THE WORLD'S FAVORITE SPARK PLUG.™

TECHNICAL INFORMATION■ The Spark Plug’s Relation To The Ignition System ■The ignition system must be capable of supplying the voltagenecessary to initiate a spark between the spark plug electrodes.In practice this means there must be considerable "ignitionreserve" to compensate for normal wear of the spark plugs andcomponents of the ignition system.Ignition reserve is the difference between voltage available(Va) from the ignition system, and voltage required (Vr) by thespark plug. If at any time Vr equals Va, misfire will likely occur.(Figure 3)VOLTAGE (kV)MAXIMUMIGNITIONRESERVEREQUIRED VOLTAGE CHANGE OVER TIMENEW PLUG USED PLUG WORN OUT PLUGINITIALREQUIREDVOLTAGETYPICALIGNITIONRESERVENOIGNITIONRESERVEMISFIREAvailable voltage (Va)Required voltage (Vr)FIGURE 3■ Factors Affecting Voltage Requirements (Vr) ■COMPRESSION PRESSUREDepending on the type of fuel, moisture content and voltagesource, required voltage increases as pressures increase.IGNITION TIMINGAt TDC (top dead center), voltage requirements will be greatestdue to high compression pressures. An advance in timing lowersVr as pressure decreases. Since the advance creates higher flametemperature, Vr is further reduced by the hotter electrodes.Retarding timing past TDC also decreases Vr as pressuresdecrease, but appreciable losses of power and temperature result.SPEED AND LOADHigh speed and load generally produces the highest Vr due toincreased cylinder pressures despite the natural increase inelectrode temperatures.ELECTRODE TEMPERATUREVoltage requirements will decrease as electrode temperaturesincrease, and increase as temperatures decrease.ELECTRODE CONDITIONSharp, new electrodes concentrate the spark arcing by offering aneasier path for current flow. As new plugs with well-definedelectrodes wear, an increase in required voltage results.ELECTRODE MATERIALCommonly used electrode materials are platinum, iridium, goldpalladiumand nickel, in descending voltage requirement.GAP SIZEWith other influencing factors being equal, required voltages willincrease as gap size increases.Multi-strike ignition: While multi-strike ignition can bebeneficial for igniting the air-fuel charge, particularly in leanburn applications, it also accelerates gap wear. To minimize wear,use the fewest number of ignition strikes necessary to achievecomplete combustion.COIL POLARITYPolarity is a design feature of each ignition system. Current flowfrom center electrode to ground electrode is referred to as"negative polarity." "Positive polarity" is current flow from theground electrode to center electrode. Most all manufacturersrecommend negative polarity for best spark plug life andperformance, since reversed or positive polarity will increaserequired voltages and substantially reduce ignition system reserve.FUEL/AIR RATIOWith other factors being equal, voltage requirements will be lowestat the ideal stoichiometric fuel/air ratio. Voltage requirementsincrease with leaner mixtures because air is a poor conductor.TURBOCHARGINGTurbocharging increases the voltage requirement because ofhigher cylinder pressures.25