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Discussion Starter #1 (Edited)
I have been researching the hei ignition module used with the
Marelliplex distributor. The module was designed by GM and is used in a GM distributor with a built-in coil.

It appears that GM does not use a resistor in their rotor. I would think that if the goal was to get as much energy to the spark plug as possible, adding a resistor to the rotor would be the last thing you would want to do.

Can anyone tell me the purpose of the 5k resistor in the rotor used on the Marelliplex ignition, and if anyone has run this distributor with a non resistor rotor.

Also, I am looking for a coil to replace the original Marelliplex coil. I would be interested to here what people have used successfully.

Thanks.

Larry
 

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The resistor is used, as are resistor type sparkplugs, to reduce RFI emissions (Radio Frequency Interference). Since an ignition coil will output only the voltage required to fire the sparkplug, any resistance in the secondary circuit has zero effect on said required voltage. The coil will simply output a bit more voltage to overcome the resistance and fire the plug anyway. Of course, if the required voltage exceeds the maximum voltage output of the coil, the plug will not fire.
 

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The resistor is used, as are resistor type sparkplugs, to reduce RFI emissions (Radio Frequency Interference). Since an ignition coil will output only the voltage required to fire the sparkplug, any resistance in the secondary circuit has zero effect on said required voltage. The coil will simply output a bit more voltage to overcome the resistance and fire the plug anyway. Of course, if the required voltage exceeds the maximum voltage output of the coil, the plug will not fire.
How does that reduce RFI?
I think it may have to do with introducing a resistive load to the inductive reactance (the nonresistive component of impedance in an AC circuit, arising from the effect of inductance or capacitance or both and causing the current to be out of phase with the electromotive force causing it).
Bob
 

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Discussion Starter #4
I was wondering perhaps it is to help the coil charge to a higher voltage. Again the Marelli Plex this a GM ignition module, but the coil used by GM is different.
 

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Discussion Starter #5
I have been doing more research, and found the patent for the resistor in the rotor.
The patent claims it is to help RF interference created by the ignition system, and as and added benefit, helps prevent crossfire in the distributer cap. 

It appears that in the Bosch and Marelli distributers the limiting factor for the maximum voltage they can handle is based on their diameter. The larger the distributer cap the higher the voltage. I read somewhere that the smaller Bosch caps can handle 19,000 volts, and the Marellies around 29,000 volts. (I will try to find the reference).
The GM HEI distributers used distributer caps about twice he diameter of the Marelli caps.

As I understand it the spark plug gap will determine the maximum voltage reached in the distributer cap. The larger the gap, the higher the voltage. This implies that to get the benefit of using an electronic ignition the spark plug gap should be increased to 35 up to maybe 40??( This is just my understanding) It seems that if the gap is too large to voltage in he distributer will be too hight, and cross-firing will occur, as well as the possibility of destroying the resistor in the rotor??

Perhaps, the resistor was not needed in a points type ignition, because they are not designed for excessively high voltages? I am guessing the with the more efficient Marelli Plex systems, the voltage could get high enough inside the distributer, where a 5k resistor could help prevent spark cross fire. ( Just a thought)
Larry
 

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This implies that to get the benefit of using an electronic ignition the spark plug gap should be increased to 35 up to maybe 40??( This is just my understanding)
I believe that you will get the most benefit if you open up the gap but it is a bit more complex than just voltage. The total energy in the spark is the voltage multiplied by the current multiplied by the duration of the spark. The more energy in the spark, the higher the spark temperature and the greater range of mixtures that the spark will ignite. There is a simpler way of looking at it: The total energy stored in the coil will be delivered to the spark. The energy stored in the coil is limited by saturation of the coil, but in general electronic ignitions and low impedance coils will make the hottest sparks, particularly at high rpm when the charging time of the coil is short.

I have a 123ignition distributor which uses a small Bosch cap. It fires an MSD6AL and an MSD Blaster coil. For years I ran with spark plug gaps in the .045" to .055" range, hence high voltages and the cap had 15k miles on it with this configuration before I changed it out. It was still working fine but the center button was worn down. I did have to use high quality plug wires as I was shocked when I touched the previous small diameter wires.
 

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I have been researching the hei ignition module used with the
Marelliplex distributor. The module was designed by GM and is used in a GM distributor with a built-in coil.

It appears that GM does not use a resistor in their rotor. I would think that if the goal was to get as much energy to the spark plug as possible, adding a resistor to the rotor would be the last thing you would want to do.

Can anyone tell me the purpose of the 5k resistor in the rotor used on the Marelliplex ignition, and if anyone has run this distributor with a non resistor rotor.

Also, I am looking for a coil to replace the original Marelliplex coil. I would be interested to here what people have used successfully.

Thanks.

Larry
ANSWER:

http://www.vishay.com/docs/30255/an0004.pdf

Bob
 

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I am an EE so I understand the Vishay information but I question the necessity of resistors. It is marketing blurb. I removed the radio from my Spider a year ago but I never suffered interference from either my Marelliplex or my 123ignition/MSD6AL/Blaster coil ignitions. I used solid core copper wire until a year ago and I used non-resistive spark plugs gapped anywhere from .040" to .055".
 

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I am an EE so I understand the Vishay information but I question the necessity of resistors. It is marketing blurb. I removed the radio from my Spider a year ago but I never suffered interference from either my Marelliplex or my 123ignition/MSD6AL/Blaster coil ignitions. I used solid core copper wire until a year ago and I used non-resistive spark plugs gapped anywhere from .040" to .055".
I am also an EE and see how the Vishay info may be valid and makes sense.

I have tried solid core wire on one of the Masi's and had lot of noise on AM radio,none on FM which makes sense.

YNMV (your noise may vary)

Bob
 

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I never listen to AM so that may account for the differences in our experiences.
 

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I just went thru my system as my coil failed, blew the rotor apart and shot out thru the cap

the gm part is off the shelf auto parts piece, mine cost $30
wiring is critical, so make sure it doesnt get reveresed

the coils come in 1.5 or3.0ohm rating
you want the 1.5--I had a bosch coil on the shelf, so popped that in

cap n rotor were sourced from Centerline
it the 80-81 version cap
bought 2 so I will never need one again

Im not an ee, but there's my .02 worth
 

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I had to- Rush was only on AM here in ETX.
That is what my wife says. Whenever I drive her car it is tuned to RUSH AM.

There is one minor downside to the "inductive resistor". The RFI is related to the rate of change of current and that is highest at the leading edge of the high voltage pulse so the wire wound resistor slows that initial ramp. But the ability of the pulse to ionize the molecules between the electrodes of the spark plug is also related to that same rate of change so slow it to reduce RFI and you reduce it's ionization capability. Under "normal" driving conditions that is not an issue but if you are running Webers and you stomp on the gas pedal the mixture goes very lean then very fat in a short time interval. If the spark does not ignite the mixture you get a stumble. So I run my ignition wide open and skip the radio.
 

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That is what my wife says. Whenever I drive her car it is tuned to RUSH AM.

There is one minor downside to the "inductive resistor". The RFI is related to the rate of change of current and that is highest at the leading edge of the high voltage pulse so the wire wound resistor slows that initial ramp. But the ability of the pulse to ionize the molecules between the electrodes of the spark plug is also related to that same rate of change so slow it to reduce RFI and you reduce it's ionization capability. Under "normal" driving conditions that is not an issue but if you are running Webers and you stomp on the gas pedal the mixture goes very lean then very fat in a short time interval. If the spark does not ignite the mixture you get a stumble. So I run my ignition wide open and skip the radio.
Hmmmm....Maybe that is where my stumble is coming from.

Bob
 

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I removed the radio from my Spider a year ago but I never suffered interference from either my Marelliplex or my 123ignition/MSD6AL/Blaster coil ignitions.
The interference is not limited to components within a vehicle but extends well beyond the vehicles exterior which could interfere with someone elses radio, TV, cell phone, pacemaker, etc.
 

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The interference is not limited to components within a vehicle but extends well beyond the vehicles exterior which could interfere with someone elses radio, TV, cell phone, pacemaker, etc.
I remember being in the pits at US30 dragstrip in the 60s listening to WLS on the radio (AM only then) and a fueler or altered would drive by with a magneto ignition and wipe out the radio signal.

Bob
 

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The intensity of electromagnetic radiation is inversely proportional to the SQUARE of the distance from the source, in other words, even without considering the shielding effect of the car bodies, the intensity at 10 ft is only 1% of the intensity at 1 foot. This is in practice trivial.

I use my cell phone in my Spider without interference problems.
 

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Rotor

So the questions for us racers using a Marelliplex to trigger Crane or MSD units with healthy coils, is there a replacement that will fit in the smaller "plex" cap that is not a resistor piece?
 

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I think that you can solder a piece of wire across the resistor which will make the resistor ineffective.
 
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