This questions seems to come up, so I pulled together a few references from my library of engine builds. Selecting a spark plug is not rocket science. The factory already knows what plug to use and your Factory Service Manual
should have this. The problem can be when you go to the auto parts store and the guy behind the counter isn't and "old" guy who knows these old cars or if the computer screen listing pulls up all kinds of different brands, designs, and prices and you and the parts guy are now more confused than ever as to what the heck to pick. But, this article is not on brand or design, but heat range and those things to consider IF you want to experiment from the factory recommended plug. You probably won't ever need to go 1 or 2 heat ranges different from the factory plug.
The term "hot" or "cold" is a reference to a spark plugs heat range. Normally, a hot plug is used in a low compression, cold engine, whereas a cold plug is used in a high horsepower, hot engine. The terms "hot" and "cold" refer to the thermal characteristics of the spark plug to transfer heat from its firing end, or the electrode, into the engine's cylinder head. All a hot or cold plug means is how hot the core temperature of the plug (or running temperature of THE PLUG) will be. It has nothing to do with combustion temperature, but the combustion temperature does have an effect on the spark plug. If you run too hot of a plug, it will allow it to retain too much of the combustion heat and it could begin to melt off the electrode. If you run too cold of a plug, it will not retain enough combustion heat and it could begin to foul the tip. Running a heat range or 2 cooler may aid in a cooler insulator (not cooler combustion chamber) where heat is your enemy and a cooler insulator could absorb just enough heat to make a difference as long as the plug still burns the air/fuel mixture and does not lead to fouling out of the plug.
The insulator tip of a spark plug is usually the hottest part of the spark plug. Its temperature is related to both preignition, and fouling. The length of the spark plug's nose and the electrode's alloy material are the primary factors that establish the heat range of a particular spark plug. Hot plugs will have a longer nose length which provides a longer path for heat transfer from the electrode to the head. Cold plugs will have a shorter nose which provide a shorter heat path for heat transfer from the electrode to the head.
A "hot" heat range plug with its longer path of heat transfer is used where combustion chamber temperatures are low. The plug transfers heat at a slower rate to avoid spark plug fouling and a more complete burning of the fuel. Spark plug fouling of the insulator tip is most likely to occur if the tip temperature drops to around 600 degrees.
A "cold" range plug with its shorter path of heat transfer is used where combustion chamber temperatures are high. The plug transfers heat rapidly to avoid overheating (melting) of the electrode or cause the electrode to become hot enough to act as a glow plug and cause preignition. Preignition is likely to occur at combustion tempertures of 1750 degrees and above.
As an example, the ideal combustion chamber temperature for a racing engine is 1350 degrees.
Heat range selection consists of selecting a plug which will keep the engine balanced thermally between fouling (too cold) and preignition (too hot) at all engine RPM's and under all driving conditions. There are five important factors that influence the heat range selection of a spark plug:
. The higher the compression ratio (cylinder pressure), the higher the combustion temperature will be.
2.) Spark Advance
. Spark advance timing has one of the greatest effects on spark plug temerature. The greater the advance at BTDC (Before Top Dead Center), the higher the combustion chamber temperature and cylinder pressure. Too great a spark advance can lead to detonation.
3.) Air/Fuel Mixture
. The A/F ratio can produce a "rich" or a "lean" mixture. A rich mixture has a larger fuel volume in relationship to its air volume. A lean mixture has a larger air volume in relationship to its fuel volume. An overly rich A/F mixture can rob power and foul plugs, but a slightly rich A/F mixture can offer a margin of safety as it absorbs heat from the air and cylinder surfaces that can offer detonation protection at full throttle and satisfy the leanest cylinder due in part that not all cylinders receive the same A/F mixture ratio. A lean A/F mixture can be dangerous because they burn more slowly and require a longer time to conduct heat away from the combustion chamber, plugs, and the piston crown. A lean A/F mixture invites preignition and detonation. (Note: I had to question a leaner A/F ratio as burning slower, as I thought it would burn faster. However, what I find is that it takes longer for the fuel to burn because there is less of it and because of the slower burn the engine temperatures rise because the ignition of the fuel is slower, and the slower burning fuel transfers more heat to the surrounding parts of the engine - the opposite of what rich A/F ratio does with regards to absorbing heat)
. Octane rating is a measure of "knock resistance." It makes a difference in the burning rate of the fuel. The higher the octane number, the higher its resistance to engine detonation. The lower the octane number, the quicker the detonation and preignition will occur. The first line of defense against detonation is high-octane fuel.
5.) Operating RPM and Temperatures
. The general operating RPM of the engine can be a contributing factor in the heat range needed. High speed versus low speed driving may require a different heat range selection. Stop and go, local driving, highway driving, or all out full throttle blasts can each have an effect on what heat range plug you may need. Engine running temperatures as well as the outside temperatures of the geographical area you live in may also require different heat ranges.
Detonation will hammer a hole in the top of your piston. Preignition will melt a hole in th top of a piston. Detonation is that "pinging" sound you typically hear, but can be easily masked by a loud exhaust system or open headers. Detonation is the uncontrolled burning of the fuel after the spark plug has fired. It is the violent collision of flame fronts within the combustion chamber. The combustion chamber should provide for a smooth flame front that burns evenly.
Preignition is when part of the combustion chamber reaches the point of incandescence (hot enough to glow) causing the A/F mixture to ignite without the aide of a spark plug. Sharp edges of a combustion chamber that hang over into the cylinder after it has been milled can be such a cause. This is why it is always a good idea to use a fine grit sandpaper to smooth out any sharp edges found on the combustion chamber or even the valve reliefs found on piston tops.
I have personally never played around with spark plug heat ranges. I have always used whatever the factory plug was and then adjusted my timing to get out any detonation if I had any. I have however tried different brands, designs, and electrode materials as new ones seem to pop up all claiming to be "the best." It seems I always fall back to whatever the factory recommendation is.