Extracting & Validating MAF Sensor Scaling from a 2005 WRX Denso 16-Bit ECU
When people talk about “tuning,” they usually jump straight to boost or ignition timing.
That’s a mistake.
If the MAF scaling isn’t correct, everything downstream lies — AFR targets, injector math, knock behavior, even load calculation. Before touching boost, before touching timing, before building AI tools, the airflow model has to be right.
So instead of guessing, we extracted and validated the factory MAF table directly from the ROM.
This is how you do it properly.
Step 1 — Lock the ROM Structure
ECU:
2005 USDM Subaru WRX
Denso 16-bit ECU
ROM ID: A4TF800F
File size: 160 KB
Critical structural rule confirmed:
file_offset = ecu_address − 0x8000
SubaruDefs describe full ECU memory space (192 KB).
ECUFlash dumps only accessible flash (160 KB).
That missing 0x8000 region is protected boot memory — not an error.
Once that translation rule was proven, table extraction became deterministic.
Step 2 — Raw MAF Extraction (No Scaling Assumptions)
We pulled raw 16-bit values from:
MAF Voltage Axis
MAF Airflow Table
First 15 voltage entries (RAW):
[254, 293, 336, 382, 433, 491, 556, 628, 704, 788, 877, 973, 1081, 1305, 1559]
That’s clean, monotonic data.
Scaling by 1000:
0.254 V → 1.559 V
Exactly what you expect at the lower end of a Subaru MAF curve.
Now airflow raw:
[12288, 12800, 13312, 13824, 14336, …]
Notice the pattern — increments of 512.
That immediately reveals airflow scaling:
airflow_gps = raw / 128
Example:
12288 / 128 = 96 g/s
12800 / 128 = 100 g/s
That matches Subaru’s typical scaling perfectly.
Step 3 — Detecting the Real Problem
Initial validation flagged:
Voltage monotonic: False
Airflow monotonic: False
Max Voltage: 63V (impossible)
Max Airflow: 600+ g/s (impossible for stock WRX)
That wasn’t a bad tune.
It was a bad assumption.
We were reading too many elements.
The table length had been hardcoded at 64.
The real MAF table length on this ROM is 48 elements.
After trimming to the correct boundary:
Voltage axis becomes perfectly monotonic
Airflow becomes perfectly monotonic
Maximum voltage ~4.7V
Maximum airflow realistic for stock turbo
That’s what proper extraction looks like.
Step 4 — Why This Matters
With stock injectors and 93 E10 fuel:
Injector scaling should remain factory
Latency should remain factory
Stoich should remain factory
If closed-loop trims move ±5%, that’s normal.
If trims drift further, MAF is the place to correct — not injectors.
Changing injector scaling to fix trims is amateur-level tuning.
Correcting measurement error is engineering.
Step 5 — Basemap Validation
We compared:
Stock ROM
Basemap reflash
Result:
MAF voltage axis: identical
MAF airflow scaling: identical
Injector scaling: unchanged
That means the basemap did not “tune around” hardware.
It’s a clean baseline.
That’s exactly what you want before building an AI-assisted calibration system.
Step 6 — What This Unlocks
Now that MAF is verified:
Load calculation is trustworthy
Open-loop fueling reflects real airflow
Knock events become meaningful
Long-term drift detection becomes possible
AI logic can operate on clean data
Without this step, everything is noise.
With it, everything becomes measurable.
The Bigger Vision
This isn’t about squeezing power.
It’s about building a system that:
Reads ROMs deterministically
Validates structural integrity
Extracts calibrated airflow models
Detects unsafe modifications
Applies bounded, stage-aware changes
The foundation is measurement accuracy.
Once airflow is correct, then — and only then — do you touch boost, fueling strategy, or timing.
Bottom Line
Before tuning power:
Tune truth.
On a 2005 WRX, that truth starts with the MAF sensor scaling.
And now it’s validated.
If you’d like, I can also generate:
A more aggressive “AI-powered tuning” version of this post
A technical deep-dive version with code snippets
Or a YouTube script version for your channel
What direction do you want to push the content next — authority, technical depth, or audience growth?