So which plug gives you the best spark?
At the moment the spark jumps the gap it causes a high frequency burst of energy, known as RFI (radio frequency interference). RFI, as its name suggests, creates static on your radio and interference with other electronic equipment, including the vehicle’s on-board electronic control units (ECUs).
Resistor plugs were developed in the 1960s to suppress some of the spark energy, thus lowering RFI to an acceptable level. Most resistor spark plugs use a monolithic resistor, generally made of graphite and glass materials, to filter the electrical voltage as it passes through the center electrode.
Since resistor type plugs actually “resist” some of the spark energy, non-resistor type plugs actually deliver a more powerful spark. It is for this reason that most racing plugs are non-resistor types. However, in most automotive applications, a resistor plug is required for proper vehicle operation. Use of non-resistor plugs in vehicles that call for a resistor type can result in rough idling, high-rpm misfire, and abnormal combustion.
Generally a resistor spark plug will not cause a noticeable difference in spark energy to affect performance. Where it will matter is when the ignition system is weak - which is not the case anymore with CDI and transistor type ignitions compared to the old mechanical points and whatever they used in OPE before the electronic revolution - or when there is high compression. I'm pretty sure higher compression requires more spark energy compared to lower compression, I don't remember if leaner or richer mixture requires more/less spark energy, relatively speaking.
Spark and Ignition | Basic Knowledge | SPARK PLUG | Automotive Service Parts and Accessories | DENSO Global Website
An explanation of the mechanism of producing a strong spark between terminals to cause combustion of the air-fuel mixture.www.denso.com
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