Anybody have a formula for determining how much resistance you need from your ballast resistor?

My specific situation is a Mopar Orange Box and a Blaster II coil. I read a post by AgressiveTed saying he used a 0.85 Ohm resistor, however I've read elsewhere that MSD suggests (not sure if it came straight from MSD) 1.5 Ohms (coil internal resistance + ballast resistor) for an 8 cylinder, and 3 Ohms for 4 cylinder. So that puts the slant at about 2.25 Ohms.

So the Blaster II coil has 0.7 Ohms of internal resistance, which would mean I need a 1.55 Ohm resistor (which I don't think anybody makes). However, that seems a bit high or maybe not.

It seems there isn't a lot of accessible information on the purpose and selection of ballast resistors. So if anybody can illuminate this topic, it would be extremely helpful.

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'66 Dodge Dart

OK,so how does that work? I understand that an 8 cylinders coil most fire more times per rpm than a 4 cylinder,but what is the difference? an 8 cylinder @ 2000 rpm and a 4 cylinder @ 4000 rpm will see the coil do exactly the same work.
So - the question is -does it really matter?

regards,Rod

Don't quite understand this enough to explain, but here goes....


The coil is an inductor. The resistance of an inductor goes up with the frequency of the current going thru it. So the more cylinders at a given rpm, the more resistance that coil has. This resistance will be added to the coil resistance and the ballast resistance.

See:

http://www.electricianeducation.com/the ... ctance.htm

this is about 1/8 down
"
Since inductors drop voltage in proportion to the rate of current change, they will drop more voltage for faster-changing currents, and less voltage for slower-changing currents. What this means is that reactance in ohms for any inductor is directly proportional to the frequency of the alternating current. The exact formula for determining reactance is as follows:

X= 2*pi*freq*L

freq in Hz, L in henries

If we expose a 10 mH inductor to frequencies of 60, 120, and 2500 Hz, it will manifest the following reactances:



For a 10 mH inductor:
Frequency (Hertz) Reactance (Ohms)
----------------------------------------
| 60 | 3.7699 |
|--------------------------------------|
| 120 | 7.5398 |
|--------------------------------------|
| 2500 | 157.0796 |
---------------------------------------- "

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Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

Thanks for all the info Ed, but what does it all mean practically? Should I go with the 0.8 Ohm ballast that comes bundled with the MSD coil, or a 1 Ohm (or higher) ballast resistor?

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'66 Dodge Dart

I'd say somewhere around 1.4

Which I think is a stock ballast................

Excerpt from a spreadsheet.........

Cyl 4 6 8
Coil 0.7 0.7 0.7
Ballast 2.3 1.4 0.8
RPM
500 3.84 3.36 3.18
750 4.26 3.98 4.01
1000 4.68 4.61 4.85
1250 5.09 5.24 5.69
1500 5.51 5.87 6.53
1750 5.93 6.50 7.36
2000 6.35 7.13 8.20
2250 6.77 7.75 9.04
2500 7.19 8.38 9.88

numbers after rpm are equivalent ohms

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Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

thanks for the electrical lesson Ed. I think I understand what happens. in reality,the faster the coil is asked to load and discharge,the greater the resistance,and lesser "strength" of spark.

my head hurts. I will read that article again a few times,and some more will sink in - I hope.

thanks again,regards,Rod

It takes time for a coil to charge up. As a coil charges the current flow increases, untill at "saturation" current flow is maximum.
A ballast resistor changes it's resistance, to control current flow. As the resistor heats up due to the current flow, the resistance increases, and current decreases.
So if you have the ballast resistor in series with the ign coil (as they should be connected), when the coil starts to charge, circuit current starts low. As the coil charges up, the circuit current flow increases, untill the it reaches the max, that the coil resistance will allow (not reluctance, that only has effect as the coil is charging). At max current flow the coil would over heat, so the ballast resistor is put into the circuit, to limit current flow, since at max current flow the ballast resistor, has a higher resistance, then at min current flow.

Ballast resistors are not needed, nor wanted with many electronic ign modules, such as the HEI. The module itself will limit the current flow.

Did I make that clear as mud?

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Charrlie_S
65 Valiant 100 2dr post 170 turbo
66 Valiant Signet 225 nitrous
64 Valiant Signet
64 Valiant 4dr 170

Expanding on Professors Ed & Charlie’s teachings:

Perhaps a look at this page will mix the mud a bit more on; how a coil, and different ignition parts do their work. I have been studying its pages in hopes that some latent memories from a basic electronics course taken while studying CE 37 years ago will be resurrected (fat chance on that). Its content is slowly sinking in to my thick scull. It is amazing how we can beam a message around the world in a millisecond, but the last quarter of an inch can take so long to get through…

At any rate, the relationship of resistor, coil, points & condenser, and electronic triggering methods are well explained & illustrated.

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67' Dart GT Convertible; the old Chrysler Corp.
82' LeBaron Convertible; the new Chrysler Corp
07' 300 C AWD; Now by Fiat, the old new Chrysler LLC

FYI - the old Mopar point-style resistors were designed to heat up and increase resistance - a clever way to partially compensate for the RL time constant. I've measured one over 0-8A and found it to be in good agreement with the FSM. Its resistance is about 0.5ohm + 0.633 ohm/amp. The electronic ignition ones are about 1 ohm and don't change resistance significantly (I measured them too).

I suspect that Chrysler decided that running the resistors red hot inside to improve the spark didn't gain you enough to justify their higher failure rate.

Josh P,

The .85 ohm resistor that comes with the MSD Blaster II coil works well. The MSD coil and resistor will bring up the coil voltage from (stock system) 6 volts at a 600 rpm idle up to 10 volts. Idling for hours in traffic was greatly improved.
I ran this set up for about 5 years with the NGK plugs gaped at .045.

This past year I switched to the 3 ohm Pertronix HEI 60,000 volt coil with the MOPAR orange box. The coil now shows a full 12 volts at a 600 rpm idle. The engine never misses a beat and is the steadiest idle I have heard from any SL6. With the Pertronix HEI coil you do not need to run a ballast resistor.
The Pertronix engineers have assured me that this is the best set up for the MOPAR ECU providing maximum voltage and spark and is comparable to the Ignitor II system.

The Ignitor III system is the best to date and you can run any of the new coils that are .5 to .7 ohms for maximum spark, primarily for racing. The 8 cylinder version has been released however, the 6 cylinder version is still in work.

Click on the red link below my name to view pictures of the 3 ohm coil.

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Aggressive Ted

http://cid-32f1e50ddb40a03c.photos.live ... %20Swinger


74 Swinger, 9.5 comp 254/.435 lift cam, 904, ram air, electric fans, 2.5" HP2 & FM70 ex, 1920 Holley#56jet, 2.76 8 3/4 Sure-Grip, 26" tires, 25+MPG

Wow awesome info everybody!

So it would seem there is a bit of differing views... The MSD 0.85 Ohm resistor that comes bundled with the Blaster II coils (I think these days its 0.80 Ohms), and the 1.4 Ohm "stock" resistor. The 1.4 Ohm resistor would come much closer to what I've heard from a guy claiming to be an MSD tech (total resistance, coil internal + ballast, should be 2.25 Ohms for a 6 cylinder).

If I understand the rest of the info given in the thread. Lower resistance would give a hotter spark and put more load on my coil (putting it, or the ECU, at risk of over heating). Conversly, using more resistance I will have less spark and my ballast resistor, coil and ECU should be more reliable. Is this all correct?

Last question, does anybody see any problem with experimenting with ballast resistors between 0.8-1.55 Ohms?

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'66 Dodge Dart

In my opinion, it is not worth messing with ballast resistors. Just us an electronic module that does not require a resistor. Let the module do the current regulation.

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Charrlie_S
65 Valiant 100 2dr post 170 turbo
66 Valiant Signet 225 nitrous
64 Valiant Signet
64 Valiant 4dr 170

+1 however, when I upgrade the, I want to go to an MSD box and that will take some saving, and I can think of a few other upgrades I'd like to do before that.

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'66 Dodge Dart

If money is an issue, I would say "do the HEI conversion". On a stock street car or mild race car, you would not see any difference between the HEI and an MSD 6A, except in the wallet. Even buying new parts for the HEI, it would cost under $100, and junk yarding would bring the cost down more.

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Charrlie_S
65 Valiant 100 2dr post 170 turbo
66 Valiant Signet 225 nitrous
64 Valiant Signet
64 Valiant 4dr 170

I just did the HEI conversion on my 65 D100 and it is amazing. Total cost of parts from the junk yard (2 coils, 2 hei modules and some connectors. I like having spares.) $38, Mopar electronic distributor I took from my "spare parts" slant in my garage, a little wiring, a power relay and some fabbing of a heat sink/mounting position and I was in business. MUCH improved idle... MUCH... and WAY peppier pick-up. Still waiting to fill the tank and check the mileage.

Just my $.02.


-Marc

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1965 D100 225 slant 6, NP435 Utiline.