Ignition coil primary resistance

[ian_uk1975]

Bought a new distributor and now need a coil. The module in the dizzy can accept a coil with a primary resistance of 0.6 Ohms. However, would there be any tangible difference if I fitted a coil with a primary resistance of 1.4 Ohms?

[honestjohntoo]

ian_uk1975 said:

Bought a new distributor and now need a coil. The module in the dizzy can accept a coil with a primary resistance of 0.6 Ohms. However, would there be any tangible difference if I fitted a coil with a primary resistance of 1.4 Ohms?

I would be fairly sure that the amplifier specification is to ensure that the resistance of any chosen coil should not be less than 0.6 ohms - thereby ensuring it can handle (switch) a maximum raw DC current of about 20 amperes from a 12 volt battery.

If so, then the amplifier will work just fine switching a coil with 1.4 ohm resistance, taking, as it will less current (approx 8.5 DC amperes) from the battery.

However, the coil resistance should not be determined by the capacity of the amplifier, but by the demands of the ignition system, in particular the maximum HT voltage - typically in the range, 10-20,000 volts.

Ideally you should source a coil based upon the engine or ignition system manufacturers' recommendation.

Then, as long as the coil primary resistance is not less than 0.6 ohms, the amplifier will be able to handle the applied load.

Expressed another way, you will not damage the amplifier, if using a coil with a primary resistance of 1.4 ohm.

If you have the stomach for it try studying the two essays on the components and testing of a typical period ignition system, here:

http://www.vintagemodelairplane.com/pages/Download...

http://www.vintagemodelairplane.com/pages/Download...

[ian_uk1975]

Thanks for that very detailed reply!

However, I'm really trying to understand if there would be any performance penalty by using the higher resistance coil? The motor is 10.6:1 CR and revs to 6,500, so nothing particularly taxing for the ignition?

[honestjohntoo]

ian_uk1975 said:

I'm really trying to understand if there would be any performance penalty by using the higher resistance coil?

It really depends upon the type of ignition/starter circuit that you have.

I'm going to assume that you do not have a ballast resistor in your ignition circuit (which would be switched out of circuit during cranking) so your circuit would be something like this



In that case (because the battery voltage usually falls to about 9 volts during cranking) the coil will see just 9 volts during cranking and 12 + volts when cranking ceases.

To get a really good spark whilst cranking, there has to be more current circulating in the coil primary circuit to generate sufficiently high voltage during that phase.

If you fit the 1.4 ohm coil, the current during cranking will be too low to generate a good spark and the engine may fail to catch, particularly when the engine is cold.

I'm thinking, therefore, that you really need the 0.6 ohm coil when there is no ballast resistor in the system.

All the above is based on the electrical phenomenon of 'induction' detailed in the articles mentioned, whereby the higher the primary current then a higher the secondary HT voltage will be attained.

So if your 3.5 lump is expecting 10 - 20,000 volts to generate a spark, then reducing the primary current by approx half will reduce the output by half.

I understand that it will be pretty hard to go back to whatever whatever ignition circuit was installed with your engine and try to find out what the original coil recommendation was.

One more thing by the way. You could decide to experiment with the 1.4 ohm coil, and in my opinion, you will NOT cause any damage to the amplifier nor any of the secondary circuit components.

I'm hoping I got this right, if not someone will be along soon to correct me!!!

[Mr2Mike]

ian_uk1975 said:

Thanks for that very detailed reply!

However, I'm really trying to understand if there would be any performance penalty by using the higher resistance coil? The motor is 10.6:1 CR and revs to 6,500, so nothing particularly taxing for the ignition?

The higher resistance primary coil is likely to require a longer dwell period to reach the specified primary current. Most OEM electronic ignition systems automatically control dwell time to take into account battery voltage variations etc. Problems will arise if the system can not provide enough dwell time to reach the specified primary current as the RPM rises. A V8 at 6000RPM has a firing event every 2.5ms, so it doesn't give a lot of time to charge a single coil.

[84Dave]

If the electronic module has an input impedance/resistance of .6 ohms, there would be a maximum transfer of energy if the coil primary resistance/impedance was .6 ohms. If that impedance is mismatched, the ignition system will have less energy transfer, which can harm ignition performance. -Dave-

[Mr2Mike]

84Dave said:

If the electronic module has an input impedance/resistance of .6 ohms, there would be a maximum transfer of energy if the coil primary resistance/impedance was .6 ohms. If that impedance is mismatched, the ignition system will have less energy transfer, which can harm ignition performance. -Dave-

Ignition coils don't work like that because it's a purely inductive load. The energy that can be stored in the coil is inherently limited by magnetic saturation.