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Thread: Engine hits "ghost" rev limit at 6350 RPM. *SOLVED*

  1. #1

    Default Engine hits "ghost" rev limit at 6350 RPM. *SOLVED*

    Setup / Background:
    MSD 6AL.
    MSD 2-Step Rev limiter (3500 RPM chip and 7600 RPM chip).
    Powerglide with Transbrake.
    Biondo Elite RPM switch for automatic air shifter.

    Typical Run/Symptom:
    Push Transbrake down, activates Low Side Rev limiter to 3500 RPM, release transbrake and car climbs to 6600 RPM clean and shifts in high gear the car climbs rpm, but as soon as it hits 6350, the car goes on some sort of rev limiter that limits rpm to 6000, then after a brief 0.5 seconds or so, it is gone and the car goes to 7000 RPM no problem.

    Things I have tried:
    Disconnected Biondo RPM Activated Switch from Tach port on MSD box and removed power fuse - No Change - Eliminates the RPM Activated Switch
    Changed the RPM chips on both sides of 2-Step (3200 RPM and 8000 RPM) - No Change - Eliminates bad chip theory
    Removed the 2-step and installed high side chip directly into MSD 6AL box and foot-braked car with no transbrake - No stumble - Possibly isolates issue to 2-Step
    Replaced 2-step with another 2-Step (older one) - Stumble is back and actually was worse - So either both boxes are bad, or there is RF interference in the wiring to the 2-Step.

    I manage to take a video of the tach on the older 2-Step to show you what happens.
    You will see the car at idle, getting on the rev limiter at 3500, then after the release of transbrake a clean climb in RPM to 6600 and then the shift.
    After the shift you will see the climb in RPM in high gear and at 6350 RPM, the RPM drops to 6000 RPM and stays there. I got off and on the gas twice which is the drops you see to 5200 or so and then you will see the rpm go back to normal and climb. If I didn't lift as I did in previous passes the RPM goes to the 6000 RPM and sits there as you can hear in a rev limiter state and then eventually about .5 second later goes back to normal.

    Let me know if you have heard of this before. Also, is it possible to get the interface schematic for the RPM Rev limiter port and the tach port? I may try to add my own filtering to protect more against noise than MSD does now. Thanks.

  2. #2
    msdtech1955 Guest

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    Have we checked any of the secondary ignition? Check the spark plug wires for resistance? Inspect the cap & rotor?
    routing of magnetic pickup leads?

  3. #3

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    I inspected cap and rotor and everything looks good. I'll check spark plug wire resistance. Routing of magnetic pickup leads is isolated from everything and it is the shielded pickup with ground.

    I did test the system with the MSD ignition tester and tried to simulate the run by the knobs and couldn't replicate the problem which has me perplexed.

    Also remember that it didn't have this system when I unplugged the 2-Step and plugged in a single chip in the MSD 6al directly. So that should eliminate all secondary ignition.

    One question I have is does the 12V wire in the 2-Step have a pull down resistor to ground internal to the box? If it doesn't I can see how noise might cause an oscillation on the floating 12V line of the 2-Step causing a dc offset equivalent to what 6000 RPM would see.

  4. #4

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    Do you use a crank trigger? I had something similar with my crank trigger and 6530 Box:

  5. #5
    msdtech1955 Guest

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    When you did the tester, the ignition was not under load like a normal run. So that leads me to believe it's something secondary ignition.

  6. #6

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    I have a similar problem with my car. I have been running the msd 6421 for a few seasons now with great results. Last weekend in Topeka it started acting up.
    Before the shift light came on (set at 7000) the car acted like it was on the rev limiter. This was only at 6500rpm. I shifted..power was back on til it reached 6500rpm
    again. The car started missing bad. The tac bounced down a few thousand rpm just like in these videos in previous posts. So did you guys fix your cars and if so what
    ended up being the problem? After a lot of testing I went ahead and ordered a new box.

  7. #7

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    Ok. Here is the latest status

    Inspected Cap / Rotor. Looks good
    Replaced Spark Plug Wires
    Replaced Spark Plugs
    Rewired all Grounds in Car
    Rewired power to MSD Box
    Removed Shift light and associated wiring (since I have the automatic shifter
    It improved but was still very noticable.

    After next pass we:
    Re-jetted carburetor (fattened) (no change)

    Next pass we:
    Increased timing from 36-38 degrees (small if any change)

    Next pass we:
    Replaced Coil (with brand new spare) (no change)

    Current Ignition
    MSD 6AL
    Blaster SS coil

    The specs for the box and coil:
    115mJ energy
    70:1 turns ratio with 480V primary voltage puts the coil output voltage of 33,600V and due to secondary resistance in the coil of 4400ohms allows for a maximum current of .300A

    After research and this testing I've come to this conclusion.
    This issue came up after installing the powerglide trans, and the car worked flawlessly with the TH400. The car weighs ~3700lbs and the engine is a 509BBC with 13.7:1 compression. Quite a heavy load for a powerglide.
    From the video in my first post, you can see that the flash on the stall is at 6100rpm, after the shift the car climbs and from what I can tell the rpm where this mis-fire occurs in high gear is at maximum load on the engine (converter locked & 1:1 ratio). Based on everything that has been done, I believe now that the coil I am using is not good enough for my application (its a street/strip coil) and while the TH400 3-speed worked well the change/increase in engine load and the change to the powerglide has allowed the coil to show its weakness.

    So where do I go from here:
    Plan is to change out Ignition Box and coil

    Proposed Ignition
    MSD 7AL-3
    Pro Power HVC or HVC 2 coil

    160mJ energy
    HVC2 Coil - 70:1 turns ratio with 570V primary voltage puts the coil output voltage of 39,900V and due to secondary resistance in the coil of 30ohms allows for a maximum current of 2.0A
    HVC Coil - 85:1 turns ratio with 570V primary voltage puts the coil output voltage of 48,450V (45,000 max on coil) and due to secondary resistance in the coil of 86ohms allows for a maximum current of 1.1A

    So my question. Which coil would you recommend? I was leaning towards HVC2.
    In addition, I am wondering if we should change to a better spark plug wire as well. Currently we are running the 8mm Spiro-Pro from Taylor which are at 350ohms/foot. The MSD wires won't fit my car so I can't use MSD wires, but I found Taylor's Thundervolt wires that have the same spark plug ends to fit my tight header clearance and they are at 40ohms/foot the same as the MSD super conductors. Would I benefit from this as well?


    On a side note, my engineering background had me interested in how MSD gained spark pulse energy in the 7AL-3 over the 6AL and found out that all that is done is increase the primary voltage out of the MSD box. Based on the energy equation E=.5CV2, where E in energy, C is capacitance and V is Voltage. By squaring the 480V from the 6AL, you come up with 230,400. If you square the 570V from the 7AL, you come up with 324,900. The ratio 324,900/230,400 = 1.41 which is the energy ratio. To verify this, I took the 115mJ of energy and multiplied it by 1.41 and came up with 162mJ which is what the 7AL-3 box is spec'd at. Which means the internal capacitance in the box didn't change. Useless information for the problem, but was fun to calculate :thumbup1:

  8. #8

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    I never found out the real issue of this malfunction. MSDTech said it will be a rotor phasing problem, but when I turn the distributor back and forth the "ghost" point doesn't change.
    I replace the locked distributor with a non locked one, used the signal from the distributor instead crank trigger and the problem was gone. I tried a new msd box, crank trigger, coil, noise filter, coil ... all bought for nothing ... the problem is only when I run with the crank trigger...

    Do you use a crank trigger?

  9. #9

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    I do run a crank trigger. The engine ran fine on the dyno to 7500 RPM.
    Not much is different from the dyno to my car. So I started there.
    They ignition on the dyno is an analog 7AL-2.
    I have tried two different 6AL-2 boxes. They both start to miss at the same rpm 6700.
    Does not matter if it is in low or high gear (Powerglide).
    Does not matter where the max limit rpm is set at still misses at 6700 RPM.
    I rewired and re routed the pick up wires and coil wires.
    I rechecked the ground and Ohms tested several places on the engine, body, chassis.
    I do have a one inch spacer under the carb (not dynoed that way) but doubt that makes a difference.
    Timing remains the same as on the dyno 36° distributor locked out.

    So then I went ahead and changed the cap & rotor.
    Shut the water pump off.
    Shut the fan off.
    took the alternator off, no difference.
    Tomorrow I'll try new plug wires even though they're all in spec you never know.

  10. #10

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    Okay so I found something out. I removed the pick up lead from the crank trigger and plugged it into the distributor. IT RUNS, NO MISS! I was thinking about the reverse polarity issues possible with crank triggers. I have an digital box and the dyno ran a analog. I understand where the polarity can cause a timing difference but not a misfire. I'll try a new crank trigger sensor and see what happens.

    New Crank Trigger Sensor fixed it. Thank you Robs455!

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