Ignition question involving use of ballast resistor

1987 Dodge Dakota 3.9L carbed
I recently posted on rec.autos.tech about the problems I was having with an MSD 5900 Blaster ignition and an MSD 8202 Baster 2 coil. It seems I
could use one or the other with no problem, either the MSD ignition with the stock coil or the MSD coil with the stock ignition, but if I had both installed I would get a stumble on accelerating from a dead stop.
From this behavior I deduced that, for some reason, the problem was being caused by too much ignition voltage. To test out that theory I installed a 1 ohm ballast resistor in line with the +12 volts going to the coil. Voila, the problem vanished and I was back to strong, smooth acceleration from a dead stop. So then I tried a 1/2 ohm ballast resistor and the stumble returned, but not as bad, so I went back to the 1 ohm resistor.
The stock coil has about a 1.5 ohm primary and the MSD coil has a .7 ohm or so primary. I don't know what impact this has on the situation but it's info that I have and I figured I might as well throw it in if I am going to all the trouble of posting this and you are going to all the trouble of reading it.
Now we get to meat of the my puzzlement. The ignition wires that I have on the engine measure about 3000 ohms per foot. For about $45 I can buy some MSD Street Fire wires that are only 500 ohms per foot. I know a bit about electricity, but not enough to know whether or not these wires would enable me to run the MSD ignition and coil without the ballast resistor. After all, the point of an MSD ignition upgrade is to get longer stronger spark to the plug.
I don't have a clue as to why the engine stumbles with both the MSD ignition and coil installed. From the symptoms it seems to me that it must be because of too much voltage at the coil tower, but why that might be causing the problem is beyond me.
Could it be because the high voltage is leaking out to ground somewhere on the way to the plugs? I checked the engine at idle in a dark area at night and saw no evidence of arcing. Could the high resistance wires be the cause? If so, what is the rationale? I can't think of one.
Well that's it. At present, I am more than happy with the ballast resistors. The truck runs great. Just curious as hell as to what might be going on.
Jack
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ROTFLMAO !!!
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Jack, The purpose of a ballast resister is to allow a lower voltage ignition coil to be used. The reason for this is that when the starter motor is engaged, battery voltage will drop to say 8 V. You will note that when wired correctly the ballast resister is bypassed at that time allowing the coil to create adequate spark during lower battery voltage conditions . When the starter is then disengaged the resister bypass is also dropped and the resister is now in play to reduce excessive heating of the coil. Electronic ignitions are very sensitive to coil configurations, as they provide coil drive and can be permanently damaged if the DC resistance of the coil is too low. Follow the manufacturer's recommendations. Steve

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Steve Lusardi wrote:

Steve, thanks for the heads up on the wiring. I didn't run the B+ wire to the coil from the starter relay because I felt that the MSD ignition and MSD coil together would provide enough spark going through the ballast resistor even with the engine cranking. It proved to be true. The engine starts up immediately with the 12+ volts from the ignition switch going to the coil through the ballast resistor. I was mainly intent at the time to test the idea that the stumble problem was being caused by trying to push too high a voltage to the plug gap. The ballast resistor was my way of reducing the voltage at the plug gap while using both the MSD coil and ignition together.
The ballast resistor is rated at 1 ohm and measures 1 ohm. The Blaster coil primary is rated at .7 ohms and measures .6 ohms. The resistance of the coil primary of the MSD Blaster coil and the added ballast resistor measures 1.6 ohms, all measured on a digital meter. The recommended primary resistance for the coil on this vehicle is 1.34 to 1.55 ohms.
I don't know what stress is being placed on the spark control computer when using the MSD Blaster coil with the stock ignition and no ballast resistor, but the engine runs great under this configuration, with very smooth acceleration a dead stop. The same is true when using the MSD ignition box with the stock coil. The problem of a stumble on accelerating from a dead stop happens only when both are used together.
With the addition of the ballast resistor the computer now sees what it was designed to see at the primary winding, so there should be no problem of the kind you are warning me about, having too low resistance at the primary.
As to what is causing the stumble on accelerating from a dead stop using the MSD ignition and coil and no ballast, I am still in the dark. After all, the engines performs great with just the coil and no ballast.
Jack

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Jack wrote:

Normally when all is working as it should, what limits voltage is the spark plug gap and the engine compression (actually the absolute pressure inside the cylinder). A wider gap or more pressure will cause the voltage to climb higher before it will cross the gap. acceleration from a stop often will produce the greatest pressure inside the cylinder.

Yes it could be that - If the voltage climbs too high too fast it finds another path instead of the spark plug gap. Ultimately, your ballast resister doesn't limit the voltage at the spark plug but it does slow down the rise in voltage which means that some other points in the secondary won't get quite as high. If stray arcing in the secondary is what is happening it would be likely that it would get progressively worse over time as the stray arcs develops a carbon trail that the current can follow more easily.
    The cause could also be in the primary there may be something that saturates in the primary or even something intended to limit voltage/current to protect against damage.
-jim

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This was especially true when the Kettering system was used. When capacitive discharged systems came along, it pretty much stopped being true. Their rise times are so short and transformer action so definite that they usually put up much more constant ignition performance.
I dont know what the multiple spark discharge systems do. Are they CD, or something else?
Some early studies showed that, all things being equal, a small single spark ignited the fuel as well as anything else. Some university studies in England showed that high performance ignition systems which gave significant gains were possible, but they were not necessarily simple and cheap either.
It is my opinion, and nothing more, that there is little to be gained from systems like the MSD for normal driving. For racing, maybe....
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HLS wrote:

What stopped being true? The statement you are responding to has never stopped being true. It may have become unimportant to know that it is true, but that is hardly the same as being not true..

How does this information relate to solving the OP's problem? The rate at which voltage is developed in the coil is going to be a function of resistance and inductance no matter what. Which is why changing resistance produces different results.

His intent is related to neither of those - it is to improve emissions.
-jim
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HLS wrote:

They're usually ordinary flyback systems, just like points systems. I never saw a CD multistrike system, but that was long ago, when every CD system was triggered by an SCR, a device that can be turned on but not turned off. But since then, high voltage, high current devices that can be turned off have become more common, so multistrike CD should be easier to implement. I had a double-strike CD system that consisted of 2 separate CD units because I didn't know how to make anything better.

True, thanks to the EPA and its stringent emissions standards that include cold weather, cold engine conditions.
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HLS wrote:

HLS, the unit that I have is the MSD 5900 Blaster. It is not multiple spark, but one strong spark through 20 degrees of rotation and it is induction discharge, not capacitive. But most, if not all, of MSD's other ignition units, with the exception of the MSD 5, are capacitive discharge.

I installed the MSD components in the hope that they would help pass California smog requirements. I had to swap out the original feedback carb for a non-feedback version because the mixture control solenoid in the feedback carb is no longer being provided by anyone as a replacement part and owners of vehicles with this carbs are left to fend for themselves. I had the non-feedback carb installed the last time I had the truck smogged and it passed, but only just. I was 2 ppm below the max for HCs.
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jim wrote:

Thanks, jim, for the feedback. I ran the engine at idle in the dark and could see no evidence of arcing. Of course this is different from accelerating the engine under a load from idle.
At any rate, if the problem is caused by current leak, do you think that changing the plug wires from 3000 ohms per foot to 500 ohms per foot would help? It would only cost me about $50 or less to find out, but I hate throwing money at a problem to see what sticks.
Jack

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Jack wrote:

My guess is the cause is in the primary not the secondary. But if it was due to a leak in the secondary you would probably have to be looking at it in the dark while it was happening.     Changing the spark plug wire resistance shouldn't have any effect. Current doesn't really flow thru the wires until the spark plug starts to arc. The tail end of the wave form will be affected but not the initial spike. Getting rid of the resistance won't change the peak voltage in a measurable way.
    It is not clear how this is going to help with your emissions. If the mixture ia a bit rich all the spark in the world is going to get the extra fuel to burn. If it is a tad lean it might help, but it doesn't sound like that is the case.
-jim

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jim wrote:

I will find out soon what the emissions are. I just got back from a freeway run and some stop and go driving and the only problem was a sag when I floored it at about 25 to 35 mph, like the accelerator pump was out of adjustment or the power valve was not opening. I tweaked the pump linkage a bit but it wasn't out that much. I checked timing and it is at spec. Checking the power valve will involve tearing into the carb, which I will save for another day. It could be that the main jets are a tad too small, but before I do anything else I will take it to a smog shop for a dry run to see what the emissions are. At least the ignition system seems to be performing okay and that was my main worry.
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The ballast resistor does more than just change the voltage, it can change the shape of the pulse through the coil as well.

No, using a wire with a lower resistance will _slightly_ increase the voltage to the plug. But honestly, the difference is very small because the voltage is so high and current so low that it takes a lot of resistance to cause much drop.

What you want is an ignition system analyzer that will let you look at the signal coming out of the coil on a CRT. This will tell you what is really happening when you put the ballast resistor in. Ask older shops around town if they still have an analyzer kicking around.

The scope will tell you, and without the scope you have no real chance of seeing what is happening. --scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

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Scott Dorsey wrote:

What role does the resistor play when voltage is cut off to the primary? This is what causes the secondary to produce its voltage, right, the sudden collapse of the primary field?
Does the inductance produced by the expanding field of the primary limit the rate of expansionof the field? Is this why there is not a high voltage produced in the secondary when voltage is applied to the primary?
Could it be that the lower resistance of the MSD coil primary over the stock coil, .6 to .7 ohms, compared to 1.3 to 1.5 ohms, coupled with the greatly increased voltage to the primary from the MSD ignition could cause the field in the primary to expand too quickly and cause a weak spark at the plug at the wrong time?

Scott, this was what I suspected. It seemed to me that the higher the voltage, the less impact high resistance wires would have. My thought was that perhaps the higher resistance wires was causing current to leak out somewhere besides the plug gap.

Thanks, scott. I really appreciate your input.
Jack
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Scott Dorsey wrote:

The modern equipment would be the Snap On Modis or the Snap On Vantage Pro.
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Scott Dorsey wrote:

And suppresses "ringing" in the circuit.
And given the fact that the electronics that drive the coil are basically analog amplifiers, it changes their output current profile directly too.
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Jack wrote:

I'm going to say MSD designed the output section of their ignition system wrong, and maybe when the resistance is low, it goes into current-limiting mode to protect itself. Another possibility is the high voltage protection for the output section (typically some zener diodes and capacitor) is interacting badly with the coil primary.

Low resistance actually doesn't help in most cases, probably because it slows the rise time (turn-on) of the current, and I've heard of many people running into trouble when they switched to low-resistance plug wires.
High voltage leakage shouldn't be a problem with modern (1980s and later) distributor caps, rotors, and wires. On the other hand after I installed a CD ignition in my 1975 Dodge Dart, 2 caps cracked in a few months. A slight modification to limit the primary voltage to under 400V stopped this.
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I would suspect that too much primary current just supersaturates the coil and it doesn't discharge properly, inhibiting the next charge/ discharge cycle.
Dan
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I suspect the pure resistance is less of a factor than the distributed impedance, wouldnt you imagine?
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I am not really sure... I think you can look at the cable as a lumped-sum anyway. It's pretty much just the resistance of the cable that will affect how much the coil will ring on the tail end of the pulse, and I think that is the big deal. The capacitance between the cable conductors and ground COULD make a difference, but they are going to be different in every installation depending on how the cables are laid out. The series inductance of the cable should be minimal. --scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

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