Spark Plug Folklore

Disclaimer: Direct from ipd’s Tech Tip archive!  This tech tip contains information from previous publications.  Products mentioned may not be available or the information may not be accurate due to changes in supply, manufacturing, or part number association.  Please contact ipd Customer Support if you have further questions.

Spark plugs are one of the more simple parts of today's vehicles and yet still are misunderstood. Heat range, firing voltage, and durability are all important factors in vehicle performance so correct spark plug application is critical.

Heat range:

Hotter plugs have longer insulator noses, which have greater surface area in contact witl the combustion gases. This keeps the plug temperature higher overall which is preferred for extended stop and go or idle conditions, a hallmark of city driving. The shank of the plug will tend to be shorter, less material to sink away heat. Hotter plugs do not generate more or greater spark. Ideally spark plug temperature for the average vehicle should run between 550-800 degrees C. Timing has a direct effect on plug temperature as well, 10 degrees of advanced timing can up plug temp by 100 degrees C.

Colder spark plugs have shorter insulator noses, which have a lesser amount of surface area exposed to the combustion chamber gases. Generally colder plugs are used for racing applications or engines with turbochargers, supercharges, nitrous, etc since they can transfer the greater amount of heat these engines produce from cylinder combustion to the head quicker. A colder plug will help to reduce ping or detonation, although the colder the plug the greater the risk for fouling or buildup during extended light throttle or idle conditions. The shank of the plug will tend to be longer, more material to sink away heat. Colder plugs do not have less or reduced spark.

Firing voltage:

Firing voltage, otherwise known as ionizing voltage, is a complex beast. Many factors affect the firing voltage including, cylinder temp, cylinder pressure, ignition system components wear, humidity, barometric pressure, and fuel mixture to name a few. New iridium spark plugs available from various manufacturers are designed to require a lower firing voltage. This translates into lower load on the ignition system and increased life span of components. Iridium also resists electrode oxidization and erosion to provide consistent performance during its' life span. Performance engines demand a high quality plug to survive the harsh cylinder conditions associated with turbos, superchargers, nitrous, etc. Many tuners are using iridium with very noticeable results justifying the greater expense for these parts.