Electrode and Gap Design

Electrode and Gap Design

When selecting the spark plug “nose” configuration, the simple rule to

remember is: The more the spark plug is exposed to the air/fuel

mixture, the easier it is to initiate combustion. Many specialized plugs

have been developed for high-end race cars, but for most dual-purpose

vehicles the choice typically boils down to either regular-gap

(conventional) or projected-nose styles.

The regular-gap plug is the traditional configuration factory-installed on

many classic musclecars. For modern high-performance work, it should

only be used if there isn’t enough clearance for a projected-nose plug.

The latter style “projects” the spark further into the chamber than a

standard plug, and will nearly always offer improved performance if

there is sufficient valve and piston clearance, although many nitrous

oxide users prefer to stay away from them because of excessive heat

buildup in the tip that can cause detonation.

Projected plugs initiate the flame-front closer to the center of the

combustion chamber, which has an effect similar to advancing the

timing. This lets the total ignition advance be reduced, decreasing the

chances of detonation while providing superior throttle response. A

projected plug’s longer core nose provides a hotter plug at low speed to

help prevent fouling. As engine speed increases, the incoming air/fuel

mixture flows across the core nose tip, providing charge cooling that

effectively reduces the heat range for increased top-end detonation

resistance. Today many race cars also used projected-nose plugs, albeit

in highly modified form from the “civilian” versions—the ground

electrodes are often cut back to help improve the flame kernel and

reduce the voltage amount needed to fire the plug.

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