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Discussion Starter #1 (Edited)
Flashover is when the voltage sent to the spark plug does not fire between the center and ground electrodes within the combustion chamber, instead, it shorts between the metal shell and the terminal nut on the plug exterior causing the engine to misfire. This can occur when the air is highly ionized (such as on a stormy day), or when the insulator is dirty. To reduce flashover, some spark plug manufacturers make the top portions of the insulator with 4 or 5 ribbed corrugations. (See the picture below)


To understand quenching and how it is reduced it is first necessary to review the basic purpose of the spark plug - to ignite the air fuel mixture in the combustion chamber. To do this your vehicle ignition system generates tens of thousands of volts to jump the gap between the center and ground electrodes. However it is good to know that it is not the actual electricity that ignites the air fuel mixture, it is the heat energy generated by that electricity or spark. Therefore when you are creating the spark you want as much of the heat from that spark to be used to ignite the air fuel mixture and not have the heat from that spark be re-absorbed by the center and ground electrodes. With a standard, flat ground electrode, the spark can be "crushed", inhibiting its growth, and thus, ignitibility - this is what they call "quenching". (see picture#2) Special center electrode designs such as Denso's U-groove (see picture #3) and NGK's V-power plugs (see picture #4) forces the spark to the outer edge of the ground electrode, placing it closer to the air/fuel mixture. What this does is allow the spark to more quickly ignite the mixture, providing more complete combustion. More complete combustion not only provides more power, it also means less harmful emissions which can reduce the life of expensive catalytic converters and oxygen sensors.
 

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