Internal combustion engines (excluding diesel engines)
This bulletin will explain what projection is and the reasons for it.
Projection is the distance the center electrode extends past the base of the metal shell. The ground electrode, extended metal shell, and gap are irrelevant to the projection value but will affect overall length of the spark plug. The purpose of the projected spark plug is to center the spark further into the combustion chamber to improve ignitability and overall performance of the ignition process. Take extra caution when considering a plug with more projection as it will extend further into the combustion chamber. See the illustration below. Take note as to the projection into the cylinder the projected tip spark plug has compared to the non-projected tip. A spark plug with excessive projection can cause piston to spark plug contact causing severe engine damage.
On modified engine applications projection is a very important specification to pay attention to. Modified engines using power adders such as nitrous, supercharger, or domed pistons require a spark plug with reduced projection. Projection does not affect the heat range of the spark plug but will affect the heat dissipation of the spark plug tip. Spark plugs can have the same heat range but have a different projection value. Using a projected tip plug in a nitrous, supercharged or an engine with domed pistons can cause the spark plug tip to overheat or possibly contact the piston and essentially melt causing damage to the spark plug and possibly creating costly engine damage.