What knock detection does
A knock sensor is basically a microphone bolted to the block. It hears everything: valvetrain clatter, piston slap, injectors ticking, and the sharp ring of detonation. The Knock Detection function in ESP turns that raw signal into something useful. You tell it which frequency to listen to, when in the engine cycle to listen, and how loud is too loud. Anything above that line is logged and flagged as a knock event.
It's also how you prove the sensors actually work. The old-school test was to add timing until the engine knocked, then take it back out. The problem is that people forget the second half. ESP's Momentary Knock feature adds timing for a fixed time and then puts it back on its own, so a test never turns into a long session of detonation.
If you're new to the topic, start with what knock is and why it matters. Once detection is working, knock control can use it to pull timing automatically.
Wiring tab
| Setting | What it does | Our advice |
|---|---|---|
| Knock Sensor 1 | Turns on the first knock input | On for any engine with a sensor fitted |
| Knock Sensor 2 | Turns on the second knock input | Normally used on V engines or anything with two banks, one sensor per bank |
| Light Output | Adds a warning light for knock events | Worth having during tuning and on track cars |
| Light Output Mode | Chooses which light does the warning | See below |
| Active State | Low: the output is off and switches on when knock occurs. High: the output is on and switches off | Match it to how your light is wired |
There are two light modes. Use CEL borrows your existing check engine light: it flashes when knock is detected and otherwise works as a normal CEL. Digital – Switched uses a dedicated light on its own output, which you pick in the Connections section (a spare DPO, for example). That output then flashes on knock.
The knock sensor wires in the Elite harness are shielded. When you make the joins, make sure the braid can't touch the center signal wire. A shorted or noisy shield ruins the signal and you'll spend hours chasing false knock.
Finding the right frequency with the spectrogram
Every engine knocks at its own characteristic frequency, set mostly by bore size. The Knock Detection tab has a spectrogram that plots signal strength across a wide frequency range so you can see where knock shows up.
- Tick Start Spectrogram. It runs for 20 seconds and then turns itself off. While it runs, most of the ECU's communication bandwidth goes to the spectrogram, so the rest of ESP may feel slow. That's normal.
- Capture the engine running cleanly at a steady load. This is your background noise.
- Capture again while the engine is lightly knocking (the Momentary Knock test below is ideal for this).
- Compare the two. You want the frequency with the biggest gap between clean and knocking: quiet when the engine is healthy, bright and obvious when it isn't. That's the best signal to noise ratio.
- Right-click that spot on the spectrogram and choose Set As Centre Frequency, or type it into Knock Frequency yourself.
Setting the listening window
The ECU doesn't need to listen all the time. It only needs to listen when knock can happen, and the window is set in crank degrees.
- Start Angle is where the window opens. The sensor hears knock some time after it actually happens in the cylinder, so the useful signal arrives later in crank degrees than you might expect. Open the window too early and you pick up normal combustion and mechanical noise. Open it too late and real knock can slip past before you start listening.
- Duration is how long the window stays open. A narrow window rejects more noise and gives a cleaner trace, but if the knock peak falls outside it you won't see it.
Neither setting has a universal right answer. We start with a sensible window, log Knock Level through known clean and known knocking runs, and move the start angle and duration until the difference between the two is as large as possible.
Momentary Knock: testing the sensors safely
Momentary Knock adds a set amount of timing (Ign Adv.) for a set time (Duration), then returns to the normal ignition map on its own. Use it to confirm the sensor, wiring and settings actually react to knock.
Do the test at light load and moderate RPM, add only a small amount of timing for a short time, and never do it under boost or high load. The goal is to hear light knock and see it on the trace, not to find out how much the pistons can take.
The Knock Threshold table
The Knock Threshold table is the line between normal noise and a knock event. Whenever Knock Level goes above the value in this table, the ECU counts it as knock. The table can be mapped in up to 3D, so the threshold can follow RPM and load, which matters because engine noise rises sharply with RPM.
How we set it:
- Tune the engine first, conservatively enough that you're sure it isn't knocking anywhere.
- Log Knock Level across the whole RPM and load range.
- Set each cell a little above the clean Knock Level at that point. Around 3 to 5 dB above is a typical margin.
- Confirm with a controlled knock test that real knock pushes Knock Level well past the threshold.
Set the threshold too low and the ECU flags noise as knock. Set it too high and real knock gets ignored, which defeats the whole point.
Troubleshooting
| Symptom | Usually |
|---|---|
| Knock flagged everywhere at high RPM | Threshold too close to the noise floor, or a detection window that includes too much mechanical noise |
| No response to a Momentary Knock test | Sensor input not enabled, wiring or shield fault, wrong frequency, or the window misses the knock |
| Spectrogram shows no clear difference | Resonant factory sensor, poor sensor location, or not enough real knock during the capture |
| Light never flashes | Wrong light mode, output not assigned, or active state reversed |
Resonant factory sensors are a common cause of poor results on older engines. A non-resonant sensor like the Haltech genuine Bosch knock sensor gives the spectrogram a broad signal to work with. The Elite knock sensor wiring guide covers the connections.
Knock detection FAQ
Can I tune timing off knock detection alone?
No. It's a safety net and a diagnostic tool. Base timing should come from the dyno, with knock detection watching over it.
Why does ESP slow down while the spectrogram is running?
Most of the communication link is busy streaming spectrogram data for those 20 seconds. It returns to normal once it stops.
Do I need two knock sensors?
On V engines and other two-bank layouts, yes, one per bank. Many inline engines get by with a single sensor in a good spot, but if your engine came with two, use both inputs.
