Choosing a Nitrous Spark Plug: The Missing Manual

Spark plug choice plays a huge role in the information you will receive from looking at the spark plug during tuning. There are many different spark plug brands and styles.

1. Determine if your head requires a gasket seat or tapered seat spark plug. You will also need to know the thread reach of the plug. Different heads require different reach plugs.
2. Determine the brand plug you will be using. The three most common brands are NGK, Autolite, and Brisk. I personally prefer to use NGK when possible. It is an affordable plug and easy to read. For many late-model EFI applications, Brisk may be the only thing available.
3. For nitrous applications, a non-projected spark plug with a nickel-alloy body and copper core is recommended. Non-projected is a term that refers to how far the insulator nose of the spark plug protrudes out of the shell and into the combustion chamber. A projected spark plug has an increased distance from the plug’s center electrode to the plug’s threaded shell, placing the ground electrode further into the combustion chamber. As a result, the projected spark plug has an increased distance that the heat has to travel through the insulator nose dissipating into the head. A non-projected spark plug has a shorter distance from the plug’s center electrode to the threaded shell, placing the ground electrode not as far into the combustion chamber. The non-projected plug will dissipate the heat that is absorbed from the combustion chamber to the head faster due to the shorter traveling distance. The nickel-alloy body is made from a softer material. The nickel-alloy plug does not last as long as a platinum or an iridium plug. However, due to the fact it will melt at a lower cylinder temperature, it is a more forgiving in the event that the tune is off. The copper core allows the plug to heat up quickly while also allowing it to dissipate the heat quickly.
4. Determine the plug temperature you need. The term hot or cold refers to the thermal characteristics of the spark plug. A hotter spark plug will heat up faster from the combustion temperatures, while a colder spark plug will provide more resistance to the combustion cylinder’s heat. Lower horse power engines produce less combustion cylinder temperatures requiring a hotter spark plug in order to maintain optimal operating temperature. Higher horse power engines produce higher combustion cylinder temperatures requiring a colder spark plug in order to resist the heat buildup and maintain optimal operating temperature. If the spark plug is too hot, it can cause pre-ignition, melting the spark plugs electrode and causing engine failure. If the spark plug is too cold, it can cause plug fouling by allowing deposits to build up on the spark plug due to weak ignition spark.

PER NGK: "The operating temperature of a spark plug varies between 450-870°C. At 450°C the spark plug reaches its self cleaning temperature; this means that carbon deposits which are produced during combustion are actively burnt off the insulator nose. When too many carbon deposits accumulate along the insulator nose carbon fouling occurs and engine misfire may occur. If the temperature of a spark plug exceeds 870°C overheating may occur leading to spark plug and possible engine damage."

The spark plugs insulator nose design is usually a great indication to the heat range of the spark plug. The hotter plug will have a long, thin insulator nose, while a colder plug will have a short, thick insulator nose. The long, thin insulator nose will heat up faster and will not transfer the heat to the shell as fast. The short, thick insulator nose will withstand the heat better and transfer the heat to the shell faster. Under most circumstances, the colder the plug is, the less protruded the center electrode will be from the shell.   

The best way to determine what temperature spark plug is needed is to consider the engine's compression ratio, overall horse power level, fuel type, and ignition system. Each of these is a factor to be considered in both naturally-aspirated and power-adder type engines. Compression and horse power generally correlate with one another. However, in a boosted engine, you will usually refer to overall horse power and the amount of boost. Both higher horse power and higher compression ratio engines generate more cylinder combustion pressure and heat. Using the correct fuel type and octane is important because it will also alter the cylinder temperatures. E85, alcohol, and higher octane race gas will require different heat range plugs for the same given horse power and compression ratio than the engine would need on pump gas or lower octane fuels.
(We will cover fuel choice in a later section.)

Spark plug companies have a scale in which they rate the spark plugs temperature.
For example: NGK uses a scale from 2-11, the lowest number being the hottest spark plug, and the highest number being the coldest spark plug.

A stock LS1 5.7 Liter engine requires a base plug temperature of a number 5 temperature rating. As you increase the engine’s naturally-aspirated horse power level, it increases the cylinder pressure and temperature. This is also the case with any type of power-adder application. On average, lower horse power applications will need a step colder spark plug for every 100 HP added. On higher horse power levels, it may need a step colder for every 50 HP added.

A stock LS1 makes in the range of 350HP= #5 plug

Slightly modified LS1 makes in the range of 450HP = #6 plug

Add 100-150 shot of Nitrous = #7 non projected-tip plug

Add 200-250 shot of Nitrous = #8 non projected-tip plug

Add 300-350 shot of Nitrous = #9 non projected-tip plug

Add 400-450 shot of Nitrous = #10 non projected-tip plug

Add 500+ shot of Nitrous = #11 non projected-tip plug

If the temperature of the plug is wrong for the engine combination, it will provide inadequate tuning information. There is no specific chart to base spark plug temperature off of. This is why reading spark plugs during the tune up process is important. (We will cover spark plug reading in a later section)

5. Spark plug electrode gap plays an important role in how well the flame core (spark) ignites the air-fuel mixture. The correct amount of gap is needed in order to have a reliable and accurate flame core between the spark plugs center electrode and ground electrode, which ignites the surrounding air-fuel mixture.
Modified engines with higher cylinder pressures will typically require a smaller electrode gap to ensure ignitability, as the voltage requirement at the electrode gap is increased due to higher combustion pressures. The larger the gap is, the greater the demand will be from the coil and ignition system, eventually exceeding the ignition system’s ability. The ignition system and electrode gap are dependent on one another in order to maintain a strong flame core. If the electrode gap is too small, the electrodes will absorb the flame core’s heat, which will extinguish the flame core and prevent combustion.