Understanding Spark Plugs

understanding spark plugs

A spark plug is a device that delivers electric current from the ignition system in the combustion chamber to ignite the compressed air/fuel mixture with a spark. Spark plugs a metal threaded shell, electrically isolated from a central electrode by a porcelain insulator. The central electrode, which may contain a resistor, is connected by a heavily insulated wire to the output terminal of an ignition coil. The central electrode protrudes through the porcelain insulator into the combustion chamber.


Spark Plug Gap

Spark plugs have a specified spark gap. The same plug may be specified for several different engines, requiring a different gap for each. The gap may require adjustment from the out-of-the-box gap. The technician installing the spark plug can adjust the gap by bending the ground electrode slightly. 

spark plug gap gauge is a disc with a sloping edge, or with round wires of precise diameters, and is used to measure the gap.


The gap adjustment can be crucial to proper engine operation. A narrow gap may give too small and weak a spark to effectively ignite the fuel-air mixture. However the plug will almost always fire on each cycle. A gap that is too wide might prevent a spark from firing at all or may misfire at high speeds, but will usually have a spark that is strong for a clean burn. A spark which intermittently fails to ignite the fuel-air mixture may not be noticeable directly, but will show up as a reduction in the engine’s power and fuel efficiency.

How To Read Spark Plugs

Reading A Spark Plug

The ability to “read” a spark plug can be a valuable tuning aid. By closely examining the insulator firing tip color, you can determine a lot about an engine’s overall operating condition.

Generally, a light tan/gray color tells you that the spark plug is operating at optimum temperature. Dark coloring, such as heavy black wet or dry deposits, can indicate an overly rich condition, too cold a heat range spark plug, a possible vacuum leak, low compression, overly retarded timing or too large of a plug gap.

Wet deposits can indicate of a breached head gasket, poor oil control from ring or valvetrain problems or an extremely rich condition—depending on the nature of the liquid present at the firing tip.

Signs of fouling or excessive heat must be traced quickly to prevent further deterioration of performance and possible engine damage.

Spark Plugs – Explained

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