Knock, knock sensors and false knock

Detonation is the end gas exploding before the flame reaches it. The sensor hears it; the threshold decides what counts.

As the flame crosses the chamber it compresses and heats the unburnt mixture ahead of it. If that end gas reaches its autoignition point before the flame arrives, it detonates: a pressure spike that rings the block, hammers the piston crown and ring lands, and blows the boundary layer off the metal so everything runs hotter. Pre-ignition is the nastier cousin — a hot spot lights the charge before the spark — and retarding timing cannot stop it once it starts.

What moves the knock limit

• Charge density (boost, cold dense air): more pressure, lower limit.
• Charge temperature: hot air knocks sooner. Heat soak between pulls is a silent killer.
• Mixture: a rich mixture (0.78–0.82) buys several degrees; lean takes them away.
• Fuel: 87 vs 91 vs 93 vs race fuel is purely a knock-limit difference in this simulator.
• Compression ratio, coolant temperature, and worn pistons all lower it.

The sensor and the threshold

A knock sensor is a microphone bolted to the block. Engine noise rises with RPM; knock stands above it. The threshold table says how far above the normal noise a reading must be to count. Too low and background noise trips it — false knock — so the ECU retards timing and power drops with nothing wrong. Too high and real knock, especially at high RPM where the noise floor is highest, goes unseen while the engine takes damage.

Knock control

When the sensor trips, the ECU pulls timing in steps and slowly gives it back. It is a safety net, not a tuning strategy. If the log shows retard being held, the map is too aggressive in that cell: fix the map. Turning knock control off to "see the real knock" just removes the net.