Thanks for that input, guys.  I have to tell you that going from a 20k-volt to a 40k-volt coil has resulted in a car that starts much, much quicker and easier when cold and when hot.  I'm not noticing any difference in HP or torque, but the quicker start was well worth the $25 (+/-) outlay for the PerTronix coil--I tap the key and the engine fires immediately and idles as it should.  And I like the idea that PerTronix designed this coil (hopefully after some serious R&D) to run with my PerTronix Ignitor original ignition.  They also make a 45k-volt coil designed to run with their PerTronix Ignitor II ignition.  I will probably increase my plug gap to .040 tomorrow and see if I notice a change in performance.  

I'm running .040 on my '55, 272, 465 holley, it runs and starts fantastic. I'm leaving it right there. Just pulled all the plugs after a summer of driving, they look great, tailpipes too. Gary

       

Gary, 55 Vicky, St. Germain, Wisconsin

 

The delivered coil voltage is more a function of the resistance in the combustion chamber than the dielectric rating of the coil itself. Probably still only using about 10,000/12,000 volts to jump the plug gap. Even a 100,000 volt coil rating wouldn't ever drive the voltage that high with an inductive type ignition.

If the coil were being powered by a CD type ignition that uses the coil as a "step-up" transformer (they put 400+ volts on the primary) - then you could get a charge high enough to stress the coil insulation.

Ted has dyno results that show the smaller plug gaps made more engine power - even with a very powerful ignition. That being the case, why stress everything with the big gap?

 
 Steve Metzger       Tucson, Arizona

I have a 312 that's bored .040 over, Blue thunder intake, mild cam from John Mummert, and Pertronix Ignitor (the original, not the II or III).  I just recently installed the Pertronix 40k volt, 1.5 ohm "Flame Thrower" coil and I'm wondering if I should continue running Autolite spark plugs gapped at .035 as I did with my old 20k volt coil, or should I try widening the gap to about .040 now that I've doubled the coil voltage?  The car is running good, but if there's room for improvement in power, gas mileage, idle/start under hot conditions, etc., I'd like to give it a try.  I'd be interested in hearing from others who have this combination and have experimented with plugs and gap sizes.  Thank you, Ron.