How can a spark plug's "heat range" be determined?

The heat range of a spark plug is primarily determined by its ability to transfer heat away from the combustion chamber to the engine component it is installed in. The characteristics of the spark plug can influence how heat is managed.

The diameter of the spark plug can indeed play a role in its heat range. A spark plug with a larger diameter typically has more surface area, which can allow for better heat dissipation. Conversely, plugs with smaller diameters might retain more heat in the combustion chamber, influencing the performance and efficiency of the engine. The design and construction elements, such as how the spark plug is engineered, contribute to its thermal performance, which is closely tied to heat range.

While the other factors—length, color, and electrode material—can provide some information about a spark plug, they do not directly denote heat range effectively. Length affects installation compatibility rather than thermal characteristics, the color can indicate combustion conditions but not the heat range itself, and the electrode material influences wear resistance and performance but is separate from heat management considerations. Thus, diameter serves as a key physical specification linked to the heat range classification of a spark plug.