is there a simple way to test a ignition coil? Options

[rmital]

got a box full of them. want to use a good one. not interested in buying a new one "if" I have 5 good ones on my shelf.

thanks..... (IMG:style_emoticons/default/flag.gif)

[Tom]

Just thought I should throw this out there.
A simple DC resistance test will only test the primary and secondary windings under not running conditions. Coils will and do break down under heat and electrical load. Many folks get really discouraged when testing ignition systems on cars that won't start or run right and when doing the spark plug outside the engine test get a spark. Well there are sparks and there are SPARKS. A strong blue spark is preferred.
I remember back in about 1970 taking a gas engine course and during the ignition part the instructor showed us an engine that had what seemed to be a good spark. Would not start! The class went thru the entire electrical troubleshooting procedure to try to get the engine to fire. Finally after trading out the coil for a new one, it fired and ran fine. The old coil was putting out a spark that was slightly weak. I wish I could remember why the weaker coil produced what to us looked like a good spark but would not fire under compression. I am sure he explained it, I just can't remember. (IMG:style_emoticons/default/sad.gif)
A few years later while playing with RD-350's, I had Don Vesco's shop port and polish the cylinders/pistons, and machine the head for increased compression. The mechanic that performed the work also told me I would have ignition issues if I did not improve the ignition system. When I asked what to do, he said go to K-mart and buy a $6.00 K-Mart coil and install it and widen the plug gap by 0.010 inches. Well I did what he said and it fired just fine. Boy that bike was fast for it's time!!!
Maybe Jake or one of the engine builders could tell us more on why coils break down under higher compression. I have heard of lots of guys that had a great running car only to rebuild it with higher compression pistons and then have issues with the stock ignition system.
Tom

[stugray]

Maybe Jake or one of the engine builders could tell us more on why coils break down under higher compression. I have heard of lots of guys that had a great running car only to rebuild it with higher compression pistons and then have issues with the stock ignition system.
Tom

Here is why:
Voltage breakdown in air (or any gas) is when a voltage potential can arc across a gap.
The bigger the gap, or the higher the pressure, the more voltage is required to jump the gap.

(disclaimer - numbers are approximate for sake of discussion)
The voltage for breakdown in air with 0.7mm gap at 1 ATM pressure (14.7 PSI) is ~3400 Volts
An engine with a static CR of 8:1 will compress the air to 8 ATM (~118 PSI) which is ~19,000 Volts
An engine with a static CR of 10:1 will compress the air to 10 ATM (~147 PSI) which is ~23,000 Volts

So to get a spark to even start in the combustion chamber with the stock compression ratio, you need greater than 20,000 volts.
As you increase the CR, you increase the voltage required to start the spark.

So if you do not bump up the voltage that the ignition system can deliver, then to get any spark at all with the higher CR, you would need to reduce plug gap.
Smaller plug gaps mean less energy delivered per spark, and performance drops off.

It gets way more complicated after spark ignition, but you get the point.

Regardless of CR, you can increase gap if you have a higher ignition output voltage.
This is why the MSD is so popular.
It allows a bigger gap.