Injector Mathematics

When upgrading turbochargers (such as converting a stock K03/K03s to a K04-001, K04-023, or Garrett GT28/G25 series) or converting to E85 ethanol fuel, the fuel delivery subsystem must be recalibrated from first principles. Modifying base fueling by artificially skewing the MAF curve (MLHFM) or hacking the VE map (KFLF) is an engineering error that corrupts the internal torque model.

In Bosch Motronic ME7.5, fueling must be calibrated strictly through KRKTE (Primary Injector Conversion Factor), TVUB (Voltage Latency Offset), TEMIN (Minimum Pulse Width), and physical MLHFM (Housing Diameter Linearization).

15.1. KRKTE — Mathematical Derivation & Injector Scaling

Physical Definition

KRKTE defines the relationship between cylinder air charge and fuel mass. Specifically, it represents the effective injector opening duration (ti, in milliseconds) required to achieve stoichiometric combustion (λ = 1.0) for a relative cylinder air filling of rl = 100%:

Where:

Master KRKTE Quick-Reference Table (1.8T 20V Platform, 3.0-Bar Fuel Pressure)

15.2. TVUB — Injector Voltage Latency / Dead Time Correction

Physical Mechanism

When the ECU energizes the low-side injector MOSFET, magnetic flux buildup in the solenoid coil requires finite time before the pintle overcomes fuel pressure and lifts off the valve seat. At lower battery voltages (e.g. during engine cranking at 8V), electromagnetic force develops slower, increasing dead time significantly:

  Dead Time (ms)
       ▲
   3.5 ┼──●  Bosch EV6 (Stock AWP)
       │      2.5 ┼────●
       │        1.5 ┼──────●────●  Bosch EV14 550cc
       │       \       0.8 ┼────────●────●────●
       │         \    \           └──┴──┴───┴────┴────┴────► Battery Voltage (V)
          6V 8V 10V  12V  14V  16V

Factory TVUB vs. High-Performance EV14 Profiles (ms)

Tuning Rule: If TVUB is set too low, idle fueling runs excessively lean during electrical load changes (e.g. radiator fans kicking on); if set too high, the engine stumbles rich during cold starts.

15.3. TEMIN & TEMINVA — Minimum Pulse Width Limits

Large motorsport injectors (such as 1000cc units) deliver substantial fuel mass per millisecond. At hot idle (800 rpm), the required stoichiometric pulse width may drop below 0.60 ms. If the factory TEMIN limit (0.78 ms) is left unchanged, the ECU clamps the minimum injection duration, causing the engine to idle rich at λ = 0.82–0.88 with oscillating idle hunting.

15.4. MLHFM — Mass Air Flow Sensor Linearization & Housing Scaling

Fluid Dynamic Scaling Formula

The Bosch hot-film anemometer (HFM-5) measures linear air velocity across the sensing element. When installing the stock sensor probe into a larger diameter housing, velocity decreases for a given volumetric flow rate in direct proportion to the cross-sectional area:

MAF HOUSING SCALING & SATURATION THRESHOLDS

Calibration Procedure for 73mm Audi TT 225 / VR6 MAF Housing