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Spark Plug Choices

by John T. Elle; courtesy of Mopars Unlimited of Arizona

There is probably more spark plug information based on emotion than there is room for in a year's worth of newsletters, but to keep it simple without the need for an alchemist, let's talk spark plugs. I might mention that I prefer Champion spark plugs and most of my data on this subject is Champion based data. If your preference is another brand, that is okay, all spark plugs have to do the same job and for the most part have to meet the same objectives.

Let's start with the first component, the shell of a spark plug. It is a threaded metal hex that is used to seal the combustion chamber and provide a means to remove and install the spark plug.

The second component of a spark plug, the insulator, has two functions. First, it insulates the secondary ignition voltage from grounding anywhere except across the gap in the combustion chamber. Its second function is to move the heat picked up in the combustion process into the cooling system.

The last major component of a spark plug is the electrodes; these are designed to conduct the spark from the spark plug wire into the combustion chamber and to provide a gap. Surprisingly enough, these definitions have not changed through the years are almost word for word identical to the same data printed in 1908. The materials will have been improved along with manufacturing but the objectives are the same.

The place to start for the correct spark plug is the application charts for your car. A quick look at the Champion application chart for Chrysler cars shows a good percentage of them take the RN12YC or 14 or 17. This indicates a resistor plug, 14mm thread, ¾ inch reach, a projected tip and copper electrodes. The numbers indicate heat range. The heat range is selected based on a duty cycle of driving over mixed road conditions and approximately 10,000 miles a year and tip temperatures measured at wide open throttle and idle. When we talk about the heat range of a spark plug, we're referring to its ability to move heat away from its tip or core nose into the cooling system. A cold spark plug would have a cooler tip temperature than a hot one. With today's fuels, we know that any time the tip of the ceramic core nose goes below 850° F, carbon will build up and the spark plug will foul. We also know that if the tip temperature of the plug exceeds 1550°, the metals will begin to break down. At approximately 1700°, the plug will glow and can become a source of pre-ignition within the combustion chamber.

Armed with this information, it becomes clear that maximum performance can be achieved with a spark plug that has a temperature of greater than 850 at idle, but no more than 1550° under wide open throttle. There are a number of variables that can affect the heat range requirement but, simply stated, modified-for-performance engines and engines that are driven at higher speeds more often can use a colder than recommended plug. I use RN9YC in my engines and the plugs last seemingly forever and the car will pass smog so for me, that is the correct plug to use on a factory recommendation of N12Y [if you have enhanced your engine's performance or normally drive on the highway]. If your engine spends a lot of time at idle and is burning a bit of oil, a slightly hotter plug than recommended will allow the deposits to burn off and avoid misfires, but it may cause detonation under conditions of wide open throttle causing pinging or spark knocking.

Today's spark plugs use resistors to eliminate radio frequency interference (RFI) which can affect on-board computer systems, AM and FM radio reception, TV broadcasts, and even airplane communications. Each time the spark jumps across a gap, an electromagnetic field is created that can interfere with radio signals, by placing a resistor in the spark plug, we can substantially reduce RFI. Furthermore, the use of non-resistor plugs with today's complex computer systems can result in drivability problems, loss of performance and can even cause the computer to store trouble codes. The same can be said about using resistor style spark plug wires.

If your choice of spark plugs makes extravagant and wondrous claims I would suggest you pass on them. Plugs are very basic in their function and properly selected will do it well. It would take a very specific application spark plug to do something different and then only with a trade off as to the normal basic function.

We strive for accuracy but we are not necessarily experts or authorities on the subject. Neither the author nor / Allpar, LLC may be held responsible for the use of the information or advice, implied or otherwise, on this site. This page is offered "as is" and without warranties. By reading further, you release the author and Allpar, LLC from any liability.

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