The knock sensor is a piezo-electric transducer bolted to the engine block that functions as a microphone, detecting the specific high-frequency vibrations associated with pre-ignition (detonation). The ECU processes this signal to retard ignition timing, preventing catastrophic engine damage.

Application#

Knock sensors are standard equipment on DOHC VTEC engines, including the B16, B18C, and H22 series, as well as most OBD2 SOHC VTEC engines.

Technical Operation#

The sensor generates a voltage signal proportional to the vibration frequency of the engine block. This signal is routed to a dedicated knock-detection circuit (often referred to as the "knock board") integrated into the ECU.

• Signal Processing: The ECU filters the input to isolate the specific frequency range characteristic of engine knock.
• Timing Correction: Upon detecting knock, the ECU applies a pre-programmed ignition timing retard map to suppress detonation.

Limitations in Forced Induction#

The factory knock-detection system is calibrated for stock engine configurations and is generally considered insufficient for high-performance or forced-induction applications.

Warning

The factory knock board is prone to false positives when used with forged internal components, which have different acoustic properties than cast pistons.

Tip

For boosted applications, the factory sensor should be treated as a secondary safeguard against poor fuel quality. It is highly recommended to utilize a dedicated aftermarket knock-detection system for precise engine protection.

Diagnostics and Reference#

Use the following resources for troubleshooting and sensor calibration:

• Voltage Reference: Knock Sensor Voltages0
• Diagnostic Procedures: ECU Trouble Codes0