Chapter 37 · Ignition, Knock Control, Dwell & Oxygen Sensing

Knock Sensor Recalibration

Why forged pistons and solid mounts cause false knock, and how to re-gate it.

Calibration symbols

DWKRZ_0 DWKRZ_3 NMOT_W KRMX BTS G61 G66

ECUs covered

06A906032LP

Size

7 symbols · 1 diagram · ~647 words

Why forged pistons and solid mounts cause false knock, and how to re-gate it.

When building a high-power 1.8T engine (> 300\text{ WHP}), installing forged connecting rods and forged aluminum pistons (manufactured from 2618 high-tensile alloy or 4032 low-expansion alloy) is an industry-standard reliability upgrade. Simultaneously, builders replace the soft factory rubber engine and dogbone mounts with stiff polyurethane (75A/85A durometer) or solid billet mounts to eliminate wheel hop.

However, on factory Bosch ME7.5 calibrations, these mechanical modifications introduce severe false knock detection. The engine pulls massive ignition timing (dwkrz > 6.0°) under light-throttle acceleration, causing sluggish performance, elevated exhaust gas temperatures, and erratic surging.

┌──────────────────────────────────────────────────────────────────────────────────────────────────┐
│                   MECHANICAL NOISE VS. COMBUSTION DETONATION ACOUSTIC SPECTRUM                   │
├──────────────────────────────────────────────────────────────────────────────────────────────────┤
│ Signal Amplitude                                                                                 │
│      ▲                                                                                           │
│ High ┼                                      ● True Knock Detonation Spike (> 1.8x Ratio)         │
│      │                                     / \                                                   │
│      │      ● Mechanical Piston Slap      /   \                                                  │
│ Med  ┼     / \  & Solid Mount Noise      /     \                                                 │
│      │    /   \─────────────────────────/       \                                                │
│      │   /    ▲                        ▲                                                         │
│ Low  ┼──/─────┼────────────────────────┼────────────────────────────────► Acoustic Frequency     │
│       1 kHz  3 kHz                   6.8 kHz (Resonant Chamber Mode)                             │
│                                                                                                  │
│ Factory KRMX Threshold: Too sensitive -> Treats piston skirt slap at 3 kHz as real knock         │
│ Recalibrated KRMX Envelope: Raises baseline noise floor, preserving 6.8 kHz detonation safety    │
└──────────────────────────────────────────────────────────────────────────────────────────────────┘

37.1. The Acoustic Origin of False Knock#

Forged 2618 Alloy Piston Slap:
- Factory rubber mounts act as low-pass mechanical filters, isolating transmission gear noise and road vibration from the engine block.
- Stiff aftermarket mounts transmit drivetrain harmonic chatter directly into the engine block casting, creating continuous broadband acoustic noise that enters knock sensors G61 and G66.
Solid & Polyurethane Engine Mounts:

37.2. Recalibrating the Reference Noise Baseline (`KRMX`)#

In Bosch ME7.5, a combustion event is flagged as detonation only if its integrated acoustic voltage exceeds the rolling reference baseline:

$\text{Detonation Flagged if: }\frac{V_{\text{measured}}}{\mathbf{KRMX}(N_{\text{mot}}, \ T_{\text{mot}})} > \text{Knock Ratio Threshold}$

KRMX (Kennfeld Referenzpegel Klopfen / Reference Noise Baseline Map):
- Address in 06A906032LP: 0x012780 (12 \times 16 grid).
- Inputs: Engine Speed (nmot_w, X-axis) and Relative Cylinder Load or Coolant Temp (Y-axis).
- Role: Establishes the expected mechanical background noise of a stock engine with cast pistons and rubber mounts.

Step-by-Step Recalibration Protocol#

Datalog Knock Activity via ME7Logger:
- Do not alter KRMX above 5500\text{ rpm}. At high engine speeds under full boost, real detonation shockwaves generate immense acoustic energy (> 6.8\text{ kHz}) that far exceeds mechanical noise. Preserving stock thresholds at high RPM ensures the ECU retains full protection against bad fuel or overboost pre-ignition.
Raise Low-to-Mid RPM Cells in KRMX:
Preserve High-RPM Detonation Safety:

Cross-referenced on shared calibration symbols, not on subject matter — these are the chapters that touch the same maps.