Chapter 48 · Boost Control & Thermal Management

Altitude & Turbo Protection

Altitude compensation, compressor maps, and turbo over-speed protection.

Calibration symbols

KFLDHBN KFMIRL LDRXN BTS LDR

ECUs covered

06A906032LP

Size

5 symbols · 2 diagrams · ~788 words

Altitude compensation, compressor maps, and turbo over-speed protection.

Internal combustion engines experience severe volumetric degradation as atmospheric barometric pressure (p_{\text{amb}}) declines at altitude. While naturally aspirated engines suffer uncorrectable power losses of approximately 3\% per 1,000\text{ ft} of elevation, a turbocharged engine can theoretically maintain sea-level power by increasing turbocharger compressor speed.

However, forcing a turbocharger to deliver sea-level manifold pressure in thin mountain air drives the compressor pressure ratio (\Pi_c) into dangerous over-speed conditions, causing extreme thermal expansion, aerodynamic choke, and centrifugal wheel burst. Bosch ME7.5 incorporates a sophisticated multi-tiered **Altitude Derating Architecture (Höhenschutz)** that dynamically calculates ambient barometric pressure and enforces strict ceiling maps to protect the turbocharger from destructive over-speeding.

┌──────────────────────────────────────────────────────────────────────────────────────────────────┐
│                    HIGH-ALTITUDE TURBOCHARGER PROTECTION ARCHITECTURE                            │
├──────────────────────────────────────────────────────────────────────────────────────────────────┤
│                                                                                                  │
│   [ Ambient Barometric Pressure Sensor: p_amb ]                                                  │
│   (Onboard ECU Baro Sensor / Key-On MAP Pre-Crank Sample: 1013 hPa -> 700 hPa)                   │
│                                  │                                                               │
│                                  ▼                                                               │
│   ┌─────────────────────────────────────────────────────────────────────────┐                    │
│   │ Altitude Correction Factor: FHO (1.00 at Sea Level -> 0.72 at 3000m)    │                    │
│   └──────────────────────────────┬──────────────────────────────────────────┘                    │
│                                  │                                                               │
│         ┌────────────────────────┴────────────────────────┐                                      │
│         ▼                                                 ▼                                      │
│   ┌───────────────────────────┐                     ┌───────────────────────────┐                │
│   │ Dynamic Maximum Boost     │                     │ Max Cylinder Filling      │                │
│   │ Ceiling: KLDHAP           │                     │ Ceiling: LDRXN_H          │                │
│   │ (Caps Absolute Pressure)  │                     │ (Limits Specified Air)    │                │
│   └─────────────┬─────────────┘                     └─────────────┬─────────────┘                │
│                 │                                                 │                              │
│                 └────────────────────────┬────────────────────────┘                              │
│                                          ▼                                                       │
│                         ┌─────────────────────────────────┐                                      │
│                         │ Minimum Selector (LDRPID Input) │                                      │
│                         │ Prevents Shaft Over-Speed &     │                                      │
│                         │ Compressor Aerodynamic Choke    │                                      │
│                         └─────────────────────────────────┘                                      │
└──────────────────────────────────────────────────────────────────────────────────────────────────┘

48.1. Compressor Pressure Ratio (\Pi_c) & Rotational Speed (N_{\text{tc}}) Physics#

Turbocharger rotational speed is governed by the volume flow rate (V) and the compressor pressure ratio:

$\Pi_c =\frac{p_{2,\text{abs}}}{p_{1,\text{abs}}} =\frac{p_{\text{charge\_pressure}}}{p_{\text{ambient}} - \Delta p_{\text{intake\_filter}}}$

Consider an ME7.5 calibration requesting p_{2,\text{abs}} = 2150\text{ hPa} (1.15\text{ bar} gauge boost at sea level):

Geographic Location	Altitude (m / ft)	Ambient Pressure p_{\text{amb}}	Specified Boost p_{2,\text{abs}}	Pressure Ratio \Pi_c	Turbo Shaft Speed (N_{\text{tc}})	Compressor State
Sea Level (Atlantic Coast)	0\text{ m} / 0\text{ ft}	1013\text{ hPa}	2150\text{ hPa}	2.12	165,000\text{ rpm}	Optimal Efficiency Island (74\%)
High Plains (Denver, CO)	1600\text{ m} / 5280\text{ ft}	835\text{ hPa}	2150\text{ hPa}	2.57	192,000\text{ rpm}	Nearing Choke Line (65\%)
Mountain Pass (Loveland Pass)	3650\text{ m} / 11990\text{ ft}	650\text{ hPa}	2150\text{ hPa}	3.31	> 230,000\text{ rpm}	Catastrophic Over-Speed & Surge

The Over-Speed Burst Threat#

The peripheral tip speed (U_t) of the compressor inducer blade is:

$U_t =\frac{\pi \cdot D_{\text{wheel}} \cdot N_{\text{tc}}}{60}$

On a 52\text{ mm} K04/hybrid compressor wheel spinning at 230,000\text{ rpm}, tip speed reaches 626\text{ m/s} (M = 1.84). Centrifugal hoop stresses on the aluminum alloy wheel exceed 450\text{ MPa}, triggering blade fatigue, shaft snapping, and catastrophic turbocharger destruction.

48.2. Bosch Altitude Derating Algorithms & Core Maps#

Bosch ME7.5 protects against over-speed by clipping specified boost pressure (pvdss) through a dedicated altitude compensation map hierarchy:

┌──────────────────────────────────────────────────────────────────────────────────────────────────┐
│                             BOSCH ME7.5 ALTITUDE BOOST CEILING PIPELINE                          │
├──────────────────────────────────────────────────────────────────────────────────────────────────┤
│                                                                                                  │
│   pvdss_w (Raw Specified Boost Target from LDRXN / KFMIRL)                                       │
│       │                                                                                          │
│       ├──► MIN[ pvdss_w , KLDHAP(p_amb, N_mot) , KFLDHBN(N_mot, p_amb) ] ──► Final Boost Limit  │
│                                                                                                  │
│   Where:                                                                                         │
│   - KLDHAP: Absolute pressure ceiling curve indexed against ambient pressure                     │
│   - KFLDHBN: Maximum pressure ratio ceiling across engine speed                                  │
│   - FHO: Linear scaling factor multiplying permissible cylinder air mass                         │
└──────────────────────────────────────────────────────────────────────────────────────────────────┘

Core Bosch Altitude Calibration Maps in `06A906032LP`#

Symbol Name	Flash Offset (032LP)	Dimensions	Axes	Units	Description
`KLDHAP`	`0x01EF30`	8 \times 8	RPM \times p_{\text{amb}}	\text{hPa}	Kennfeld Ladedruck-Höhenbegrenzung (Max permissible absolute boost vs. ambient pressure)
`KFLDHBN`	`0x01EE82`	8 \times 8	RPM \times p_{\text{amb}}	\text{hPa}	Höchstzulässiger Ladedruck bei Normalbetrieb (Maximum continuous operational boost limit)
`FHO`	`0x0113B8`	6 \times 1	p_{\text{amb}}	Factor	Höhenfaktor für Füllungsbegrenzung (Altitude load scaling factor: 1.00 \to 0.70)
`LDPBN`	`0x01EF10`	1 \times 1	-	\text{hPa}	Min ambient pressure threshold for altitude diagnostic mode (650\text{ hPa})

48.3. Calibration Guidelines for Tuned & Big-Turbo Vehicles#

When tuning 1.8T engines operating in mountainous regions:

Never Disable KLDHAP: Amateur tuners frequently set KLDHAP to maximum (2550\text{ hPa}) to prevent the ECU from "pulling boost" on the dyno. At sea level, this causes no immediate harm; however, driving the car into mountainous passes forces the turbocharger past its safe rotational envelope, causing immediate turbo failure.
Re-Profile KLDHAP for Larger Turbos: Larger turbochargers (Garrett G25-550, BorgWarner EFR 6758) have vastly higher compressor flow capacity and broader efficiency maps than stock K03s. The pressure ratio ceiling can be safely raised in low ambient pressure rows (p_{\text{amb}} = 800\dots 700\text{ hPa}), allowing safe mountain power delivery while maintaining shaft speed within the turbo manufacturer's recommended 145,000\text{ rpm} ceiling.
Barometric Pressure Sensor Diagnostics: If the onboard ambient pressure sensor fails or drifts, ME7.5 triggers DTC P1604 / 18012 (Internal Control Module Barometric Pressure Sensor Circuit Malfunction) and permanently locks boost control into emergency limp mode (LDR-Notlauf), clamping wastegate duty cycle to 0\%.