Spark Plug Heat Range

I know that is something that is not discussed much here so I thought I'd bring it up. I'd like to hear everyone's opinion and practical experience as well as share my own.

My first true experience with the wrong heat range was when I added a 150 shot of N2O to a Ford I built when I was 18 (OK fine, Flame on peckerheads, but this is how experience is gained, lol!). Anyway, I couldn't understand what the little "specks" were on the center electrode insulator when I'd pull a plug after a nitrous run. needless to say, I blew a head gasket shortly thereafter and had to pull the heads. When I did, I noticed that one edge of a few of the pistons looked like it was chewed by a small mouse.

This being the early 80's and widespread use of nitrous was fairly rare and there being no internet as we know it now, I figured it was due to too much advance and backed off 4 degrees. Went back to the strip and promplty comprimised another head gasket. I limped back home with a couple jugs of water and changed the gasket again. The chewing was worse and I had a three or four (my memory fails me, lol!) cracked plug insulators.

I called NOS, and they said "Drop your heat range 2 numbers". End of problems but I still had to re-piston the ford.

So, this being my experience I thought I'd also add what Autolite says about heat range:

Heat range

The term spark plug heat range refers to the speed with which the plug can transfer heat from the combustion chamber to the engine head. Whether the plug is to be installed in a boat, lawnmower or racecar, it has been found the optimum combustion chamber temperature for gasoline engines is between 500°C–850°C. When it is within that range it is cool enough to avoid pre-ignition and plug tip overheating (which can cause engine damage), while still hot enough to burn off combustion deposits which cause fouling.

The spark plug can help maintain the optimum combustion chamber temperature. The primary method used to do this is by altering the internal length of the core nose, in addition, the alloy compositions in the electrodes can be changed. This means you may not be able to visually tell a difference between heat ranges. When a spark plug is referred to as a “cold plug”, it is one that transfers heat rapidly from the firing tip into the engine head, which keeps the firing tip cooler. A “hot plug” has a much slower rate of heat transfer, which keeps the firing tip hotter.

An unaltered engine will run within the optimum operating range straight from the manufacturer, but if you make modifications such as a turbo, supercharger, increase compression, timing changes, use of alternate racing fuels, or sustained use of nitrous oxide, these can alter the plug tip temperature and may necessitate a colder plug. A rule of thumb is, one heat range colder per modification or one heat range colder for every 75–100hp you increase. In identical spark plug types, the difference from one full heat range to the next is the ability to remove 70°C to 100°C from the combustion chamber.

The heat range numbers used by spark plug manufacturers are not universal, by that we mean, a 10 heat range in Champion is not the same as a 10 heat range in NGK nor the same in Autolite. Some manufacturers numbering systems are opposite the other, for domestic manufacturers (Champion, Autolite, Splitfire), the higher the number, the hotter the plug. For Japanese manufacturers (NGK, Denso), the higher the number, the colder the plug.

Do not make spark plug changes at the same time as another engine modification such as injection, carburetion or timing changes as in the event of poor results, it can lead to misleading and inaccurate conclusions (an exception would be when the alternate plugs came as part of a single precalibrated upgrade kit). When making spark plug heat range changes, it is better to err on the side of too cold a plug. The worst thing that can happen from too cold a plug is a fouled spark plug, too hot a spark plug can cause severe engine damage

In conculsion, what brought this up is that I had installed stock heat range plugs in my latest engine without thinking (DUH)! I'm out tomorrow to grab some plugs one range cooler due to my added compression and relatively short duration cam.