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digger440 12-04-2012 12:05 AM

How much current can a electronic module handle???
In the cafe build I am doing I was looking at going COP using ford modular motor coils, specifically these: Which have a .66 ohm primary resistance

I read here that 1981 on coils were 6V wired in series with .7 ohms primary resistance

Using ohms law 2 stock coils @ 12V with .7 Ohms primary resistance each, wired in series, yields an 8.7143 amp load

while the aftermarket DIS coils @12V with .66 Ohms primary resistance each, wired in series, yields a 9.09091 amp load

A 0.37661 amp difference, would any electronic guys care to chime in? I know transistors can have severely reduced service life with even just a little more load then they are supposed to have. Now I am not going cross country on this thing, but I do not want to have to keep a spare ignition module under the seat either (this is not a early 90 GM I am building)

StephenB 12-04-2012 06:11 AM

There is more to it than Ohm'sche resistance. Your 2 x 0.7 or 0.66 limits your current to the coils, so with lower resistance comes higher current and eventually higher coil charge using the same dwell time. So I would agree that from that perspective there should be enough margin to run the 6V/0.66 Ohm coils. Don't forget though, that with heat the resistance decreases and at 3000rpm + your voltage is more about 13V than 12V. So its more like 10A current.

Now as to the coil inductance: the coils discharge upon shortening the primary side to ground and that creates a strong temporary force field which in turn creates a high voltage at the spark plugs and a back EMF. The force is dependent on the inductance and charge of your coil and that could be a limiting factor. That high voltage on the output (8KV+) does create a higher voltage at the primary side as well which has the potential of damaging your ignition module when running an electronic ignition or arcing of points when running a Kettering ignition.

I have tried it with different coils, but never long enough to see any long term problem.

(Disclaimer: due to my mental state I can not be made responsible for any damage resulting from using the provided information)

disston 12-04-2012 08:29 AM

You do realize don't you that those are COPs? That is Coil On Plug units. The coil sits directly on top of the plug. It does not have a plug wire or a plug cap in the sense of the systems we are used to. I think they will need some kind of bracket to stabilize them. I do have a Ford but mine is the vintage just before the COPS. I hear the guys with COPs complain about damage from water if they wash the engine. This might be an issue with a motorcycle's electrics out in the wind as it were.

Bill Harris 12-04-2012 08:33 AM

+1. It ain't the resistance as much as it's the inductance.


supershaft 12-04-2012 10:25 AM

In my experience, not much. Run your primary resistance as high as or higher than what the ICU was designed for.

digger440 12-04-2012 03:10 PM

Thanks for the replies, especially StephenB, they are appreciated. With such a simple ignition system I may go ahead and try it. If the stock module poops the bed I will just adapt something a bit more heavy duty in its place.

and disston I am very aware these are COP coils that is in fact why I wanted to use them. I remember seeing that somewhere? Oh it was in the first line of my post, RIF and all that. :doh

But, with all (honestly) friendly sarcasam aside the reason there are problems with these coils and water in thier factory environment is because they sit down inside a hole that is not sealed. the water goes into the spark plug well and gets drawn up the boot through steam and capillary action. if left long enough it will even draw up iron oxide from the plug base making a nice path for the spark to follow to ground outside the cumbustion chamber. People should be more liberal with Dielectric grease when changing plugs it saves a lot of headaches.

I cannot tell you how many of these I changed (or had to dry out) over the years. And yes, I am aware they require a support bracket but I have an Idearrrrrrrr :wink:

RecycledRS 12-04-2012 06:54 PM

That last statement kind of begs for some follow up pics when completed :wink:

digger440 12-04-2012 08:07 PM


Originally Posted by RecycledRS (Post 20179660)
That last statement kind of begs for some follow up pics when completed :wink:

I have been a total photo-whore with this bike so trust me when I build it I will post pics. Click on the link in my sig to see what I have done and where I am going with it.

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