Heat Range: Heat range deals ONLY with the temperature of the insulator of the plug (the white part). We typically only care about the temperature of insulator, as that is what gets hot enough to pre-ignite, and consequently, that’s what gets measured.
Gap: Gap has a lot to do with ignitability. The bigger the gap, the better combustion. HOWEVER, this comes at a cost. The bigger gap requires MORE voltage to spark, putting a much higher strain on your ignition system. With a turbo charged motor, the gap is usually smaller. This is due to the fact that higher cylinder pressures make it harder for the plug to fire, thereby increasing voltage requirements (this is why 2.5L NA is 0.044″ gap and the 2.5L turbo is 0.030″ gap). Can your ignition system handle the increased load?? Good question.
Corrugations: Ever wonder why those ribs are on a spark plug? They serve to increase the di-electric strength of the spark plug. The high voltages of modern ignition systems could cause an arc, known as flash-over, from the terminal of the plug to the metal shell of the plug. The corrugations essentially increase the surface distance from the tip to the ground of the plug.
Multi Ground strap plugs: Ever wonder what good the multiple ground strap plugs, such as bosch platinum +4, are good for? These plugs have a bit longer life and a higher fouling resistance. However, they can have a decrease in ignitability, reducing performance slightly.
Knocking and Preignition: These are SEPERATE events. Knocking occurs when the combustion flame happens much too early, causing the flame front to “slap” the piston. Preigntion occurs when the spark plug (or something else in the cylinder) gets too hot and ignites the air/fuel mixture before the spark does. This can happen only a couple of degrees before the spark plug fires, so it is possible to have pre-ignition and not know it. Pre-ignition can, and usually does, eventually lead to knocking.
Resistor (The “R” printed on the insulator) Many spark plugs have a special conductive glass seal between the center electrode and the terminal stud. This seal acts as a resistor in the plug which reduces the transmission of pulses of energy to the ignition cables. These pulses can cause Radio Frequency Interference (RFI) with electrical components in the car. For some newer cars, resistor plugs are required for effective communication between the plugs and the electronic ignition.
Radio Frequency Interference (RFI): When the spark plug creates a spark, a high frequency burst of energy is created. If this energy was to travel through the ignition wires, it could cause interference with other sensitive electronic devices, such as the radio or electronic control units. The resistor in the spark plug reduces this energy before it causes interference with other electrical components.
Fouling: Fouling occurs when a plug becomes contaminated with fuel, oil, or other contaminates that prevent the plug from generating a spark. Most plugs today are designed to reduce fouling and become self-cleaning when they reach 500 degrees Celsius. However, short trips, low speed driving, improper spark plug heat range (too cold), improper timing, or an oil leak in the combustion chamber can cause a spark plug to become fouled.
Bridging: Over time, contaminants can build on the surface of the spark plug if it does not reach self-cleaning temperature often. These buildups can grow between the electrodes until they are actually connected by a bridge of contaminants. This will often cause misfire.
Flashover: Occurs when the spark does not jump between the electrodes within the combustion chamber, but instead jumps between the metal shell and the terminal on top of the plug. This will always cause a misfire since the air/fuel mixture will not be ignited
Quenching: The purpose of a spark plug is to introduce enough heat into the combustion chamber to initiate a smooth burn of the air/fuel mixture. Quenching occurs when that heat generated by the spark is reabsorbed back into the ground electrode, the center electrode, and the ceramic insulation.
