Tech Support - Spark Plugs Overview

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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

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