My parents have a 64 e type coupe and a 67 e type convertible. They were both running NGK plugs. Last year both cars were converted to pertronix ignition due to repeated issues with the points on the distributors. Instantly both cars felt like they had more zing and the engines picked up a lot quicker. Then the 64 started to sound like it was missing. Since I had recently installed the pertronix, I started trouble shooting the timing, plug wires and firing order. Everything checked out. I took it to our local classic car mechanic and he put an oscilloscope on it. He found cylinder 3 was not firing well and others were not optimal. He pulled the cylinder 3 spark plug and it was wet. He cleaned it and put it back in and the car instantly ran better. He then said we should run champion RN12YC. He explained the electrode protrudes further into the cylinder so the spark won't jump. Sure enough I bought a set and put them in the 64 and it runs brilliantly now. I got a second set and put them in the 67 and it runs even better than it did before (and it was running ok before).

So the argument for the Champion RN12YC is the electrode goes deeper into the combustion chamber and therefore has better and more controlled detonation. I will not be going back to the NGK plugs. It's amazing how much of a difference the plugs have made. For 9 months I've been trying to figure out what was going on with it. Just needed $18 worth of plugs and $100 worth of knowledge (he didn't charge that much, but he earned it).

 

It sounds like you went to a "hotter" plug which is a bit longer than a "colder" plug. That's not a Champion vs. NGK thing but rather the proper plug for the new ignition system. Not sure why your Pertronix would require that but it obviously does!

 

Yes, definitely a hotter plug. The NGK I used was a recommendation I found on this site or one of the other forums. Seemed ok at first, so didn't have a reason to suspect the plug wasn't right for our cars. Still, it seems fixed now, so I'm happy.