Ignition coil, how good?

[Earl Svorks]

  When the time comes to look at your, or any ignition coil , first you might establish whether or not it's windings are intact. If they are and the unit seems to make some spark, the next question is just how good or strong is that spark. There are a few things that can cause a poor coil output, not always the coil itself. Normally it's difficult to test a coil in a dynamic mode. I like a 3 inch piece of an 8 gauge spoke with the threaded end stuck into the plug cap. With the engine running you can let an arc pass to the end of the spark plug and see how long a spark can be had at whatever RPM. This is of course a comparative value and it helps if you've done this test on 50 different bikes so you know what is typical.
   To test the coil off the bike , or to compare the output of one coil to another, or simply to evaluate a coil for it's ability to produce good secondary voltage at elevated RPM, you could go with something like what I have made.
  With this I can track RPM, input voltage and current draw. Output voltage via arc length. I have recently added a bit of vintage automotive instrumentation with which I can read the actual secondary output voltage.  I can turn the 8 cyl  distributor at up to 1440 RPM, that equates to over 11000 crankshaft RPM 
That is an aftermarket  coil from a VW that is shown  It makes more voltage than your average stock MC coil at 32KV. This would jump a gap of 25mm.  I was  surprised to find the best performance from any of the several stock MC coils I have tested,( including a couple of individual coil over spark plug units) was a coil from an FJ This was good for 15mm.  I will retest using the meter recently acquired to see  actual voltage reading.
 

[Pat Conlon]

Thank you Simon, very interesting.
What primary voltage were you feeding the FJ coil? 12 volts or 13, 14?

The reason I asked is, with the coil relay mod where a dedicated circuit is run from the battery to the coils, in my case the voltage on the coils can be as high as 14.5 volts (I have a Transpo adjustable VR set at 14.5)
Before the coil relay mod, I was only seeing around 10.5 volts at the coils.(voltage drop thru the oem wiring harness)

Folks with the coil relay running the oem voltage regulators can see even higher voltages at the coil, of over 15 volts. (same as charging voltages)

I wonder how big of an impact higher primary voltage has on the coil spark gap performance?

Using equal source voltage, I wonder how the stock FJ coil would compare with a Dyna 3 ohm green coil?
The Dyna coil supposedly being 40kV output:
http://4into1.com/dynatek-3-ohm-green-coils-dual-output-dc1-1/

[Earl Svorks]

   Hey Pat,
  40kv, that is a lot of power! As it now is, I'm just using a large MC battery, from a Wing or maybe Sportster I think it's visible in one pic. As long as it has a reasonable charge and will maintain 12 plus volts I'm happy. I do have the option of a ballast resistor (1 ohm I think) that can be used in series with the feed. In any case I think the coil output when primary voltage is lower than 12 is at least as important as what it makes @15 plus.
About the most critical time when a hot spark is really needed but seldom available is when you are starting from dead cold after not running for a week.  I was taught that  battery cranking voltage must not go below 9.6 V. Over the years I have seen many cold no start problems in automotive, for that matter, any engine on gasoline with electric start using points or electronics whose troubles were rooted in the ballast resistor bypass circuit.
Cranking voltage at the ignition coil(s) is perhaps the most critical issue in the ignition system at startup. It's also very often overlooked or missed  when trying to diagnose
hard starting symptoms.  It is also very easy to cure with nothing more than one simple diode and a foot or 2 of wire.
   Cheers
  Simon

[PaulG]

I'll bet you built this in your backyard too!  That'll keeps the little brats off your lawn!   


https://www.youtube.com/watch?v=FY-AS13fl30


Wonderful to see there are still some mad scientists alive and well out there.     Even though I don't understand a whit of it.