Marinette Boat Forum

I used AC plugs R42XLS.

Ditto here. A text book written by Dr. Chris Jacobs PHD, the engineer that founded Jacobs ignition systems, had this advice about spark plugs. I'm paraphrasing here, but it is basically boils down to the 3 biggest spark plug manufacturers used in domestic V8's. These 3 have very different properties based on the type of metal used for the base of the plug, i.e., the alloy of that metal. The first are Champions, which use a soft metal. Chryslers used Champ's from the factory. The advantage of using the soft metal is that they will usually fire no matter how fouled they get. The disadvantage is they wear out faster, in that the electrode on the bottom burns off at a faster rate over the life of the plug. Champ # in the proper heat range is RN9YC. Next in line Dr. Jacobs recommends are AC Delco. The metal part is in between, wears good (long life) and fires properly as long as everything else is in order (good spark and properly tuned carb) These are what I use, the R42XLS. Finally are AutoLite's. The metal part is very hard, can take a lot of abuse (pinging for example) and they last a long time. Disadvantage is they won't fire well unless your engine is in perfect tune. Dr. Jacobs book was published by HP Books, publisher of many auto, hot rod and performance texts. Gets into the science and engineering principals, but breaks it down into layman's terms. Good read. Not sure if it's still in print, Summit Racing used to sell it. The year of your engine depends on the spark plug gap and proper timing, having to do with the different fuel systems used ('79 and below used Carter carbs, '80 and up used GM Rochester carbs). Hope this helps.

Thanks
Bill Schneider