Jeep Engines - Oljeep

4.0L POWER TECH IN-LINE 6 (IGNITION SYSTEM) 243The second letter, or the first one before the two digits of theheat range, defines the “reach” of the plug. This is dictatedby the number of spark plug threads in the cylinder head.For most engines there are only three. The “N’ stands for the“long-reach” plug (.750”), the “J” is the “short-reach’’ plug(3/8”), and the combination prefix BL or single V stands fora .460” reach plug. There are special plugs available fromChampion that use 10 and 18 mm threads, also.The two number digit used in the middle is the heat rangeof the plug. The heat range is basically its performancenumber. This is what determines whether the plug is goingto perform at its best in your application. For the momentwe’ll skip over the discussion of heat ranges and return toit later.The first letter to the right of the heat range defines the“nose shape” or configuration. There are three basictypes -regular, retracted, and projected. The projected corenose (electrode) is defined by a “Y.” All our plugs are“Y’s -projected nose.On older, standard design plugs, the last position is blank.Most of the newer plugs use a “C” which stands for “CopperPlus Design.” This copper feature can’t be seen by casuallylooking at the plug, but it allows the plug to be moreversatile. “C” plugs are able to cover more heat ranges or,stated somewhat differently, the “C” allows the plug of agiven heat range to do its job plus the job (performancecharacteristics) of a one or two step colder plug.That brings us back to the heat range. Before we get into thepractical side of heat ranges, let’s cover the theory. Thereare three basic heat range groups that are used for specificapplications. The numbers from 1 to 25 are used forautomotive applications. Numbers 26 to 50 are used foraviation, and 51 and up are used for racing, competition,and other special applications. Since we’re concerned withautomotive racing and performance, we only have toconcern ourselves with the first and last categories.Basic heat ranges are quite easy to figure out-the higherthe number the hotter the plug, the lower the number thecolder the plug. For example, a JlOY plug is colder than aJ12Y. The exception to this is racing plugs. ALL racingplugs are colder than production plugs although they havebigger numbers. Once into the racing plug category, the“lower the number the colder the plug” rule still holds.Why all the fuss about heat range? In performanceapplications, the heat range is key to the plug’s overallperformance. The hotter the plug’s heat range, the better itperforms at low speed. The hotter plug offers betterdriveability. On the other hand, the colder plug offers betterhigh rpm operation. The colder plug performs better in highoutput engines and at W.O.T. (wide open throttle). Coldplugs used on the “street” (low rpm use) will foul quickly.Hot plugs used at high rpm or high output will “miss” andcause the engine not to put out its peak power or allow it toreach peak rpm.So, how do you pick the best heat range? It can be done bytrial and error. DaimlerChrysler racing books give variousrecommendations as to the best performance approach.DaimlerChrysler service and owners manuals specifyproduction plugs. In general, if you don’t have morespecific information available, a performance engineshould use plugs two heat ranges colder than the productionrecommendation. A race engine should be one or two stepscolder than the performance plug.Re-gapping standard spark plugs to a wide gap can be doneeasily with the Accel plug gapping tool. If this tool isn’tused, extreme care should be used when opening up a largeplug gap. The best way is to file down the center electrode.This approach has the drawback of short plug life.Champion makes plugs specially designed for the largergaps. Otherwise, use the Accel tool.