Author Topic: Ignition coils and a question (or two about spark plugs)  (Read 5384 times)

beachbear

  • Full Member
  • Regular
  • **
  • USA, HI, Kailua
  • Posts: 65
I read in the forum posts about ignition coils as they are posted. The dominant opinion about them seems to be: 'hotter is better'. This is certainly appealing and, as I understand it, true after a fact. A hotter coil should make for a more intense spark across the gap in the combustion chamber. If a manufacturer claims a coil develops 40,000 volts, the fuel/air mixture shouldn't stand a chance.

In one of my careers, I was a trainer for an engine manufacturer, (Outboard Marine Corporation: defunct since 2000) I learned ignition systems operated a little differently. The voltage needed to ionize the combustion chamber and push current across the spark plug gap only required 8,000 to 12,000 volts depending upon pressure and fuel/air load. Without regard to a coil's ability, once the threshold has been reached, the electrical path that makes up the secondary circuit, past the plug gap to ground is going to be made. If it only takes 8,500 volts to jump the gap, then 8,500 volts is all the coil will make. Sharp edges on the plug electrode are good for directing the spark across the gap, oil and carbon soot is bad, etc.

The advantage gained from a Crane or Pertronix kit comes from the increased, reliable time current can flow through the primary without overheating the coil. I don't see anywhere in Crane's documentation that multiple sparks are generated by their system, so I maybe current flows longer across the gap through the RPM range. If we watch the spark cross a gap, I believe our interpretation of a bluer or hotter spark is the result of duration as the potential across the secondary is depleted over a longer time (we see it a split second longer).

I don't find any information from any manufacturer explaining how canister ignition coils develop more potential with the same design used since ... a long time ago. The voltage limit across the primary is the same. Once the coil is saturated with current, the voltage potential doesn't change. The coils themselves are still in an oil bath with the secondary wrapped around the primary which in turn is wrapped around an iron core (I think they're steel plates actually). I'm not claiming that Mallory is selling snake oil. Just that I don't know, and would gladly learn differently if I'm wrong.

Does my understanding of ignition coils make sense? If it does, should the only issue with ignition coils be that they break down over time? An OEM coil is just as good as a Crane PS40 when new is compared to new? I ask because there is a lot of experience within this forum.

My question to those who use a Pertronix or Crane kits has to do with spark plugs. If dwell is constant and reliable, electronic ignitions can push the coil to a higher potential across the RPM band. Do you gap your plugs at .045", or .040" instead of .035"? As I recall, a hotter spark plug means more insulator is exposed to the combustion chamber and/or maybe the plug extends a bit further into the combustion chamber. Which plug do you prefer for use as a hotter plug?
« Last Edit: July 19, 2016, 09:47:44 by Beachbear »
David White
'69 280sl automatic, 3rd owner since '82

jeffc280sl

  • Guest
Re: Ignition coils and a question (or two about spark plugs)
« Reply #1 on: July 19, 2016, 22:10:09 »
Used the Crane system and blue coil.  Worked fine.  Switched to 123 distributor some years ago and its perfect.

hauser

  • Guest
Re: Ignition coils and a question (or two about spark plugs)
« Reply #2 on: July 20, 2016, 03:37:22 »
I'm with Jeff on this one.  The 123 is absolutely the best compared to anything else out there.

Tyler S

  • Associate Member
  • Platinum
  • ******
  • USA, CA, Encinitas
  • Posts: 1613
Re: Ignition coils and a question (or two about spark plugs)
« Reply #3 on: July 20, 2016, 16:46:07 »
Upgrading the coil and removing resistance lets us run more gap and also less likelyhood of ignition misfire due to resistance or fouling.  But this upgrade only works on systems that have had their points replaced with an electronic trigger. The switching is done by a transistor. Running pertronix still requires resistance in the circuit to protect the pertronix module.
1968 (67) 250sl. 4 speed manual. DB180 Silver
1955 220 Cabriolet A. White Grey
2019 E450 Wagon. Majestic Blue
1936 Ford PU Flathead V8. Creme on tan interior.
1989 Volkswagen T3 Westfailia Campmobile. Dove Grey (blue)

Scottcorvette

  • Full Member
  • Gold
  • *****
  • United Kingdom, England, Norfolk
  • Posts: 603
Re: Ignition coils and a question (or two about spark plugs)
« Reply #4 on: July 21, 2016, 05:00:46 »
As I understand it the only purpose of a ballast resistor is to protect the points from damage, a ballast resistor drops the voltage to the points to 9 volts, but there is a bypass wire to allow 12 volts to the coil whilst cranking. On electronic ignition the ballast resistor can be removed as there are no points to hurt, in fact on a Corvette which has a ballasted wire to the coil instead of a resistor, when we fit an electronic ignition we have to give it  anew 12 volt supply otherwise it doesn't run as well.

I had a quick look here http://www.pertronix.com/docs/instruction-sheets/1141.pdf and it says on here for best results to remove the ballast resistor.

I fitted a 123 ignition to my wifes Karmann Ghia a while back, and it was a massive improvement, although it was replacing an old and probably bit worn out distributor, so how much of the improvement is newness and how much is electrickery, I don't know.

We also increase the plug gap from 035 thou on points cars to 040 on pertronix or MSD, factory electronic ignition runs an 045 thou plug gap, can't remember if I changed the gap on the VW or not, I think I'll run the 123 on the merecedes if it ever gets to that stage....

Meiang

  • Guest
Re: Ignition coils and a question (or two about spark plugs)
« Reply #5 on: November 30, 2016, 17:20:48 »
Spark Plugs

Bosch +4 Platinum or Super 4 are the best around (4 electrodes around the centre electrode).  They are self cleaning around the centre electrode which is really important for older engines where mixtrure control is not as precise as modern injectiion/ignition systems during warm up.

When you have an expensive old vehicle like the SL why comprimise on spark plugs.