Cylinder Spread
The difference in average knock retard between the cylinder with the most knock and the cylinder with the least. A large spread isolates the problem to a specific cylinder.
Why it matters
When one cylinder consistently knocks harder than others, it points to a hardware issue specific to that cylinder: bad spark plug, failing injector, carbon buildup, or coolant flow restriction.
What to look for
Warning at 2.0 degrees spread. On a Subaru EJ257, watch cylinder 4 specifically (hot side, prone to ringland failure).
Related terms
When the ECU detects detonation (uncontrolled combustion), it pulls ignition timing to protect the engine. Any amount above 0 degrees means knock was detected.
Uncontrolled combustion where the air-fuel mix ignites from pressure/heat instead of the spark plug. Causes knock and can destroy engines.
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