Spark plugs have been around as long as internal combustion engines have, and are often a misunderstood component.  This basic guide is designed to assist the technician, hobbyist, or race mechanic in understanding, using, and troubleshooting spark plugs.

Spark plugs are a "window" into the engine, and can be a valuable diagnostic tool. The spark plug displays the condition inside the combustion chambers of the engine. The experienced tuner can use spark plugs to find the root cause of problems, determine air-fuel ratios, and to increase vehicle performance.

Spark Plug Basics
The primary function of the spark plug is to ignite the air/fuel mixture within the combustion chamber under any operating condition.

Spark plugs must provide a path and a location for electrical energy from the ignition coil to create a spark used to ignite the air-fuel mixture. A sufficient amount of voltage must be supplied by the ignition system to spark across the spark plug gap. This is called “Electrical Performance.”

The spark plug firing end temperature must be kept low enough to prevent pre-ignition, but high enough to prevent fouling. This is called “Thermal Performance”, and is determined by the heat range selected.

NGK Spark Plugs Heat Rating

The spark plug heat range has no relationship to the electrical energy transferred through the spark plug. The heat range of a spark plug is the range in which the plug works well thermally.  The heat rating of each NGK spark plug is indicated by a number; lower numbers indicate a hotter type, higher numbers indicate a colder type. 

Heat rating and heat flow path of NGK Spark Plugs

Some basic structural factors affecting the heat range of a spark plug are:

• Surface area and/or length of the insulator nose
• Thermal conductivity of the insulator, center electrode, etc.
• Structure of the center electrode such as a copper core, etc.
• Relative position of the insulator tip to the end of the shell (projection)

The major structural difference affecting the heat rating is the length of the insulator nose.  A hot type spark plug has a longer insulator nose.  The insulator nose of a hotter spark plug has a longer distance between the firing tip of the insulator, and the point where insulator meets the metal shell.  Therefore, the path for the dissipation of heat from the insulator nose to the cylinder head is longer and the firing end stays hotter.  The insulator nose of a hotter spark plug also has a greater surface area that is exposed to more of the ignited gases and is easily heated to higher temperatures.  A colder spark plug functions in an opposite manner.

The heat range must be carefully selected for proper spark plug thermal performance.  If the heat range is not optimal, then serious trouble can be the result.  The optimal firing end temperature is approximately between 500°C (932°F) and 800°C (1472°F).  The two most common causes of spark plug problems are carbon fouling (< 450°C) and overheating (> 800°C).  

Causes of Carbon Fouling:

• Continuous low speed driving and/or short trips
• Spark plug heat range too cold
• Air-fuel mixture too rich
• Reduced compression and oil usage due to worn piston rings / cylinder walls
• Over-retarded ignition timing
• Ignition system deterioration

Pre-delivery fouling

Carbon fouling occurs when the spark plug firing end does not reach the self-cleaning temperature of approximately 450°C (842°F).  Carbon deposits will begin to burn off from the insulator nose when the self-cleaning temperature is reached.  When the heat range is too cold for the engine speed, the firing end temperature will stay below 450°C and carbon deposits will accumulate on the insulator nose.  This is called carbon fouling.  When enough carbon accumulates, the spark will travel the path of least resistance over the insulator nose to the metal shell instead of jumping across the gap.  This usually results in a misfire and further fouling. 

If the selected spark plug heat range is too cold, the spark plug may begin to foul when the engine speed is low or when operating in cold conditions with rich air-fuel mixtures.  In some cases, the insulator nose can usually be cleaned by operating the engine at higher speeds in order to reach the self-cleaning temperature.  If the spark plug has completely fouled, and the engine will not operate correctly, the spark plug may need to be cleaned / replaced and the fouling cause identified.           

Causes of Overheating:

• Spark plug heat range too hot
• Insufficient tightening torque and/or no gasket
• Over-advanced ignition timing
• Fuel octane rating too low (knock is present)
• Excessively lean air-fuel mixture
• Excessive combustion chamber deposits
• Continuous driving under excessively heavy load
• Insufficient engine cooling or lubrication

The most serious result of selecting a heat range that is too hot is overheating.  Overheating will cause the electrodes to wear quickly and can lead to pre-ignition.  Pre-ignition occurs when the air-fuel mixture is ignited by a hot object/area in the combustion chamber before the timed spark event occurs.  When the spark plug firing end (tip) temperature exceeds 800°C, pre-ignition originating from the overheated insulator ceramic can occur.  Pre-ignition will dramatically raise the cylinder temperature and pressure and can cause serious and expensive engine damage.  When inspecting a spark plug that has experienced overheating or pre-ignition, blistering on the ceramic insulator and/or melted electrodes can sometimes be found. 

As a general guideline, among identical spark plug types, the difference in tip temperature from one heat range to the next is approximately 70°C to 100°C.

Tip Temperature and Firing End Appearance

Some factors to consider in selecting the proper heat range spark plug

There are many external influences that can affect the operating temperature of a spark plug.  The following is a brief list to consider in avoiding reduced performance and/or expensive engine damage.

Engine Speed and Load

• If the engine is to be operated at high RPM, under a heavy load, or at high temperatures for long periods a colder heat range may be needed.
• Conversely, if the engine is to be operated at low speeds or at low temperatures for long periods, a hotter heat range might be needed to prevent fouling.

Air-Fuel Mixture

• Excessively rich air-fuel mixtures can cause the plug tip temperatures to decrease and carbon deposits to accumulate, possibly causing fouling and misfires.
• Excessively lean air-fuel mixtures can cause the cylinder and plug temperatures to increase, possibly resulting in knock and/or pre-ignition.  This may cause damage to the spark plug and/or seriously damage the engine.
• If an air-fuel ratio meter or gas analyzer is not available, it will be necessary to visually inspect the spark plugs frequently during the tuning process to determine the proper air-fuel mixture.

Fuel Type / Quality

• Low quality and/or low octane fuel can cause knock which will elevate cylinder temperatures.  The increased cylinder temperature will cause the temperature of the combustion chamber components (spark plug, valves, piston, etc.) to rise, and will lead to pre-ignition if the knock is uncontrolled.
• When using an ethanol blend fuel with high ethanol content in high performance applications, a colder heat range may be necessary.  The spark timing can be advanced further because ethanol blend fuel has a higher resistance to knock (higher octane).  Due to the decreased knock, there will be less audible “warning” from knock before the spark plug overheats and pre-ignites.
  Some types of fuel additives in lower quality fuels can cause spark plug deposits that can lead to misfires, pre-ignition, etc.

Ignition Timing

• Advancing ignition timing by 10° will cause the spark plug tip temperature to increase by approximately 70° to 100°C.
• A colder heat range spark plug may be necessary if the ignition timing has been advanced to near the knock level.  Higher cylinder temperatures near the knock level will bring the spark plug firing end temperature closer to the pre-ignition range.

Compression Ratio

• Significantly increasing the static/dynamic compression ratio will increase cylinder pressures and the octane requirement of the engine.  Knock may occur more easily.  If the engine is operated near the knock level, a colder heat range spark plug may be necessary due to the resulting increased cylinder temperatures. 

Forced Induction (Turbocharging, Supercharging)

• A colder heat range spark plug may be necessary due to the increased cylinder temperature as boost pressure (manifold pressure) and subsequent cylinder pressure and temperature increase.

Ambient Air Temperature / Humidity

• As the air temperature or humidity decreases, the air density increases, requiring a richer air-fuel mixture.  If the air-fuel mixture is not properly richened, and the mixture is too lean, higher cylinder pressures / temperatures, knocking, and the subsequent increase in the spark plug tip temperatures can result.
• As the air temperature or humidity increases, the air density decreases, requiring a leaner air-fuel mixture.  If the air-fuel mixture is too rich, decreased performance and/or carbon fouling can result.

Barometric Pressure / Altitude

• Air (atmospheric) pressure and cylinder pressure decrease as altitude increases. As a result, spark plug tip temperature will also decrease.
• Fouling can occur more easily if the air-fuel mixture is not adjusted to compensate for the altitude.  Higher altitude = less air = less fuel.

Types of Abnormal Combustion

Pre-ignition

• Pre-ignition occurs when the air-fuel mixture is ignited by a hot object / area in the combustion chamber before the timed spark event occurs.
• When the spark plug firing end (tip) temperature exceeds 800°C, pre-ignition originating from the overheated insulator ceramic can occur.
• Is most often caused by the wrong (too hot) heat range spark plug, and/or over-advanced ignition timing.  An improperly installed (insufficient torque) spark plug can also result in pre-ignition due to inadequate heat transfer.
• Pre-ignition will dramatically raise the cylinder temperature and pressure and can melt and hole pistons, burn valves, etc.

Knock

• Occurs when part of the air-fuel mixture in the combustion chamber away from the spark plug is spontaneously ignited by the pressure from a flame front originating from the spark plug.  The two colliding flame fronts contribute to the “knocking” sound.
• Knock occurs more frequently when using low octane fuel.  Low octane fuel has a low resistance to knock (low resistance to ignition)
• Knock is related to ignition timing.  (Knock is sometimes referred to as “Spark-knock”.)  Retarding the ignition timing will reduce knock.
• Heavy knock often leads to pre-ignition.
• Heavy knock can cause breakage and/or erosion of combustion chamber components.
• Knock is sometimes referred to as “ping” or “detonation”.

Misfires

• A misfire occurs when the spark travels the path of least resistance instead of jumping across the gap.  Misfires can be caused by the following:
  
  
  1. Carbon fouling
  2. Worn or deteriorated ignition system components
  3. Too large of gap size
  4. Spark timing excessively advanced or retarded
  5. Damaged spark plugs (cracked insulator, melted electrodes, etc)
  6. Mismatched ignition system components (plug resistance / wire resistance, ignition coils / igniter modules, etc.)
  7. Insufficient coil primary and/or secondary voltage – voltage required to jump the spark plug gap higher than coil output

Source:  http://www.ngksparkplugs.com/