Coil-on-plug advantage

[brianl703]

Originally Posted By: Rabbler
With these premium long life spark plugs and low quality fuels I doubt an engine would be running very well after 80k with an older round coil conventional ignition system.


Ford's EDIS systems specified 100k sparkplug intervals when they switched to platinum plugs around 1995.

Also, even though a coil may be capable of higher firing voltages doesn't mean it will always fire at those higher voltages. It takes a certain amount of voltage to jump the gap and once that voltage is reached that's as high as it gets. It's only when the sparkplug is worn that the higher voltage capability of the coil is used.

 

[Donald]

I was also told by a knowledgeable mechanic that newer ignition systems only put out the "required" spark. So (made up examples) if you are idling maybe you get a short quick spark, but if accelerating at 70 MPH you get a much longer or higher voltage spark. This reduces the wear on all components when you do not need a super spark.

 

[brianl703]

Not just newer ones, all ignition systems do.

 

[punisher]

Spark voltage has always been proportional to cylinder pressure. It's at idle when you need the longer duration spark due to low mix velocity/density. It's part of the reason you see manufacturers using the multi sparks (a la MSD) at idle/low engine speeds.

 

[Donald]

Originally Posted By: punisher
Spark voltage has always been proportional to cylinder pressure. It's at idle when you need the longer duration spark due to low mix velocity/density. It's part of the reason you see manufacturers using the multi sparks (a la MSD) at idle/low engine speeds.


But did the old coil and points vary anything to engine needs?

 

[Donald]

I just looked at my 2001 (new to me) Jeep Cherokee 6 cyl and cannot even see spark plug wires or spark plugs. I assume they are under the plastic cover on the passenger side of the engine.

 

[Lazy JW]

Originally Posted By: Donald
Originally Posted By: punisher
Spark voltage has always been proportional to cylinder pressure. It's at idle when you need the longer duration spark due to low mix velocity/density. It's part of the reason you see manufacturers using the multi sparks (a la MSD) at idle/low engine speeds.


But did the old coil and points vary anything to engine needs?


Actually, yes, kinda sorta. But only within the limits of a bonehead simple inductive system.

As noted above, voltage will only rise up to the amount required to jump the gap, then the coil discharges through the plug, producing the spark. Assuming the coil had adequate time to fully saturate, there is only a certain amount of energy available with which to "spark". If more energy is required to jump the gap, there will be less energy available to "push" the spark duration out so the voltage will be higher but the spark will be shorter.

At idle when the dynamic compression is lower the voltage requirement will also be less which leaves more available energy in the coil to stretch out the spark duration, just exactly what we need.

12 volt systems are superior in most every way but I have owned and driven a number of 6 volt vehicles/tractors that ran just fine and poor ignition is not the main reason why car makers switched to 12 volts; large, high compression engines needed stronger starters to crank them up. Most 12 volt breaker point systems limited the voltage down to nearly 6 volts at the coil with a ballast resistor (some increased the internal resistance of the coil) to keep the current down to acceptable levels.