How knock control works
Knock is uncontrolled combustion that hammers the pistons and bearings. Knock sensors are vibration sensors bolted to the block, essentially microphones, and the ECU listens to them through knock detection. When the filtered signal crosses the knock threshold, knock control steps in and retards ignition timing, which is the quickest way to stop detonation. Then it gradually hands timing back.
That fast reaction is the short term part. On top of it, knock long term trim can learn a lasting retard in areas where knock keeps coming back, which covers you if part of the tune is simply too aggressive.
The settings
| Setting | What it does | Our advice |
|---|---|---|
| Mode: Banked | For engines with more than one knock sensor | Use it on V engines and long inline engines with two sensors |
| Mode: Unbanked | For a single sensor | Most four-cylinder installs |
| Short Term Retard | Degrees removed when the knock signal passes the threshold. Default 5° | Leave at default until you've proven the threshold is right |
| Short Term Decay Rate | How fast that retard is given back, in degrees per complete engine cycle. Default 0.5°/ECyc | Slower decay is safer on a new tune |
| Retard Sample Rate | Limits how many retard actions can happen within a set time | Stops one knock event being counted several times and stacking up retard |
Worked example
With the defaults, a knock event removes 5°, and 0.5° comes back every engine cycle, so it takes 10 cycles to recover fully. One engine cycle on a four-stroke is two crank revolutions. At 6000 rpm that's 50 cycles per second, so the full 5° returns in about 0.2 seconds. At 2000 rpm it takes about 0.6 seconds. That's why decay is set per cycle rather than per second: the recovery naturally speeds up with engine speed.
It is not a tuning tool. If logs show knock control working regularly, fix the cause: timing, fuel quality, air temperature or mixture. Don't just rely on the ECU to keep pulling timing.
Before you trust it
- Confirm sensor wiring and shielding using knock wiring. A noisy signal leads to false retard.
- Set the threshold from real logs so normal engine noise sits comfortably under it.
- For multi-bank engines, look at bank long term trim too.
Need a sensor? We stock the genuine Bosch knock sensor, and the Pro Tuner Knock Ears kit lets you listen in on the dyno.
