Detection vs control
Haltech splits knock into two halves. Knock Detection, covered here, is the listening side: which sensors are active, which frequency the ECU pays attention to, and which slice of each engine cycle it examines. Knock Control is the reacting side, with the retard amounts and long-term learning. That lives in its own section; see Knock Control.
A Nexus supports up to two knock sensors. Detection also includes a spectrogram for viewing what the sensors hear, and a momentary knock test for proving the system reacts.
Setting up the detection window
| Setting | What it does | Our advice |
|---|---|---|
| Knock Frequency | The frequency the ECU filters for when looking for knock | Pick the frequency with the biggest gap between a knocking and a clean engine, found with the spectrogram |
| Start Angle (BTDC) | The crank angle where the listening window opens | Too early and normal combustion noise leaks in; too late and real knock is missed. Test and refine |
| Duration | How many crank degrees the window stays open | Narrower rejects more noise but risks missing the peak. Test and refine |
| Remove Background Noise | Subtracts the noise measured outside the window from the reading inside it | Usually worth enabling. It tends to give a flatter, easier threshold table |
Finding the knock frequency
The goal is signal-to-noise ratio: the frequency where knock stands out most from everything else the engine is doing. Watch the spectrogram while the engine is known to be knocking lightly, then compare it with the same conditions without knock. The band that lights up only during knock is your target. You can right-click on the spectrogram and choose Set As Centre Frequency to drop that value straight into the setting.
Cross-checking with audio makes this much more reliable. A set of Pro Tuner knock ears lets you hear what the spectrogram is showing, which is the best way to confirm you're looking at knock and not valvetrain noise.
Start angle and duration
Knock happens after combustion has started, so the window needs to sit where the pressure rise is. There's no universal answer; engine design, bore size and timing all move it. We start with a moderate window, log knock level against known clean and known knocking runs, and move the start and width until the clean runs stay low and the knocking runs stand out clearly.
Momentary Knock: prove it works
It's worth confirming the sensors and wiring are working before you rely on them. The Momentary Knock feature briefly adds ignition advance to provoke a reaction, then puts timing back automatically.
- Ign Adv: how many degrees of advance are added.
- Duration: how long the extra advance is held before timing returns to normal.
Click the Knock button to trigger it and watch the knock level channels.
This test deliberately invites detonation. Use small advance amounts and short durations, at light load, and stop as soon as you've seen a clear response. Never test at high boost or heavy load.
Wiring settings
The Wiring node enables the inputs and the optional warning light:
- Knock Sensor 1: enable when a sensor is wired to Knock 1.
- Knock Sensor 2: enable for a second sensor, typical on V engines or anything with two banks.
- Light Output: enable if you want a dash light when knock is detected.
- Light Output Mode:
Use CELflashes an already configured check engine light on knock and otherwise leaves it working normally.Digital - Switchedgives you a separate, dedicated knock light output that you'll need to wire.
For the physical side, see Knock Control Wiring. If you're adding sensors, a genuine Bosch knock sensor is a solid choice. Mount it on a flat, clean, machined surface and torque it correctly; a loose sensor gives a poor signal.
Troubleshooting
| Symptom | Usually |
|---|---|
| Knock level rises with RPM even when clean | Mechanical noise in the window; tighten the window or enable background noise removal |
| No response to momentary knock | Sensor not enabled, wiring fault, or the wrong frequency |
| Constant false knock at one RPM | A valvetrain or accessory resonance; confirm by ear and adjust the frequency or window |
Once detection is clean, set your knock threshold and then move on to the control side.
FAQ
Do I need two knock sensors?
One is often enough on a four-cylinder. On a V engine, or a long inline six, a second sensor gets closer to the cylinders the first one can't hear well.
Can I trust knock detection without knock ears?
Only once you've verified it. Use audio at least during setup so you know the ECU is reacting to real detonation.
