Updated: Nov 20, 2019
Since there seems to be a bit of a taboo behind spark plugs and heat ranges, I'm going to hopefully shed some light on the situation for everyone! From mopeds to lawn mowers, this will cover what makes a spark plug "hot" or "cold" along with a data graph on the numbered NGK Heat ranges. What is a Spark Plugs Heat Range?
Spark plug heat range refers to the efficiency in which a spark plug can transfer heat from within the combustion chamber to the engine head. No matter where the spark plug is to be installed, moped,motorcycle or chainsaw, studies have shown that the best temperature for combustion to occur is at 500°C–850°C (932F - 1562F) . In those designated temperatures the combustion chamber will remain cool enough to prevent pre-ignition (knocking) or plug tip overheating, all while singeing and removing the deposits left behind by the combustion process that cause plug fouling. What is a fouled spark plug? A spark plug becomes fouled when the insulation around the center electrode becomes covered with oil, carbon or fuel. The contamination of the insulator creates a path of less resistance allowing the voltage (spark) to travel through the gunk back to the ground, rather than bridging the gap to the side electrode.
Tons of things can contribute to spark plug fouling for instance your air/fuel ratio may be too rich. Your two stroke mixture being too rich could also foul a spark plug. Your points may be out of adjustment as well, either firing too late or too early, creating a not so ideal atmosphere for combustion. In many cases especially with mopeds, spark plugs just can't be cleaned enough to work properly again.
Are special spark plugs needed after modifying an engine? It depends on the modifications that were performed to an engine. In our case with mopeds we're often changing the displacement, compression ratio, fuel ratios, exhaust, or fuel octanes so these would constitute to a plug change. Mods that usually do not require a plug change, would be a change of silencers, gearing or free flowing air filter. Little mods like these will not change the heat in the combustion chamber, there for the stock spark plug would suffice in this situation.
However, when compression is raised with a big bore kit or a stroker crank, this would significantly raise the heat within the combustion chamber there for requiring assistance from the spark plug to remove more heat. A colder heat range spark plug with a smaller plug gap to ensure proper fuel ignition will occur, will ease your fears of melting through pistons or the dreaded seized engine! Often a hotter heat range is not usually recommended except when severe oil or fuel fouling is occurring. What do the numbers on the spark plug mean? The random numbers and letters found on the side of a spark plug identify the features of a plug, such as plug type, heat range, build, thread length and diameter along with the construction of the firing end. NGK Heat ratings go the opposite as to what you would think, the lower the number the hotter the plug, the higher the number the colder the plug. What are the NGK Spark plug ranges?
This table provides a great reference for Cross referencing of NGK, DENSO, Champion and BOSCH spark plugs and mapping spark plug temps with their numbers. In the end no moped or engine will be exactly the same, so certain builds may not perform as well in Colorado as they do in Vermont. So each build should be treated on a case to case basis to achieve the perfect plug. If you have any questions or think I may be incorrect please feel free to share in the comment section or feel free to reach out!
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