There are three steps to consider when choosing a race plug.

Step One: Shell Design

Determine the plug type that the cylinder head/piston will accept. Factors to consider are thread diameter and pitch, thread length, shell seat and hex size.

Step Two: Electrode Design

There are a number of factors to consider regarding electrode design and configuration. Should the plug use a fine-wire center or standard electrode? Should the plug have a full-coverage “J-Gap?” Should the choice be a cut-back or an angled ground wire? Should the nose be projected or not? One rule of thumb is to attain as much projection into the cylinder as possible, although in some cases, piston clearance could prohibit the use of projected plugs.

Step Three: Heat Range

Correct heat range is critical to maintaining peak performance throughout the event. A plug that is too hot can result in preignition or detonation. A plug that is too cold could cause an engine to stumble, misfire or foul. Factors to consider when choosing the heat range of the plug include temperature, cylinder pressure, horsepower, fuel delivery (turbo, supercharged), fuel type and piston-to-head clearance.