Author - Mike BumbeckRxSpeed Published - Aug 14, 2017
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.
