Using spark plugs to tune race engines has been done as long as stock car racing has exist
If you consider your spark plugs little more than disposable engine parts to be replaced for every event or two, you're missing out on a valuable engine-tuning tool. Not only can the correct plug choice help your engine run efficiently, spark plugs are also a fantastic diagnostic tool. The ability to "read" a set of plugs is an old-school skill, but it is still invaluable at the track.
One tuning factor inherent in spark plugs that is largely misunderstood is what's known as a plug's heat range. This isn't the amount of spark output, but rather the amount of heat a spark plug dissipates from its core. A hot plug uses a large insulator nose area to absorb heat from the combustion process and hold it, while a cold plug has a smaller insulator area and is designed to dissipate the heat absorbed from combustion into the head and cooling system.
This is important because no internal combustion engine is perfectly efficient. Worn oil rings will allow oil into the combustion chamber. For example, the air/fuel charge in the combustion chamber doesn't burn completely, or all of the burnt residue may not be evacuated from the chamber during scavenging. Regardless of what it is or where it came from, there is going to be carbon buildup in the combustion chamber over time. When that carbon sticks to the surface of the valves or the piston top, it is no big deal, but a spark plug is much more fragile, and buildup can cause fouling. That's why plugs are designed to operate at a certain temperature range (usually 500 to 900 degrees C) while the engine is running. At this temperature range, the plug becomes self cleaning and burns off any carbon buildup.
Spark plug heat ranges must be tailored to your engine. A spark plug with a high heat rang
The trouble comes when a plug is above or below this range. If it's too cold, carbon will be allowed to build up until it fouls the plug. With the exception of power loss from a dead cylinder, this causes no damage to the engine. If the plug gets too hot, however, you have bigger problems. A plug that is too hot will pre-ignite the air/fuel charge before the plug actually fires. Pre-ignition leads to detonation (otherwise known as knocking), an extreme buildup of pressure in the combustion chamber that can cause severe engine damage. To avoid this, find the spark plug with a heat range that's hot enough to burn off carbon buildup, but not hot enough to cause detonation.
Of course, it's not as easy as going to the parts store and picking up a set of plugs with 500 to 900 degrees C marked on the box. Because there are so many variables involved in racing engines that affect a spark plug's operating temperature, it's difficult to determine which plug will best fit a specific application without testing. One plug will run perfectly in one type of engine but be too hot or too cold in a different engine. Variables that affect a plug's operating temperature include compression ratio, type of fuel used, cubic inches, and combustion efficiency, among other things. You generally want to use a plug that's as hot as possible without causing detonation. Your engine builder should be able to guide you in the right direction, but it's not hard to find out for yourself.
When you are tuning an engine, it's best to always start off with a cold set of plugs because the only damage you are going to do is to the plug, not your engine. Move up one heat range at a time until the plug stops fouling and is covered in a thin gray covering after a long run. If you are confident you can detect pre-ignition early, keep pushing until you detect the start of detonation and then back off one heat range. When testing, be careful not to start with a hot plug. If you start with a set of plugs that are too hot, you can potentially melt an electrode and drop plug fragments into the cylinder, which can hang a valve open or score the cylinder walls. Also, you run the risk of sending the engine into detonation.
Spark plug with ash buildup.
If you read many automotive magazines, you've probably already seen an entire article devoted to plug indexing and how important it is in making power. Believe it or not, that may be true with a '55 Chevy but less so with a modern racing engine. Indexing plugs is a method of ensuring the open end of the ground electrode faces the center of the combustion chamber (the point of the ground electrode is pointed between the intake and exhaust valves). This assures that the flame front moves across the widest section of the combustion chamber before it reaches an obstruction in the form of the chamber wall.
This is important in older, stock engines because it helps make up for inefficient combustion chamber and port designs. Indexing plugs in this situation may help produce an increase of several horsepower, but in a modern racing engine, it should not make much difference at all. The ports and combustion chamber are designed better and are more efficient at burning the air/fuel charge.
If you race a limited stock class that requires OEM cylinder heads and allows nothing more than cutting new seats, you might consider indexing the plugs. If you race a class that allows modern aftermarket cylinder heads, indexing should not show much value. If indexing does significantly help your engine, be aware that it is probably a Band-Aid at best, thus begin looking for the real reason for the engine's inefficiencies.