Re: amazingly…cheaper for 4 !!

[David Stanton]

Yes – The laws of magnetic inductance are confusing. I’m a retired electrical Engineer and can confirm there are many factors that will affect current through the points and output voltage and current. Here is a list of all I can remember:

-DC Resistance Primary winding
-DC resistance Secondary winding
Note DC Resistances of copper windings are affected by temperature, and wire size.
– Permeability of the iron core (symbol is greek letter "u" pronounced "mew") Iron cores must be a special steel to have no permanent magnetization carryover – what is known as a square hysteresis curve is undesirable.
– Inter-winding capacitance
– stray capacitive coupling from windings to core
– How close to core saturation the core is driven.
– Mutual inductance of the device (varies some with load)
– inductance of primary
– Inductance of secondary
– stray eddy currents in the core (insulation coatings on iron laminations reduce eddy currents)
– number of winding turns primary
– number of winding turns secondary
Probably the most reliable number is the ratio of number of winding turns in the primary vs secondary for voltage step up. This also affect current via the Amp-turns ratio.

All of these things come together to give complex impedances.

Note inductances are affected by how close to the iron core the windings are wound – affected by insulating spaces and thickness of enamel insulation on the winding wire.

The capacitance of the condensor also affects erosion of the points.

OK – so all this being said, the message here is each manufacturer’s ignition coils will be a bit different because there are just too many variables. So even if you have a you have a lot of lab electronics equipment (impedance bridges, etc,) you have to put them in a motor and see how they perform.

I’ve seen electronic inductors from different vendors that have identical spec sheets perform different in some circuits I’ve designed.
Dave