The electrode gap of a spark plug dictates the spark magnitude. An incorrect electrode gap may affect engine performance as the spark magnitude may be insufficient to ensure complete combustion of the air-fuel mixture.
The electrode gap is meticulously controlled when a spark plug is manufactured, and due to the very large selection of plugs, in most cases the electrode gap is set to suit the vehicle as recommended in our catalogue. In some rare cases however, a specific electrode gap may be required for some vehicles which is not commonly manufactured. As such the electrode gap may need adjustment, and it is always best to quickly consult you owner’s manual (Please refer to Electrode Gap Adjustment).
Modified engines with higher compression or forced induction will typically require smaller electrode gap settings (to ensure ignitability in these denser air/fuel mixtures) as the voltage requirement at the electrode gap is increased due to higher combustion pressures.
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Thank you again to NGK for going into detail,most of the NGK plugs supplied to me are pre-set by your manufacturers and almost all the plugs set to correct specification to vehicle.