GM HEI ignition modules

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AC sports
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GM HEI ignition modules

Post by AC sports »

I'm running one of these on a high compression 4 Cyl vintage racer that sees a red line of 7800 rpm. Problem is at high rpm ignition is erratic and scattered. I'm trying to avoid the expense of an MSD unit right now. I see Accel offer I HiPo module. Anyone have experience with them? I've heard they fail prematurely.
Can I expect a performance gain through ignition stability with one?
Any other recommendations / suggestions?
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Re: GM HEI ignition modules

Post by F-BIRD'88 »

Make sure the sparkplug gap is no bigger than .035".
If the diz cap is small diameter this is even more critical.
Check diz rotor to cap phasing alignment.
Be sure engine is gro:nded to both the frame and ,he car body. Be sure the HEI ;s seeing full 12v ++ voltage.
The ignition coil should have similar primary resistance as the oem gm coil has.
If the CR is 12:1+ reduce sparkplug gap to .030"
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Re: GM HEI ignition modules

Post by Walter R. Malik »

AC sports wrote: Wed Jun 26, 2019 10:57 am I'm running one of these on a high compression 4 Cyl vintage racer that sees a red line of 7800 rpm. Problem is at high rpm ignition is erratic and scattered. I'm trying to avoid the expense of an MSD unit right now. I see Accel offer I HiPo module. Anyone have experience with them? I've heard they fail prematurely.
Can I expect a performance gain through ignition stability with one?
Any other recommendations / suggestions?
Those are Inductive Ignitions so, they are current sensitive. The pertronix module #D72000 has low RPM multiple sparks and works very well at higher RPM. Their D2070 works well also but, doesn't have the multiple spark or rev limiter features.

Be sure there is at least 12 volts present, (without a good charging system it is less that 12 volts), at the module and at least a 12 gauge wire, (preferable 10 gauge), supplying the current to the module.

Be absolutely sure that rotor phasing is correct.
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Re: GM HEI ignition modules

Post by dfarr67 »

Maybe try Davis Unified Ignitions?
Stay with ACD modules- the Accell will leave you high and dry.
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Re: GM HEI ignition modules

Post by Schurkey »

WHAT PICKUP COIL and RELUCTOR are you using? Weak or broken magnets on a stock GM pickup coil can cause weak signal to the module.

Are you sure it's wired with proper polarity? The GM pickup coils are set up with one large blade and one small blade to assure proper connections.

Yeah, the GM module would like to see ~13-ish volts with the engine running.

WHAT IGNITION COIL
are you using? It ought to be a match to the module--about .5 ohms primary resistance. It should fire an HEI spark tester (larger air-gap than a Points-calibrated spark tester.)

WHAT MODULE are you using? 4-pin? 5-pin? 7-pin, or 8-pin? 4-pin would be by far the most likely. Mounted to a heat-sink with thermal paste?
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Re: GM HEI ignition modules

Post by GARY C »

Pertronix would be my choice.
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Re: GM HEI ignition modules

Post by Tuner »

I would use either an original '70s-'80s era genuine GM 990 module or the MSD 83647. The 83647 actually makes a hotter spark than any other HEI module I know of. The original GM 990 module will function to much higher RPM than any V8 engine used in a normal race car (whatever that is LOL) as long as the coil is powered through a 10 ga. wire, so your 4 cyl. will not be limited at all. The MSD similarly depends on good power supply, volts and amps.

For high RPM plug gap .025" or maximum 030" is best with any induction ignition, the arc doesn't stay in the gap as the coil discharges anyway. As the arc burns, air motion in the cylinder takes it out of the gap and it follows the ionized gasses to the end of the threaded shell.

A close plug gap lowers the breakdown voltage and makes it easier for a good arc to get established in the combustion gasses instead of the energy bleeding off through the deposits on the ceramic. Less energy expended to breakdown the gap (start the arc) leaves more remaining for longer arc duration during the "glow" portion of the spark.

For a race application, you can power the coil + directly from the battery + with a 10 ga. wire and remove the small pink wire from the cap + and connect it to the ignition switch. The tenth-amp (0.1) module amplifier power is switched with the small pink (red?) wire and the large amperage load is direct to the coil, avoiding any resistance of a jiggling toggle switch.

The module turns the coil off if it does not get successive trigger signals representing engine speed above very low RPM well below any normal cranking speed.

The MSD module multi-sparks during cranking and retards the timing for cranking below 400 (or so) RPM (8 cylinders). Once above 350~400 RPM (8 cyl) it does not retard or multi-spark until it sees zero RPM again. I like that MSD 83647.

Shurkey's questions are important, how are you triggering this, what kind of reluctor, the dwell time of some modules is affected by the reluctor waveform and voltage, and what kind of coil.

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Re: GM HEI ignition modules

Post by mag2555 »

What steps have been done in the first place to nail the issue down to the ingition system and not high rpm harmonics in the Cam shaft or its drive!
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Re: GM HEI ignition modules

Post by Walter R. Malik »

Tuner wrote: Thu Jun 27, 2019 2:16 am I would use either an original '70s-'80s era genuine GM 990 module or the MSD 83647. The 83647 actually makes a hotter spark than any other HEI module I know of.
MSD offered an even hotter one than that but, they haven't sold it since 2009.
Those 20 amp modules would overheat in continuous operation.

It was PN 86321 if anyone can find one as "new old stock".

I was told by MSD in 2011 that the PN 86347 is a milder derivative of this and didn't burn out; completely replacing their PN 83645.
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Re: GM HEI ignition modules

Post by gmrocket »

Walter R. Malik wrote: Thu Jun 27, 2019 6:11 pm
Tuner wrote: Thu Jun 27, 2019 2:16 am I would use either an original '70s-'80s era genuine GM 990 module or the MSD 83647. The 83647 actually makes a hotter spark than any other HEI module I know of.
MSD offered an even hotter one than that but, they haven't sold it since 2009.
Those 20 amp modules would overheat in continuous operation.

It was PN 86321 if anyone can find one as "new old stock".

I was told by MSD in 2011 that the PN 86347 is a milder derivative of this and didn't burn out; completely replacing their PN 83645.
I found this out the hard way in a boat...way off shore.
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Re: GM HEI ignition modules

Post by AC sports »

The ignition system is a factory Marelli plex.
It uses a Marelli 0.7-0.8.ohm primary resistance HE coil. Pick up is magnetic and oem new. Installed correct with 13-13.7v going to the coil + when running. This system runs a GM style 4 pin module installed on an alloy heat sink with insulating paste. It shares the heat sink with the coil. Timing scatter can be seen with the timing light and engine revving. It's not camshaft harmonics I'm pretty sure otherwise I'd know about it. I've been around these engines a long time.
I'm considering the replacement pertronix flame thrower 3 module with limiter and multi spark. I run solid core ignition leads. Does anyone know if this will create any issues? The car runs carburettors and it obviously doesn't have a radio..the only electrics are the ignition.
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Re: GM HEI ignition modules

Post by bill jones »

---several things I think need changed are:

1---I feel the module should definitely be isolated away from the coil.

2---solid core wires have gone by the wayside since the invention of electronic ignitions.

---might want to try a different set of modern low ohm resistance spiral core sparkplug wires and see what that does.

3---need to cut one or maybe two nice sized windows into the distributor cap to where you can witness the rotor tip using a timing light---and both sides of any one internal coil wire post terminal---to see if the spark happens when the rotor is aligned--if not you'll need to rotate the trigger until it lines up and then reset the timing.

---also want to be able to use the timing light to see clear into the reluctor and the trigger to see how they are aligned----making sure the rotation orientation is correct and/or the magnetic trigger wires are wired up correctly.

4----if the cap happens to be like a GM HEI and has the flat inner terminals---these flats can be bent slightly towards the rotor to reduce the gap.


5---13 to 13.7 volts sounds like you are using alternator power rather than battery power as a supply to the ignition--so you might want to try a large capacitor or a condenser to act as settling place to smooth out the pulsing spikes.
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Re: GM HEI ignition modules

Post by Schurkey »

AC sports wrote: Fri Jun 28, 2019 9:40 am The ignition system is a factory Marelli plex.
It uses a Marelli 0.7-0.8.ohm primary resistance HE coil.
Consider trying a GM HEI ignition coil and see what happens.
AC sports wrote: Fri Jun 28, 2019 9:40 amPick up is magnetic and oem new. Installed correct
HOW do you know the electrical polarity is correct? I'm also curious about the shape of the "teeth". GM uses triangle-shaped teeth on the reluctor, and triangle-shaped teeth on the pickup coil. Others use squared-off teeth. The waveform produced is somewhat different.
AC sports wrote: Fri Jun 28, 2019 9:40 amwith 13-13.7v going to the coil + when running. This system runs a GM style 4 pin module installed on an alloy heat sink with insulating paste. It shares the heat sink with the coil.
IGNITION coil, not pickup coil? The GM Vortec engines have the ignition coil and module next to each other on a heat sink, but I don't have a photo of how they're oriented. For that matter, the original HEI had the pickup, module, and ignition coil in close proximity to each other.
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Re: GM HEI ignition modules

Post by simon010 »

AC,

worthwhile checking the backlash on either a block mount or top-mount dizzy in Fiat/Lancia TC's (and SOHC) - because - they thrust with the drive gear, which changes the timing !

both have a simple cross-pin, which can be knocked out, and then "shim" to minimise the end-float. this helps a lot with stability ......

also - fyi - there is a good section on the xweb's forum - and someone posted a "neat" way of putting a GM HEI into the std bosch box ......looks factory !

hope that is useful
rgds
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