Spark Plugs

[JoeThaMan]

Spark plugs have two jobs: to produce a very high temperature spark to ignite the air/fuel mixture as quickly as possible.and to fine tune the temperature of the combustion chamber via what is know as the heat range of the plug.

The shape of the plug electrode is important. The sharper the electrodes (firing and ground electrodes) become, the easier it is to reach the plugs "ionization" point, or it's ability to jump the gap. Electricity likes to travel along the path with least resistance, so it is easier for a spark to jump from a sharp point rather a rounded point. The easier it is to reach the plugs ionization point, the more uniform ignition will be.

There is the problem associated with the width of the electrode. The width of the electrode causes an affect called shrouding. The intent is for the spark to occur, ideally, in a a hemispherical pattern. The wider and, hence more shrouded, the electrode, the greater the chance the spark will not be hemispherical and the chance of piston rock and detonation. Thus, the sharper the electrode (i,e., as un-shrouded as the practical), there is the best chance of having maximum contact between the spark and the air/fuel mixture,
making it ignite more easily and the burn spread more smoothly.

Thus, it seems that the sharper the electrodes, the better. This would be true except for two problems having to do with heat. If this tip is very sharp, it will get extremely hot because there is so little metal to heat and there is not enough area to conduct that heat away. Think of it, first, as a needle with its tip in the hottest part of a flame; the tip
will get red hot, but you han hold the "cool end" of the needle for several seconds. Now, place a thick wire into the flame, the heat transfer up the shaft of the wire to your finger will make you release the wire long before the tip becomes red hot. Thus, too sharp of a tip will melt the electrode. If the temperature of the tip reaches the melting point of the metal of which it is made. Below are the melting points of some commonly used metals (Celsius):

Zinc == 420
Aluminum == 660
Copper == 1083
Steel == 1400-1500
Platinum == 1772
Iridium == 2410

At this point, the votes seem to favor iridium. However, there is another factor to Consider. This consideration is the volume of metal at the spark tip. If the temperature even instantaneously reaches the melting point, some of that metal will disappear. You can see that Platinum and Iridium coated plugs can withstand significantly higher temperatures, and thus can have sharper tips than their steel or copper counterparts. If some of the metal does disappear from a very sharp tip, then you've actually opened up the spark gap some. To prevent that from happening, we have to start with a wider tip, such that any small amount that is eroded will not change the size or geometry of the tip by too much. Now, re-read the diatribe above concerning shrouding.

Next, too sharp a tip will create a "hot spot" in the combustion chamber. Even if you don't reach the melting point of the metal, you can still get it glowing hot. If that tip is still glowing red hot when the next compression stroke comes about, that residual heat could
ignite the air/fuel mixture before the spark is supposed to occur. This is pre-ignition. To avoid this, we want a wide area near the tip to conduct as much heat away from the tip as possible. Here are some of the thermal conductivities of some commonly used metals (Watts / centimeter*Kelvin) :

Zinc == 1.16
Aluminum == 2.37
Copper == 4.01
Steel == 0.70 - 0.82
Platinum == 0.716
Iridium == 1.47

One can easily see why Copper is the metal of choice for the core of the spark plug. It's just about the best thermal conductor on earth.

So far, what we conclude it that we want is the sharpest tip possible such that it does not melt the electrode nor does it stay so hot as to cause pre-ignition.

With this introduction, below is a brief discussion of the common plug types:

Bare Copper: Low melting temperature and the tips will vaporize away - they have a very wide tip so each little bit that disappears will not change the gap size greatly, but they still must be inspected often to make sure there is sufficient electrode material left. These plug also wear quicky around the electrode creating rounded edges, making it more difficult for the plug to reach it's ionization point. They are great for very hot running engines which must avoid pre-ignition at all costs since the wide tip will not stay hot - high boost forced
induction and nitrous engines come to mind.

Platinum: Similar in construction to the copper plugs, but with a thin coating of Platinum sputtered onto the electrode tips, about 0.010" thick (a human hair is about 0.005" thick). Because of the high melting point of Platinum, the tips can be made significantly sharper
without fear of the gap changing shape. But the copper core is still sufficient to whisk the heat away fairly quickly. These are great all-around plugs, particularly for use in medium-heat engines (low boost and NA engines) - they should last a very long time. Very high heat engines should probably not use them because the sharper tips may not conduct enough heat away to prevent pre-ignition under adverse conditions.

Iridium: They are much like platinum plugs just with iridium in place of the platinum. Because of the extremely high melting point of iridium, they can have very sharp tips without risk of melting and they should last a very long time. These would be best for high-rpm NA engines where the sharpest tip is needed for the best spark and there is little danger of pre-ignition. However, for a high heat engine, the sharp tips may not conduct enough heat away to prevent pre- ignition under adverse conditions.

A little bit of research by the original poster and the numerous debaters and nay-sayers would have revealed the above information. I do credit Joe Schesso for having presented some data, where as the others are restating hearsay and observations without basis in fact - the best word for this is "conjecture."

Bottomline: plugs require considerable design and should be selected not on one tid-bit of information (conductivity, melting point, shape, etc.) but upon application. Lastly, no matter what plug one selects, always check the gap before installing, always use a
calibrated torque wrench to tighten your plugs (over- or under- tightening is probably the most common cause of spark plug failure), buy from a reputable manufacturer, learn to read your plugs, and check them often.