes this.
  14. sporty-to-GN

    sporty-to-GN Well-Known Member

    Yep, when trying to get the 2nd ring out this crack opened up to 1/32 and an inch and 1/2 of ring land nearly fell out. IMG_20180722_162655.jpg
  15. BQUICK

    BQUICK Well-Known Member

    Too much timing and/or not enough octane......

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