I am currently running a stock distributor on my 2.0 w/ Djet. It has the pertronix ignitor module made for my distributor with the vacume advance. I also had a flame thrower 3ohm coil made by pertronix. I am not sure how many ohms a stock bosch coil is, but my question is...

most high performance coils such as mallory, crane, even the flamethrower (not the one I have) comes with a resistance of .06 or less. Why is this, and what does the ohms do for you anyway. I am thinking to put one of the VERY low ohm coils on my engine to see if any better performance.

If you dont mind also explaining why i threw out my ballast resistor when I originally put the pertronix in. (as per installation instructions) what the heck did that do for a stock coil thats not needed for a 3 ohm coil?

_Very_ short version:

Stock Kettering ignition uses a coil. It's a nice 19-century way of turning battery voltage into the BIG voltage needed to jump a spark gap at cylinder pressure. It's an energy storage device. During the 'dwell' time, it charges. When the points open, it discharges acting like a transformer.

Capacitive-Discharge ignitions do not use coils, although the pulse transformers they use frequently look like coils of olde, and people often call them that. But they're pulse transformers.

With points and a Kettering ignition, ALL the ignition power runs through the points. You use a ballast resistor to keep the points from burning up with to much current.

Coils have fairly high primary resistance because they need a lot of turns on the transformer to turn 12V into 20,000V. A CDI starts at 400-ish V and doesn't need as much help from the transformer part. Plus, since it's not used to actually store energy (the capacitor in the CDI box does that) it doesn't need to build up as big a magnetic field.

with a CDI, the points aren't carrying all the ignition power, they simply switch the CDI electronics.

I have nothing more to say about pertronix (beyond my opinions that are still in several threads here) other than to speculate it would be wise to use whatever they recommend, lest you loose the Magick Smoke.

In general, I prefer to stick with whatever the spark box maker recommends - I use a Crane pulse transformer with my Crane HI-6, and I used MSD High-Vibration "coils" when I ran an MSD.

When we ran the race cars, I set up a hi-output coil with an optical trigger. I did not use a ballast resistor. Found out two things:

1.) Putting 12 VDC to the system produced one hell of a spark through the distributor that burned out the center electrode in the dizzy cap.
2.) The brillance of the spark in the dizzy cap triggered the optical pickup and caused spark to jump across the electrodes.

I added an old style ballast resistor which I believe effectively drops the voltage to the coil to 9 VDC from 12 VDC with no further problems.

The Bosch Blue coil that is run with the systems has an internal resistor to drop the voltage to the primary coil.

From what I understand, the 3 ohm coil will produce the correct amount of current and will not burn out the pertronix. If you use a low resistance coil, you will increase the current through the pertronix and burn it out.

( from a technical point. Ohms law: voltage / ohms = current. So, 12v / 3 = 4 amps. 12v / .06 = 200 amps. Or smoke.). This does not take into consideration the resistance of the pertronix.

-- brett

some cars use the ballest resistor when running, but bypass the resistor during cold start. Some British cars are configured this way. this allows a hotter spark during start up, and a lower power spark during running. if the high power spark was used for any longer than starting (ie cranking the motor) the point would burn up. a nice way to deal with a cold start issue

Ohms =Volts/Amps ...... So ohms (resistance) is the value of Volts (potential) divided by Amps (current)

some one that knows old school . thank you

From my high school shop class in 1964. . . .extremely simplified. . . . meaning lots of electrical magic properties and theories have been omitted. . .

All ignition coils in the begining were really 6 volt coils. . . when 12 volt systems came out in the 50's for most cars, the same old coils that were used from the early cars were used. . the new 12 volt wiring included a Ballast Resistor, whose job it was to drop the input voltage to the coil to 6 volts. . . an interesting side circuit was added from the ignition switch to the coil which bypassed the resistor during cranking only. . . this gave the coil a direct 12 volts with the theory that it would make the spark hotter during startup. . . [Yeah a 6 volt coil will work on 12 volts but it will overheat]

Input voltage/ current is what determines the gage of the wire used in the windings. . the ratio of the number of primary [low voltage side] winding to the number of secondary windings [Hi tension side] is what determines the output voltage to the plug

Ignition Points need a condenser for 2 reasons. . { The job of a condenser is to store energy} the condenser stores voltage and keeps the points from arching when they open. . plus , when the points open, the condenser causes an immediate spike of no energy into the primary, then the magnetic field in the primary windings collapse quickly, which causes the secondary winding to discharge into the spark plug,
I know from trouble shooting past electrical issues, that a car with a weak spark can be caused by a bad condenser alone.

That secondary discharge needs somewhere to go. . ie the spark plug. . without proper discharging the secondary high voltage will try to arc from the high tension terminal to the terminal that the points connect to. . . we're talking in excess of 20,000 volts. . . if it can't find that path, it will eventually arc inside the coil case and destroy the coil. . thats why you should always have a spark plug connected to a working coil..
. . .

Now tell me how to check a coil for ohms. Do I put the meter on the + and center (output)?

measure from the + to the - for the primary winding. . this is wound with large wires with relatively little windings and the reading will be around 1 ohm, which for all practical purposes is a dead short. .
to measure the secondary windings go from the spark plug lead to the - terminal .. this is a lot of windings and a relative small guage wire so this will have a greater resistance than the primary

looking at the diagram, it is possible to measure from the spark lead to the positive lead but you will then be measuring through both sets of windings

Back to your original question, why does the performance coil have a lower primary coil resistance. . because it is probably wound with larger gauge [ think diameter] wires. . the larger the wire with less resistance + more current flow which in turn make for a coil with a bigger secondary winding thus a bigger charge and thus a bigger spark, but it sucks more current and the coil runs hotter. . it's all relative. .

toolguy,
Your memory is pretty good, but one small point is wrong.The condenser is already discharged while the points are closed. Looking at your diagram, see the condenser has ground on both sides when the points are closed? In that condition, it will discharge. So what actually happens is the condenser starts to charge upon the points opening, assisting in the collapse of the magnetic field and extending the length of the high voltage "spark" to the spark plug. This happens due to the condenser and primary winding forming an LC circuit, which oscillates until the energy is dissipated.
My memory is not so good. I felt something in your explanation was not quite right, but I had to look it up to confirm.
And, the condenser is vital to the points not arcing to death upon opening.

jsayre914,
If you want a more detailed explanation see Electronic Ignition Overview here
jetav8r.com.
I can't get the link part to work. Just google jetav8r.com and you will see it.
Tom