Cracked Coils

[Steve Tinker]

Plug gap can certainly affect coil life...... A coil is basically a transformer - low voltage in (LV), high voltage (HV) out.

If your perfect plug gap is +/- 1.5 mm, the HV will jump the air gap easily causing a nice fat juicy spark.
If you were to set the air gap to double, say 2.0 mm, the HV would have difficulty jumping that size air gap and the spark would be intermittant, causing a misfire, especially when the engine is under load.
Increasing the gap to an even bigger gap of 3.0 mm, the HV cannot jump the distance at all and the coil cannot discharge its voltage which can lead to the coil overheating and eventual breakdown.

Adding coil cracks (full of moisture) to the recipe will cause tracking across the insulator reducing the HV which automatically reduces the strength of the spark - hence intermittant misfires.

One of the advantages of the Porsche specified Beru / Bosch sparkplugs is that they have 4 (sometimes 2) earthing electrodes which helps maintain the set gap to the HV centre electrode.
Of course, if you wear out the centre electrode by running the plugs beyond the recommended intervals, the air gap will increase, but you would (should) have misfires before the coil actually fails.....