Why Spark Plug Wire Boots need to be Fastened Securely
Back in the day, “the day” being when gasoline was $0.32 per gallon and we changed ignition points at each 10,000 mile tune-up, ignition systems were not as high strung as they are today. When reading the voltage on a running engine using an oscilloscope one could expect to see voltage spikes in the 10,000 to 25,000 volt range, as described in “Automotive Diagnosis and Tune-Up” by Guy Wetzel, published in 1965. Modern ignition systems are capable of two or three times that voltage.
Spark plug voltage is a function of the voltage required to bridge the spark plug gap. That is, when it is triggered the magnetic field in the ignition coil collapses and discharges, the voltage rises until it is high enough to jump the gap. Therefore the measured voltage will be whatever was required to complete the circuit.
The environment in the combustion chamber of pre-EPA regulated engines was less demanding than it is today. Fuel mixtures were richer and less contaminated by exhaust. Fuels themselves were laced with tetraethyl lead (TEL) which slowed the burn rate and permitted greater ignition advance, higher compression ratios and prevented knock. These conditions combined to make it easier for the spark to jump the plug gap compared to today.
Modern combustion chambers are more tightly controlled. Today’s fuel mixtures are leaner to reduce emissions and exhaust gas is deliberately introduced via Exhaust Gas Recirculation (EGR) in order to control emissions of nitrous oxide. Testing described by Obert in his book, “Internal Combustion Engines” demonstrated a marked increase in voltage requirement when lean fuel mixtures were tested. Wider spark plug gaps were added to prevent lean misfires which increased the voltage requirement further.
With this background consider the modern ignition system, whether a coil-per-plug system or a distributor type of system. Sufficient voltage must be delivered to the plug gap in order for ignition to occur. But electrical voltage is a lazy animal and will seek the easiest pathway to ground. To force the voltage to find ground through the spark plug gap the entire system must be secure.
Starting with the spark plug, the spark plug insulator itself is the first line of defense. Most spark plug insulators are designed with annular rings from connection point to the steel jacket to prevent voltage from “flashing” along the surface to ground.
The connection point of the plug wire to the spark plug is frequently located in a tight area, surrounded by the cylinder head, valve cover or exhaust manifold. The voltage will find it much easier to reach ground through the aforementioned components than jump the spark plug gap.
Oil, grease and moisture will provide an excellent path for voltage to leak to ground and not jump the gap if permitted to collect on the spark plug surface. Moisture will also cause the formation of corrosion at the connection to the spark plug.
In recent years a new method of knock detection has been developed. ION sensing was developed by Delphi Automotive and has been marketed to many auto and motorcycle manufacturers. The system depends upon a good connection at the spark plug to detect the onset of knock and correct it by retarding spark advance as required. The system was described in a Delphi Automotive white paper and a Harley Davidson service manual.
The lowly spark plug boot is an important component of the secondary side of the modern ignition system. The spark plug boot provides a level of insulation between the spark plug connection and its surroundings. When properly installed the spark plug boot seals out the environment and maintains the critical, clean connection between the plug and plug wire. With today’s computer controlled engine systems, secure electrical connections throughout are critical.
