Modern engine management and ignition systems help contemporary internal combustion engines make more power than ever but that 20-horsepower banger from 1908 and 300-plus horsepower turbocharged screamer from last week both use remarkably similar looking spark plugs to complete the combustion hat trick of fuel, air, and spark. The ingeniously designed spark plug has kept the horsepower fires burning for well over a century. Choosing the right spark plug for peak power and efficiency starts with an understanding of its primary electrical and secondary thermal dual purpose.
Spark: Primary Ignition
The primary function of the spark plug is to channel voltage through its center and produce a spark to ignite the fuel and air mixture. Voltage controls the strength of the spark. Ignition timing sends the voltage through the spark plug so that it bridges the gap with a spark microseconds before the piston reaches the top of its compression stroke. The resulting burn pushes the piston back down in the cylinder. Horsepowers are made.
Power: Operating Temperature
The secondary function of a spark plug is to transfer heat away from the combustion chamber and through the metal it is screwed into. The heat transfer rate is often referred to as the spark plug heat range and leads to talk and arguments of cold and/or hot spark plugs. Heat range is crucial. The correct heat range brings the spark plug tip up to an ideal self-cleaning operating temperature between about 900 to 1450 degrees and keeps it there so carbon and crud burn away.
Cold and Hot: Foul or Melt
Cold spark plugs transfer heat quickly. Less surface area is exposed to combustion, so more heat is transferred into the cylinder head. The electrode side of a cold spark plug heats up more slowly than that of a hot spark plug. Using a spark plug that's too cold in heat range can cause crud and carbon to build up on the electrode, which can result in misfires or no spark jump at all as the spark plug is not hitting the temperature it needs to clean itself.
Hot spark plugs transfer heat more slowly. Greater exposed surface area in the combustion chamber is subject to more heat. The electrode side of a hot spark plug heats up more quickly than a cold plug. Using a spark plug too hot in heat range can result in excessive temperature, detonation, melted electrodes and pre-ignition. Detonation is bad. Pre-ignition from red hot electrodes prematurely igniting the fuel and air mixture is piston melting levels of worse.
Spark intensity is a function of ignition and voltage and has nothing to do with hot or cold heat ranges. A hot spark plug does not produce a hotter spark. Screwing in a set of hot spark plugs with hopes for a hotter spark is an incredibly bad idea. See above. Turbocharged or supercharged engines that generate increased combustion chamber heat from more atmosphere and fuel can benefit from a colder spark plug. As a general rule the experts at NGK suggest running one heat range colder spark plugs for every 75-100 horsepower added. Step up the ignition system to find a hotter spark.
Choice is Good
Pondering spark plug selection reveals a vexing array of electrodes, materials and surface types from the standard copper core single electrode to octo-tipped spark plugs promising horsepower from nano scale space technology. Back In reality semi-surface, multi-electrode and race-spec spark plugs are designed for a narrow range of operation. Indexing and electrode shape can affect combustion but don't expect giant performance miracles.
If your engine and ignition system were designed to take advantage of the 100,000 mile durability offered by iridium, platinum or similar rare metal tipped spark plugs then spend some scratch and get the best. If your ride left the factory with standard spark plugs then rare and expensive metals offer no benefit. Manufacturers and the folks in the white lab coats that designed the engine in your ride make choosing the right spark plugs easy. Year. Make. Model. Engine. Done.
Racecar you say? The spark plugs in a nitro-burning Top Fuel dragster or funny car might survive 1000 feet and a few seconds before melting into header plasma. In between the horrors of nitromethane and the daily driven appliance are the rest of us. Mild mods don't call for anything other than the spark plugs your ride left the factory with. A few steps into the cold heat range might lead to a foul but too far into the hot range can result in engine destruction.
Heat range ratings vary by manufacturer. NGK spark plugs have a low number for hot and high number for cold. Bosch are rated the other way around. Spark plugs are usually pre-gapped but consult the service manual for the correct electrode gap and check conventional spark plugs with a feeler gauge if required. Exception! Do not gap iridium, platinum or similar noble metal spark plugs. That stuff is ready out of the box - especially if that box says "do not gap".