If there is a component on a Pontiac engine that gets little or no thought, it's the spark plug. We refuse to spend much money or time on them, but unless your Pontiac is powered by a GM diesel, the engine won't run without one in each cylinder. The spark plug has a paramount impact on the performance of the engine, especially in a high-powered Pontiac.
Function
The spark plug introduces ignition energy into the combustion chamber and ignites the air/fuel mixture between the electrodes. It is positioned at the point most suitable for igniting the compressed air/fuel mixture. It must introduce the ignition energy into the combustion chamber without developing a leak and without overheating under all conditions. It must also enable reliable cold-starting, prevent misfires, and endure long periods of operation.
When the engine is running, the spark plug absorbs heat from the combustion gases with every firing. It is then exposed to the much colder air/fuel mixture on the next induction stroke. So the spark plug must endure continuous heat cycling. On a cold first-start, the tip of the spark plug is either very near or at ambient temperature. In some parts of the country, this can be as low as minus 40 degrees F on a winter day. But as soon as combustion begins, the tip and electrode are exposed to a flame of around 1,000 degrees F, a swing of 1,040 degrees F almost instantly! The spark plug must hold up to this thermal shock over tens of thousands of miles of operation.
Spark plugs must also withstand the mechanical demands created by the pressure in the cylinder bore while maintaining a gas-tight seal. This requires strength, particularly of the ceramic, which is subjected to mechanical stress during installation as well as operation. The spark-plug shell must absorb the tightening forces of an over-zealous mechanic without permanent distortion. If that weren't enough, the part of the spark plug projecting into the combustion chamber is exposed to the high-temperature chemical processes inside a cylinder bore.
When the spark plug is connected to a GM electronic-ignition system, energy flow of up to 30,000 volts bridges the gap between the two electrodes. It is essential that the plug resists insulator arcing. Under certain temperature conditions, the deposits resulting from the combustion process (e.g., soot, carbon residues, ash from the fuel and oil additives), will conduct electricity, but arcing or breakdown across the insulator under such conditions must be prevented, even at the high-induced ignition voltage. The electrical resistance of the insulator must perform up to 1,000 degrees C, with only minimal deterioration over the course of the plug's life.
Spark Plug Construction
Spark plugs consist of metal, ceramic, and glass, and each material has different properties. Spark plug design makes full use of the positive properties of these materials. The terminal stud, insulator, shell, and electrodes represent the main elements. The steel terminal stud is melted gas tight into the insulator with a special conductive-glass seal, which also serves as the electrical connection to the center electrode. On the end projecting out of the insulator, the terminal stud then attaches to the secondary or high-voltage ignition wire.
Made of a special ceramic material, the insulator separates the center electrode and the terminal stud from the shell. The dense micro-structure of the ceramic ensures a high resistance to electrical breakdown. To limit flashover or an unwanted electrical path, the surface of the connection side of the insulator is glazed to reduce moisture and dirt.
The insulator houses both the center electrode and the terminal stud. Demands placed on a spark plug contrast with those of most insulators. Though each manufacturer may use a different insulator material, most use aluminum oxide with small amounts of other materials. After it has been heated and glazed, the ceramic insulation can perform under the requirements of an ignition system.
Made of steel, the shell secures the spark plug to the cylinder head. The upper part of the shell is hexagonal for wrenching during installation, and the lower section of the shell is threaded. An electroplated nickel coating on the surface of the spark-plug shell prevents corrosion, keeps the threads free, and prevents seizing. This is especially important in aluminum cylinder heads where the threads are easily damaged.