Why do car manufacturers recommend resistor spark plugs?
One reasons is actually emissions. Since the resistor is an obstacle it forces the spark voltage to be higher, assuring combustion in a lean mixture. Also resistor plugs are MUCH cheaper to produce. You will never find resistor plugs in serious race cars, yet these cars use some of the most sophisticated engine management and data acquisition systems. But these cars have no EMI problems. Why? The spark happens inside the combustion chamber where he is completely shielded by the metal cylinder head. No EMI can escape the combustion chamber.
