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Thread: Ignition ECU operation, HEI

  1. #1
    Senior Member Bitsyncmaster's Avatar
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    Ignition ECU operation, HEI

    One of my customers sent me a photo of his GM HEI 4 pin module he plans to use in his DeLorean. That got me thinking so I took some measurements of the stock ignition voltage on the negative side of the ignition coil. Correct me if I say something wrong to my thinking.

    FYI:
    You drive current into the ignition coil to saturate the core (iron) magnetically. So more current or keeping the current on for longer time will only waste power and increase heating of the coil. The coil would take a fixed time of current to saturate it (lets say it takes 2.0 ms to saturate). So it would be best to have that timing at all engine RPM. So voltage on that negative coil side should be zero volts for only 2 ms and +12 volts for the rest of the time.

    When current flow is stopped, the magnetic field collapses and that creates the high voltage spark.

    So I measured the dwell on that negative coil signal. With the meter set on 4 cylinder dwell at 850 RPM I read 62 deg and at 2500 RPM I read 60 deg. Checking that signal on my oscilloscope, the power on time (zero volts) was 11 ms and 21 ms (you get two timings because of uneven fire PRV) at 850 RPM. At 2500 RPM I got 4 ms and 7 ms.

    So it looks like the stock ignition ECU has no dwell change to speak off. The GM HEI claims to have a dwell change which would keep the ignition coil running cooler and therefor not require dropping resistors if the correct coil is chosen. So I'm going to do some playing with the GM HEI 4 pin module. I know Bill uses the Ford Duraspark unit with success but the Ford unit holds the dwell constant.
    Last edited by Bitsyncmaster; 05-12-2016 at 08:19 AM.
    Dave M vin 03572
    http://dm-eng.weebly.com/

  2. #2
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    That's an Internet rumor that is untrue. I accepted it myself until I hooked a dwell meter up to my car. Duraspark dwell is unique in that it *DECREASES* as RPM's go up. Near as I can figure it's part & parcel with large diameter Duraspark distributor caps and wide rotor blades -- increases the length of spark duration.

    I measured a stock Bosch module at 54 degrees while cranking, 44 degrees at idle, and 46 degrees at 3,000 RPM.

    Duraspark is 56 degrees while cranking, 50 degrees at idle, 44 degrees at 3,000 RPM.

    6 cylinder Duraspark cap and rotor versus DeLorean (that Duraspark needs a larger cap to avoid cross firing is another Internet rumor that's untrue -- cap is bigger because the rotor blade is wider):

    DurasparkCap.jpg DurasparkRotor.jpg

    Note the absence of a resistor in the Duraspark rotor button -- to the best of my knowledge no American car has ever had a resistor in its rotor.

    Bill Robertson
    #5939
    Last edited by content22207_2; 05-12-2016 at 08:33 AM.

  3. #3
    Senior Member Bitsyncmaster's Avatar
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    Quote Originally Posted by content22207_2 View Post
    That's an Internet rumor that is untrue. I accepted it myself until I hooked a dwell meter up to my car. Duraspark dwell is unique in that it *DECREASES* as RPM's go up. Near as I can figure it's part & parcel with large diameter Duraspark distributor caps and wide rotor blades -- increases the length of spark duration.
    #5939
    The dwell reading is the primary current nothing to do with the high voltage currents.
    Dave M vin 03572
    http://dm-eng.weebly.com/

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    The only dwell that doesn't change is breaker points. That's one advantage of electronic ignition: the ability to change dwell as RPM's change.

    Bill Robertson
    #5939

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    Quote Originally Posted by Bitsyncmaster View Post
    The dwell reading is the primary current nothing to do with the high voltage currents.
    High voltage depends upon dwell. If the coil can't recharge fast enough, secondary voltage will drop. That's one of the limiting factors high rev'ing race engines deal with.

    Bill Robertson
    #5939

  6. #6
    Senior Member Bitsyncmaster's Avatar
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    Quote Originally Posted by content22207_2 View Post
    The only dwell that doesn't change is breaker points. That's one advantage of electronic ignition: the ability to change dwell as RPM's change.

    Bill Robertson
    #5939
    True. I would not consider a few degree change to be very valuable info. The meter accuracy with the shorter timing of higher RPM probably suffers. Our PRV is even harder on dwell meters accuracy with it's uneven fire engine.
    Dave M vin 03572
    http://dm-eng.weebly.com/

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    I suspect an analog dwell meter can handle the uneven firing order better than a digital meter -- needle just can't respond fast enough. Kind of like averaging.

    Bill Robertson
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  8. #8
    Senior Member Bitsyncmaster's Avatar
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    Quote Originally Posted by content22207_2 View Post
    I suspect an analog dwell meter can handle the uneven firing order better than a digital meter -- needle just can't respond fast enough. Kind of like averaging.

    Bill Robertson
    #5939
    Your probably correct but my Snap On digital dwell meter holds pretty steady values.
    Dave M vin 03572
    http://dm-eng.weebly.com/

  9. #9
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    See if this helps: http://www.bgsoflex.com/igncoil.html

    Bill Robertson
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    This morning I brought my school bus back to the house to get it ready for a field trip tomorrow. It's an old gas burner that runs as well as a 2016 Honda, partly due to its HEI (factory original Duraspark module -- I just swapped out the coil and wired it to charging voltage):

    SchoolBusIgnition.jpg

    (Autolite carburetor, circa 1960's, in the background also helps -- Holley monstrosity the bus came with didn't work worth 2 cents).

    Bill Robertson
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