Pre - Ignition is a bad thing. It is also frequently referred to as "knocking" because of the sound it makes at low intensities. It is also frequently called "detonation." The Knock Sensor and Knock Board present in some engines/ECUs attempt (poorly) to detect Pre Ignition and change engine parameters (usually: pull timing) to stop/prevent it.

Pre-ignition is an uncontrolled, violent, explosive burning of fuel that happens when engine conditions (heat, pressure, Air Fuel Ratio, etc) are such that fuel spontaneously combusts in an uncontrolled manner without help from a Spark Plug.

Pre-ignition will destroy your engine.

Common causes of pre-ignition:

• too lean of a Air Fuel Ratio, especially under Boost.
• too much Spark Advance (especially with too high of a Compression Ratio or with too much Boost)
• too much Boost (especially with a poor Combustion Chamber design or too high of a Compression Ratio)
• too high of a Compression Ratio for the Fuel Octane gas that is being used.
• poor quality gasoline

Links:

• http://www.streetrodstuff.com/Articles/Engine/Detonation/index.php

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