In normal combustion, the flame ignites at the spark plug and spreads evenly through the combustion chamber. Knock (or detonation) occurs when the unburned end-gas at the trailing edge of the flame front self-ignites — before the flame reaches it. This uncontrolled ignition creates a pressure wave that strikes the piston, cylinder wall, and bearings.
Brief, rare knock events are still tolerable. Sustained heavy knock leads to: piston damage (scoring, cracks), conrod bearing failure, cylinder head damage, and in extreme cases engine failure. Modern ECUs detect knock significantly earlier than it becomes audible.
Every modern petrol engine has knock sensors (piezoelectric accelerometers) mounted on the cylinder block. They detect structure-borne vibrations and deliver an analogue signal to the ECU. The ECU analyses this signal in a specific frequency band (typically 6–16 kHz) and recognises the characteristic high-frequency oscillations of knock events.
Modern systems such as Bosch "KSS" (Knock Sensor System) use digital signal processing per crankshaft angle — enabling the ECU to assign knock to individual cylinders rather than just detecting it globally.
When the ECU detects knock in a cylinder, it immediately retards that cylinder's ignition timing — typically 1.5–3° per event, up to a maximum of 10–15°. Simultaneously it slightly enriches injection to lower combustion chamber temperature. If knock subsides, the ECU advances timing back in small steps toward the optimal value.
The result is a permanent "probing" at the knock limit: the ECU maximises ignition advance (= maximum efficiency and power) while safely preventing knock. This system allows modern engines to run on lower-octane fuel — at the cost of some power and efficiency.
The octane rating (RON) determines the knock resistance of the fuel. A remap calibrated for 98 RON will run on 95 RON — but knock control will retard the ignition timing and cost up to 5–8% of the power gain. For maximum benefit from a high-performance remap, always use the fuel grade specified by the tuner.
At ecufiles.io we specify the minimum recommended octane rating for every petrol remap. A file designed for 98 RON will run safely on 95 RON — but not with the full promised power figures.
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