Posted by Alan Bowes on March 20, 1998 at 10:00:21:
In Reply to: Spark Plugs Again posted by T.Kralovic on March 18, 1998 at 23:50:12:
This won't give you an instant answer, but I believe that it is still the best approach for plug selection:
The best way to eliminate most trial and error is to get an original plug and match the length of the thread and the reach (the distance that the electrode extends beyond the end of the plug). That will at least ensure that the spark is in the correct location. As to heat range, try a set of plugs for a while and then check them. If they are fouling, you could go to a "hotter" plug. If they are burning, go to a "colder" plug. This assumes, of course, that your mixture is correct and that you are not burning oil or losing compression. What I mean by "hotter" and "colder" is this: A hotter plug has a wider and deeper open area or pocket between the metal wall of the plug and the ceramic insulator for the electrode. This allows more of the combustion flame to circulate around the insulator, hence the electrode/insulator run hotter. A colder plug tends to have a narrower and/or shallower pocket around the electrode/insulator, which allows less flame activity around the electrode/insulator, and it allows more heat transfer into the metal plug wall and cylinder head, hence the electrode/insulator run cooler. There are other schemes, such as electrode diameter, that can affect heat range, but for the most part, the above should apply.
By the way, a plug with a longer reach (where the tip extends out farther from the plug) tends to run hotter, but do NOT use the reach length as a method for selecting a hotter plug. If you change the reach, the spark will be in the wrong location for optimum ignition. If you need a hotter or cooler plug, do it right and get one with the same reach, but with more or less space around the electrode/insulator. This should be easy, since usually only one digit of the plug number will change, and any good parts person should know which one represents hotter or cooler.
Alan