Controlling the operating temperature of the plug’s firing tip is the single most important factor in spark plug design. “Heat range” is the relative temperature of the spark plug’s core nose, and it is determined by the length and diameter of the insulator tip, as well as the ability of the plug to transfer heat into the cooling system. A “cold” plug transfers heat rapidly from its firing end into the cooling system and is used to avoid core nose heat saturation where combustion-chamber or cylinder-head temperatures are relatively high. A “hot” plug has a slower heat transfer rate and is used to avoid fouling under relatively low chamber or head temperatures. What’s confusing is that a “hotter” (higher performance level) engine requires a colder plug because more power equals higher cylinder temperatures.
Critical factors affecting heat range include:
• Air/fuel mixture: Lean air/fuel ratios raise cylinder-head temperatures, requiring a colder plug. Rich air/fuel ratios require a hotter plug to prevent fouling. Mixtures that cause the plugs to read lean may contribute to pre-ignition or detonation. If not running an electronic engine management system, it pays to tune slightly on the rich side to avoid detonation.
• Spark advance: Ignition timing has one of the greatest effects on plug temperatures. It becomes more critical as compression ratios increase. More timing raises combustion temperatures, calling for colder plugs.
• Compression ratio: Increasing the mechanical compression ratio raises cylinder pressure, resulting in higher cylinder temperature. The higher the compression ratio, the colder the spark plug needs to be. According to Champion Spark Plugs, for normally aspirated, gasoline-fueled engines, a good rule of thumb is to go about one heat range colder for each full point in compression ratio increase from 9:1 through about 12.5:1, and two heat ranges colder for each point increase between 12.5:1 and 14.5:1. Beyond 14.5:1, 3-4 heat range reductions per point may be needed.
• Gasoline quality: With musclecar-era leaded gas, the lead is attracted to the hotter (core-nose) part of the plug, causing glazing. The spark runs down the core nose instead of jumping the gap. Going to a slightly colder plug helps prevent lead-glazing. However, with today’s cleaner-burning oxygenated unleaded gas, an equivalent engine needs to run plugs about 1-2 heat ranges hotter than originally specified (many plug manufacturers have revised their catalogs accordingly).
• Methanol: Methanol has a higher octane level compared to gasoline (allowing an increased compression ratio), contains 50 percent oxygen by mass (requiring a much richer air/fuel ratio), and has a reduced latent heat of evaporation (which cools the incoming air/fuel charge and allows a denser mixture). The net effect is to require a plug that’s at least one step colder than normal for an equivalent gasoline-fueled application.
• Nitrous oxide: N2O raises cylinder temperatures and may require a plug 1-2 heat ranges colder. Lower output street systems may get by with standard heat ranges if nitrous use is held under 10 seconds.
• Supercharging/turbocharging: With increased pressure and temperature in the chamber, two or more heat ranges colder may be needed. Extreme high-boost race-only applications may need a surface-gap plug.
• Sustained acceleration: Prolonged acceleration or high-speed driving