Chapter 43 · Air Path: Throttle, Flex Fuel, MAFless & Intake

Intake Resonance (SRU)

Helmholtz resonance in the intake, SRU maps, and turbo conversion deletes.

Calibration symbols

ESKONF MIIST DFPM

ECUs covered

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3 symbols · 1 diagram · ~690 words

Helmholtz resonance in the intake, SRU maps, and turbo conversion deletes.

While turbocharged 1.8T transverse platforms (AWP, AUQ, AUM) utilize a fixed-length cast aluminum intake manifold, several closely related platforms within the Volkswagen Audi Group Bosch ME7 family—specifically the 2.8L 24V VR6 (BDF) and 3.2L VR6 (BFH on Golf R32) managed by Bosch ME7.1 / ME7.1.1, as well as naturally aspirated 2.0L 8V engines (AZG, AVH) and European longitudinal 1.8L platforms—incorporate a variable-geometry intake manifold.

This subsystem—designated in Bosch engineering as SRU (Saugrohrumschaltung)—employs vacuum-actuated internal rotary barrel valves or change-over flaps driven by solenoid N156. Understanding its acoustic physics and software calibration is critical when tuning naturally aspirated ME7 vehicles or converting them to forced induction.

┌──────────────────────────────────────────────────────────────────────────────────────────────────┐
│                   BOSCH ME7 VARIABLE DUAL-RUNNER INTAKE TOPOLOGY (SRU)                           │
├──────────────────────────────────────────────────────────────────────────────────────────────────┤
│                                      [ Throttle Body Airflow ]                                   │
│                                                   │                                              │
│                                                   ▼                                              │
│                            ┌─────────────────────────────────────────────┐                       │
│                            │    SRU Vacuum Solenoid Valve (N156)         │                       │
│                            └──────┬───────────────────────────────┬──────┘                       │
│                                   │                               │                              │
│         Low RPM (< 4200 RPM)      │                               │ High RPM (> 4200 RPM)        │
│         Flap CLOSED (De-energized)│                               │ Flap OPEN (Energized)        │
│         Air forced down LONG,     │                               │ Air takes direct SHORT path  │
│         narrow resonance runners  │                               │ into intake valve ports      │
│                                   │                               │                              │
│                                   ▼                               ▼                              │
│                 ┌──────────────────────────────────┐   ┌──────────────────────────────────┐      │
│                 │ Long-Runner Acoustic Inertia     │   │ Short-Runner Low Flow Resistance │      │
│                 │ • Helmholtz Ram-Charging         │   │ • Maximizes Peak Air Mass Flow   │      │
│                 │ • Boosts Low-End Torque (+25 Nm) │   │ • Delivers Maximum Top-End Power │      │
│                 └──────────────────────────────────┘   └──────────────────────────────────┘      │
└──────────────────────────────────────────────────────────────────────────────────────────────────┘

43.1. The Acoustic Physics of Helmholtz Intake Resonance#

The air column within an intake runner behaves as an acoustic oscillator:

Low-to-Mid RPM (1200\text{ to }4000\text{ rpm}):
- Long runners present high fluid friction and acoustic choking.
- The ECU energizes solenoid N156, applying vacuum to the actuator to rotate the flap open.
- Air bypasses the long runners, taking a short, direct path (L \approx 180\text{ mm}) into the cylinder head, unlocking maximum mass airflow for top-end peak horsepower.
High RPM (> 4200\text{ rpm}):

43.2. Calibration Parameters in the Bosch ME7 Architecture#

NSRMIN (Schaltdrehzahl Saugrohrumschaltung):
- Minimum relative engine load threshold (typically 60\%\dots 70\%).
- Under light-throttle highway cruising, even above 4500\text{ rpm}, the flaps remain in the long-runner position to maximize swirl, fuel mixing, and thermal efficiency.
NSRHYST (Schalthysterese):
RLSR (Mindestlast Saugrohrumschaltung):

43.3. Converting Naturally Aspirated ME7 Platforms to Turbocharger Induction#

When installing a turbocharger on a 24V VR6 or 2.0L engine, the dual-runner factory manifold is replaced with a cast aluminum or fabricated short-runner turbo plenum lacking the internal N156 flap.

Clean Software Deletion & Driver Repurposing#

Disable Diagnostic Monitoring:
- The low-side DMOS driver channel previously assigned to N156 is rated for 2.5\text{A} continuous current.
  - A high-output secondary fuel pump relay (energized only when boost exceeds 15\text{ psi}).
  - An exhaust cutout butterfly valve (opened above 4500\text{ rpm} at wide-open throttle).
  - An instrument cluster shift light telltale.
- Tuners can inject custom C167 machine code to repurpose this physical ECU pin to command:
- A high-output secondary fuel pump relay (energized only when boost exceeds 15\text{ psi}).
- An exhaust cutout butterfly valve (opened above 4500\text{ rpm} at wide-open throttle).
- An instrument cluster shift light telltale.
Repurposing the N156 Driver on the PCB: