Ignition Coil Resistance

[codfishface]

Sunday morning question for those playing at home.

I recently ditched my EST ignition for a HEI setup and all is good.

Just got hold of a Pertronix Ignition Coil and it is rated at 1.5 ohms primary resistance.

The old Bosch unit is rated 0.5 to 0.7 ohms primary (Haynes Manual).

Will twice the resistance give me a weaker or stronger spark?

Will my dissy assplode?

Should I not even install it and go to the pub instead?

[ari666]

lol. hei units dont use a balist resistor. thats the only resistance i have any clue about in regards to coils. if you are talking pole to pole resistance then you have lost me completely and shouldve just gone to the pub

what ya really wanna know is their capacatance (spelling?) cos thatll tell you if you get a better spark.

other than that i have no clues bro. sorry

[codfishface]

No worries ari, I'm onto the "no ballast" on an Electronic Ignition setup.

Local publican didn't get his usual donation coz I was googling with a pint of home brew.

Found this at a BMW motorbike forum -



"Coil resistance
________________________________________
I researched the issue of coil resistance and its influence on ignition systems some time ago:

Earlier points ignitions systems used coils with coil resistance in the order of 1.5Ohm to 4Ohm or higher: dual coils from Honda Goldwing (2.2Ohm) or Dyna Dual Coils (green 3Ohm, grey 2.2Ohm or black 5Ohm) as well as the highly praised HD -83A with 2.3Ohm.

High energy/low resistance coils with 0.5Ohm to 1.5Ohm are used in modern electronic ignitions such as Dyna or BMW's stock electronic ignition used from 1981 onwards: dual coils such as BMW 2V Dual (0.6Ohm), BMW 4V Dual, Dyna Dual (blue 0.7Ohm or brown 1.5Ohm), Andrews 6377B etc

BMW Airheads coil resistance history looks as follows:

1969-1980 - 2 single coils with a combined 2.2Ohm coil resistance on a contact ignition
1981-1983 - 2 single coils with a combined 1.2Ohm resistance on an electronic ignition
1985-on - one dual coil of 1.2 Ohm resistance on an electronic ignition, later replaced by a dual coil with 0.7 Ohm.


So running the HD coil with 2.3Ohms in an electronic ignition circuit would not harm the ignition, but restrict the current flow in the primary circuit in a way that the spark created through the secondary circuit might be a bit weaker compared to using a high energy coil. It will work though as we all know.

Here is some more physics related information:

An ignition coil is an autotransformer with a high ratio (approx 100:1) of secondary to primary windings. It works as an inductor: when the primary winding is connected to 12V (the battery), it is "charged" and within a couple of seconds a magnetic field is built up as a result of the current. The strength of this field is proportional to the current. The maximum current is determined by battery voltage and contact resistance, thanks to Mr. Ohm.

(1) I = U / R

Where I is Current, U is Voltage and R is Resistance.

This charging is not instantaneous and the time largely depends on the windings' resistance and inductance.

Since primary and secondary coil "share" some of the windings, a voltage of a couple of thousand volts is created at the output during "charging", not enough to create a spark though.
The way to put a long wire into a magnetic field is to wind it into a coil and concentrate the magnetic field through the coil. So here you have it: your ignition coil.




A shorter wound wire is used to charge the coil via electromagnetism, and a long wire to create the 10,000s of Volts to get a spark. Here another physical law applies thanks to Mr. Faraday: If you place an inductor (a piece of wire) in a CHANGING magnetic field then a voltage will be produced: the longer the wire, the shorter the change, the higher the voltage.

A contact breaker in form of points or electronics changes the field rapidly.

(2) V = L x dI

Where V is Voltage, L is Inductance and dI is the rate of change of the current.

The abrupt change in current will produce a very short, very voltage spike. The change of current is on the primary side as well, but is the secondary side where 10,000s of volt are created. Again, less current is flowing on the primary side the higher the coil resistance on the primary side.






So, the lower coil resistance (HD -83A) allows a smaller maximum current compared to a high energy coil with 0.6Ohm as per (1). As a result, less current change produces a spark with a lower voltage as per (2)-> call it a weaker spark. But it is still a couple of kV, therefore it still works! "



So i guess,

Higher resistance = weaker spark = cooler running coil.

I'll throw it in during the week and see how it goes.