Service bulletin. Why does this work?

[Barry Wolk]

If #6 and #8 are too close together, and the wires are, for discussion sake, 1" apart in a square pattern the hypotenuse of a square with a 1" spacing the hypotenuse is 1.4". With the material of modern wires, does this still make sense?

My car was running horribly. I had slow oil leak from a valve cover gasket that was dripping on one of these blocks. It appeared that the wires were shorting to each other through the dirt that had stuck to the oil.

So, were they really correcting the problem, or did the problem correct itself with new wires?

[Mark Norris]

For "induction firing" to ever happen the magnetic field collapse after the spark discharges would have to be appreciable and that could only happen if the current was high (since magnetic flux is proportional the the rate of current collapse). However the average current in the HT cable is very small (in inverse proportion to the winding ratio in the coil). Also the further the second HT cable is from the first then there is an exponential reduction in the flux field and the adjacent positions in the insulator are already much further apart than any HT clips you'll find in a modern car.

I suspect you are correct Barry, despite the source its a myth based on kitchen physics and the problem was in fact cured by the repositioning of the failing HT cables before/after the insulator block or indeed new cables if they were fitted.

I've yet to find a reliable source for "inductive misfiring" due to HT cable proximity ...in every case its far more likely to be arching from one cable to another due to failing HT insulation or more likely arching to ground (ie. the block). In either case the spark energy is compromised and the cable insulation is further damaged.

[Milsteads Garage]

Spark plug wires have come a long way since the original ones our cars were equipped with. Our spark plug wires used to have metal cores which meant the outer sheath did all the work of insulating and it didn’t always work. Remember the whole radio buzzing? That was RFID caused by the high voltage going through the plug wires. With old spark plug wires, you had to keep them all separated and out of the way of one another because as the service bulletin says 8 and 6 fire one after the other and with 2 electrical pulses firing pretty much at once, the spark will jump between wires because the insulation at the time could not always hold the electricity. New technology plug wires are resistance core wires which do a much better job of keeping the electricity in the wire so this service bulletin doesn’t really mean anything if you are using resistance core wires.