Beyond normal combustion, there are two main types of abnormal engine combustion that occur beyond the bounds of the spark event that is initiated by the sparkplug in a spark ignition engine. These are Engine Knock and Pre-ignition.

Engine Knock – which may be called pinking, pinging, detonation or rattling.

In normal combustion, the spark event occurs, then the flame front that began at that sparkplug expands though the combustion chamber until there is no more fuel of an appropriate air/fuel ratio to burn in the combustion chamber.

Engine Knock occurs after the spark event, and before the air/fuel mixture in the combustion chamber is completely consumed. If the spark event occurs too early, radiant heat and cylinder pressure builds up to the point that at another area within the combustion chamber, the air/fuel mixture self ignites causing another flame front to move out from that point.

Now you have two flame fronts moving towards each other. This is, of course a completely abnormal situation. When the two flame fronts collide, Engine Knock occurs.

People describe the sound of Engine Knock as “gravel in the sump” or “rain falling on a tin roof”.

Factors That Increase Engine Knock: Low octane fuel High compression ratio High turbo boost Overly advanced ignition timing Lean air/fuel ratio

Things To Do To Decrease Engine Knock: Use fuel with a higher octane Reduce the compression ratio to an appropriate level for your fuel and/or boost pressure level. Reduce the ignition timing to a value below where engine knock occurs. Increase the amount of fuel being delivered at the point of detonation.

Correctly setting the ignition timing and engine fueling are the two easiest and generally the most often used ways to stop your engine from going over the knock limit.

Pre-ignition usually occurs due to hot spots either on the piston crown, inside walls or roof of the combustion chamber or having a sparkplug with the wrong heat range. These hot spots have enough heat energy to ignite the air/fuel mixture as the piston is moving up the cylinder towards top dead centre (TDC).

At a point – before the correct ignition timing point – the hot spot causes the air/fuel mixture to ignite.

Unlike we described earlier in the Engine Knock section, reducing the delivered ignition timing will not stop Pre-ignition, as it is not caused by the correct spark event. If this is occurring, you will need to check the heat range of your sparkplugs and checking the shape and cleanliness of the combustion and piston crown to ensure there are no places for potential “hot spots”.

If you are interested, you can find tools to help you determine when your engine is experiencing Engine Knock at