Spark Plugs

Spark plugs are on of the most misunderstood engine components. Spark plugs are the window into your engine(your only eyewitness into the combustion chamber)and can be used as a valuble diagnostic tool. The spark plug displays symptoms and conditions of an engine's performance.

1. To ignite the air/fuel mixture.

2. Remove heat out of the combustion chamber.

Spark plugs transmit electrical energy that turn the fuel vapors into electrical energy. A large amount of voltage must be supplied by the ignition system in order to have a spark move across the spark plug gap. This is known as electrical performance.

The temperature of a spark plug's firing end must be kept low enough to prevent pre-ignition but high enough to prevent fouling. This is known as thermal performance and is determined by the spark plug's heat range. The spark plug works as a heat exchanger by pulling heat from the combustion chamber to the engine's cooling system. The heat range is defined as the plug's ability to dissipate heat.

Heat transfer is determined by several factors:

1. Insulator nose length is the distance from the firing tip of the insulator to the point where the insulator meets the metal sheel. Since the insulator tip is the hottest part of the plug, the tip temperature is primary factor for pre-ignition and fouling. Spark plug tip temperature must range between 450-850 degrees Celsius. If temperatures fall below 450 then the tip will not be hot enough to burn off carbon and combustion chamber deposits. If temperatures are higher than 850 the plug will overheat which may cause the porcelain to blister or the tip to melt. The difference from one heat range to the next is the ability to remove approximately 70-100 degrees Celsius.

2. Gas volumne around the insulator nose.

3. Materials/construction of the center electrode and porcelain insulator.

There are three basic determinations for spark plugs: good, fouled or overheated. The borderline between fouling and optimum operational performance is 450 degrees Celsius which is the plug's self-cleaning temperature(accumulated carbon and combustions deposits are burned off.)

Nose length determines heat range, the longer the insulator, the less heat absorbed and the further heat has to travel into the cylinder head water passages. This means the plug has a higher internal temperature and is determined to be a hot plug.

A cold plug has a shorter insulator nose and absorbs more combustion chamber heat. Heat travels a shorter distance and allows the plug to operate at a lower temperature. A colder plug is required when an engine is modified for performance, subject to heavy loads or run at high rpms for long periods. The colder plug removes heat more quickly, will reduce pre-ignition(ping) and reduces the chance of firing end burn-out. Engine temperature affects the spark plug's operating temperature but not its heat range.

External Influences on Spark Plug's Operating Temperatures

Air Fuel Mixtures:

• Rich air/fuel mixtures cause tip temps to drop which cause fouling
• Lean air/fuel mixtures cause tip and cylinder temps to increase causing pre-ignition, detonation, spark and engine damage (burnt pistons)

Higher Compression Ratios/Forced Induction (Supercharger/Turbocharger):

As compression increases, a colder heat range plug is required as well as higher octance gas, careful attention to ignition timing and air/fuel ratios.

Advancing Timing:

Advancing ignition timing by 10 degrees increases plug tip temps by 70-100 degrees Celsius.

Engine Speed and Load:

When an engine runs at a constant higher rpm range or carrying heavy loads, a colder heat range plug should be installed.

Ambient Air Temp:

As air temp falls, air density/volume becomes greater which means leaner air/fuel mixtures. This causes situtation causes higher cylinder pressures/temps and increases the plug tip temp. So, more fuel should be added.

Humidity:

As humidity increases, air intake decreases which results in lower combustion pressure/temps which reduces plug's temps. Air fuel mixture should be leaner depending on ambient temp.

Barometric Pressure/Altitude:

The higher the altitude the lower cylinder pressure/temps which reduces plug tip temp. To adjust for altitude, change fuel jetting or air/fuel mixtures to put more air into the cylinder.

Combustion Abnormalities

Pre-Igniton:

• Defined as ignition before the pre-set ignition timing mark.
• Causes: hot-spots(carbon deposits that remain hot after combustion) in the combustion chamber. Too much timing, low octane fuel, lean air/fuel mixture, high compression, poor engine cooling system function.
• Solution: higher octane fuel, colder plug, richer fuel mixture, lower compression (reduce supercharger/turbocharger pressure.) recurve distribitor or retard ignition timing, check engine cooling system.

Detonation:

• Plug's worst enemy
• Can break insulators or break off ground electrodes.
• Pre-ignition usually leads to detonation.
• Plug tip temps can spike to over 3000 degrees F in a race engine.
• Caused by hot spots which ignite the fuel mixture before the plug fires. As the piston is moving upward by the connecting rod, the pre-ignited fuel mixture tries to move the piston down. Since the piston can not go down or up, the piston will rattle side-to-side. The resulting shock wave causes a pinging sound.
• Engine damage is caused by excessive heat when detonation occurs.
• Plug tips are damaged by both excessive heat and the shock wave.

Misfire:

• Defined as not haveing enough voltage to fire the plug at the proper moment of the power stroke.
• Weak spark energy may be due to: defective coil, too much compression, incorrect plug gap, fouled plug, wrong timing.

Fouling:

• Caused by low spark plug tip temps which can not burn off carbon, fuel, oil or other combustion deposits.
• Wet spark plugs must be changed.
• When replacing fouled plugs, determine the cause of the fouling before installing new plugs.

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Installation Tips

• Check gap before installing new plugs.
• Gap plugs according to factory specs unless the engine has been modified. Gap is specific to each engine design.
• When increasing gap for high performance ignition systems (MSD box, etc) never go plus/minus more than 0.008" in order to retain parallel surfaces between ground and center electrodes.
• Use the proper tool to gap plugs, never use pliers.]
• High compression engines require smaller gaps and high power ignition systems.
• A spark plug can only transfer heat when it is fully seated in the cylinder head. Undertorqued plugs can lead to gas leakage or hot plugs.
• Overtorqued plugs create distortion (warping) of plug's internal seals which lead to poor heat transfer and hot plugs.
• Make sure cylinder head plug threads are clean before installing new plugs. Use anti-seize compounds sparingly on threads.
• Iron and aluminum heads have different torque specs.