Chapter 31 · Air Path: Throttle, Flex Fuel, MAFless & Intake

MAFless / Speed Density

Converting to speed density: FRLFN rescaling, trim unlocking, fault suppression.

Calibration symbols

LAMSONI_W PVDKS_W MSHFM_W NMOT_W CDLMM DKV_W FRA_W FRLFN KRKTE LAMFA TI_B1 DFPM FR_W FTBR +5 more

ECUs covered

06A906032LP

Size

19 symbols · 1 diagram · ~670 words

Converting to speed density: FRLFN rescaling, trim unlocking, fault suppression.

In high-output motorsport builds (> 350\text{ WHP} on K04-hybrid, Garrett GT28/G25, or Precision turbochargers), the factory Mass Air Flow (MAF) sensor becomes a major mechanical and electronic liability:

Intake Flow Restriction: The 63\text{ mm} (or 73\text{ mm} BAM) MAF housing chokes the compressor inlet.
Voltage Saturation: Peak mass airflow exceeds the maximum ADC measuring ceiling (5.00\text{V} / 655\text{ kg/h} / \sim 285\text{ g/s}), causing the ECU to under-report cylinder filling at high boost.
Sensor Element Contamination: Blow-by oil mist, water-methanol spray, and turbulent pre-compressor reversion coat the delicate heated platinum film, skewing fueling calibration.

While amateur tuners simply unplug the MAF and accept factory limp-mode defaults, Bosch ME7.5 contains an advanced physical Intake Manifold Pressure Filling Model (Saugrohrmodell). By calibrating this subsystem, the ECU can be converted to operate in a pure, factory-clean Speed Density / Alpha-N mode with full closed-loop wideband lambda trimming.

┌──────────────────────────────────────────────────────────────────────────────────────────────────┐
│                   BOSCH ME7.5 SAUGROHRMODELL (SPEED DENSITY) CHARGE CALCULATION                  │
├──────────────────────────────────────────────────────────────────────────────────────────────────┤
│           [ Engine Speed (nmot_w) ]         [ Throttle Plate Angle (dkv_w) ]                     │
│                        │                                   │                                     │
│                        ▼                                   ▼                                     │
│           ┌─────────────────────────────────────────────────────────────┐                        │
│           │            FRLFN Characteristic Filling Surface             │                        │
│           │      Base Volumetric Efficiency: VE = f(nmot, dkv)          │                        │
│           └──────────────────────────────┬──────────────────────────────┘                        │
│                                          │                                                       │
│                                          ▼ Base Relative Filling (rl_base)                       │
│           [ Manifold Absolute Pressure (pvdks_w / MAP Sensor G31) ]                              │
│           [ Intake Air Temperature (tans / IAT Sensor G42) ]                                     │
│                                          │                                                       │
│                                          ▼ Corrected via FTBR (Gas Law: P = rho * R * T)         │
│           ┌─────────────────────────────────────────────────────────────┐                        │
│           │      Synthesized Relative Cylinder Air Charge (rl_w)        │                        │
│           └──────────────────────────────┬──────────────────────────────┘                        │
│                                          │                                                       │
│       ┌──────────────────────────────────┴──────────────────────────────────┐                    │
│       ▼                                                                     ▼                    │
│ [ Fuel Injection Duration ]                                      [ Ignition Advance Map ]        │
│ ti_b1 = rl_w * KRKTE * LAMFA                                     KFZW = f(nmot_w, rl_w)          │
└──────────────────────────────────────────────────────────────────────────────────────────────────┘

31.1. The Mathematical Mechanics of the Saugrohrmodell#

When the MAF sensor signal (mshfm_w) is absent, the ECU calculates relative cylinder filling (r_l) from the ideal gas law:

$r_l = \mathbf{FRLFN}(N_{\text{mot}}, \ \alpha_{\text{dk}}) \times \left(\frac{P_{\text{ds}}}{P_{\text{ambient}}} \right) \times \mathbf{FTBR}(T_{\text{ans}})$

Where:

FRLFN (Kennfeld Normierte Füllung): The primary 3D volumetric efficiency matrix (Address 0x01A240 in 06A906032LP, 16 \times 16 grid). It maps engine speed and throttle angle to base cylinder filling under standard temperature and pressure.
P_{\text{ds}} (Saugrohrdruck): Manifold absolute pressure measured by the charge pipe MAP sensor (G31 / Pin 101).
FTBR (Temperaturkorrekturfaktor): Density correction factor inversely proportional to absolute intake air temperature in Kelvin: $\mathbf{FTBR} =\frac{293.15\text{ K}}{273.15 + T_{\text{ans}}\text{ (°C)}}$

31.2. Calibration Protocol for Permanent Speed Density Conversion#

To eliminate the MAF sensor cleanly without triggering limp mode or losing closed-loop fueling adaptions:

1. Rescale the `FRLFN` Volumetric Efficiency Surface#

In 06A906032LP, open FRLFN at 0x01A240.
In big turbo applications with high-lift camshafts or ported cylinder heads, low-RPM volumetric efficiency drops slightly while high-RPM volumetric efficiency increases by 15\% to 25\%.
Log wideband lambda (lamsoni_w) and short-term fuel trim (fr_w). Smoothly adjust cells in FRLFN until fr_w fluctuates within \pm 2.5\% across all throttle positions.

2. Unlock Closed-Loop Adaptive Trimming in MAFless Mode#

In factory software, when the MAF fails, the ECU disables long-term fuel trim learning (fra_w).

Codeword CW_LRA (Lambda Regulation Adaptation Mode):
- Set bit CW_LRA.0 = 1 (Enables additive and multiplicative lambda trim adaptation during Alpha-N backup mode).
- This ensures the wideband LSU 4.2 sensor continues continuously adapting to ambient weather and fuel density variations.

3. Suppress MAF Sensor Diagnostic Faults#

Codeword CDLMM (Codeword for Air Mass Meter Diagnosis):
- Using me7_dtc_manager.py (DOC 34), set fault classes for P0102 (MAF Low Input) and P0103 (MAF High Input) in array CLA* to 0x00.
- The Check Engine Light remains completely dark, and readiness monitors pass unconditionally.
DFPM Error Classes: