# Full mechanical-odo Bonneville permutation grid — cross-diff (Aug 2026)

Downloaded and cross-diffed every official mechanical-odo Bonneville
calibration TuneECU ships for the exhaust x fuel grid (up to VIN 739050):
stock production/aftermarket, Arrow 2-in-1, Arrow 2-in-2, each at E10 and
E25, across both catalog batches (`t202`/`t203`). 12 files, all reconstructed
to the flat 384 KB ROM with `reconstruct_rom.py` and diffed byte-for-byte.

IDs used: `20187 20188 20191 20192` (stock), `20262 20263 20264 20265`
(`t202` Arrow), `20313 20314 20315 20316` (`t203` Arrow).

## t202 vs t203: same tune, reissued

Every `t202`/`t203` pair with an identical catalogue description (e.g.
`20262` vs `20313`, both "Arrow 2 in 1, E10") differs by only **10 bytes**,
in two groups:

- 8 bytes at `0x4FFFA-B/E-F` and `0x5FFFA-B/E-F` — right at the `0x50000`/
  `0x60000` block boundaries, almost certainly per-block ID/checksum
  metadata (consistent with `DEVICES.md`/`README.md`'s prior finding that
  the `caXX` footer bytes are map-ID metadata, not a calibration checksum).
- 2 bytes at `0x53626-27` — one real 16-bit calibration value changed. Small,
  single-cell revision.

**Conclusion: `t202` and `t203` aren't different calibrations, just a
catalog-ID reissue with one tiny tweak.** Resolves the earlier open question
("which of 20262 vs 20313 is correct for us") — it doesn't matter, use
either.

## Fuel delta (E10 -> E25) is real, consistent, and localized

Matched pairs (same exhaust, VIN range, everything else equal, only fuel
spec differs):

| Pair | Exhaust | Diff |
|---|---|---|
| `20262` vs `20263` | Arrow 2-in-1 | 4809 bytes (1.22%) |
| `20264` vs `20265` | Arrow 2-in-2 | 4562 bytes (1.16%) |
| `20313` vs `20314` | Arrow 2-in-1 (t203) | 4809 bytes — **identical set** to `20262`/`20263` |
| `20315` vs `20316` | Arrow 2-in-2 (t203) | 4562 bytes — **identical set** to `20264`/`20265` |
| `20187` vs `20191` | stock production | only 60 bytes (0.02%) |

The E10->E25 delta for Arrow 2-in-1 vs Arrow 2-in-2 overlaps 93% (4455 of
4562-4809 bytes) — same fuel-table region touched regardless of exhaust,
which is exactly what you'd expect (ethanol compensation lives in the fuel
tables, not the exhaust-specific cells). The stock-production E10->E25 delta
being tiny (60 bytes) while the Arrow one is ~80x larger is odd at first
glance, but `20187` carries no VIN-range/fuel spec in its description at
all — it and `20191` likely aren't a clean matched pair the way the Arrow
E10/E25 pairs are (`20191` adds a VIN-range qualifier `20187` lacks). Treat
`20262`/`20263` as the trustworthy E10-vs-E25 reference, not `20187`/`20191`.

## Exhaust delta and fuel delta are NOT independent/disjoint

Checked whether the "stock -> Arrow 2-in-1" delta and the "E10 -> E25" delta
touch separate bytes (which would let us compose them by simple
copy/overlay):

- Exhaust delta (`20187` -> `20262`): 7548 bytes
- Fuel delta (`20262` -> `20263`): 4809 bytes
- **Overlap: 4149 bytes** — the large majority of the fuel delta's footprint
  is inside the exhaust delta's footprint too.

**This means naive delta composition (baseline + delta A's changed bytes +
delta B's changed bytes) doesn't work cleanly** — both mods rewrite
overlapping cells in the same VE/fuel tables (expected: richness-for-exhaust
and richness-for-ethanol are both corrections to the same underlying fuel
surface, so of course they land on the same cells). You can't just XOR two
independent byte-level diffs onto a base map; you'd overwrite one mod's
change with the other's rather than combining them. Real composition would
need table-level math (read both as VE values, apply both corrections
arithmetically), not byte-patching — a materially bigger undertaking than
`toggle_devices.py`'s simple flag flip.

## What this means for "build our own Arrow + SAI/O2-delete tune"

**Good news:** the exhaust x fuel grid is *already solved* — TuneECU
engineers already shipped the exact validated combination you want
(`20262`/`20313`, Arrow 2-in-1, E10). No synthesis needed there; picking the
catalog entry is strictly better than trying to reconstruct it from deltas.

**Bad news, unchanged from `COMMUNITY_TUNING.md`:** none of these 12 files
vary the SAI/O2-delete axis — TuneECU never shipped an Arrow-exhaust variant
with devices off. The grid we now have doesn't help fill that gap, because
the gap isn't on this grid. We're still bottlenecked on the single external
`2009AIRBOXBONNY.hex` reference (not currently in the repo, needs
re-fetching) for that axis, and even that reference conflates "no airbox"
with "no SAI/no O2" in one diff (per `DEVICES.md`), which the exhaust-vs-fuel
overlap finding above suggests may be a *real* problem, not just a
theoretical one — table-level effects don't separate cleanly by simple
byte diffing when two mods land on the same cells.

**Revised recommendation:** don't try to hand-synthesize "Arrow 2-in-1 +
real SAI/O2 delete" from byte deltas — the overlap finding shows that's not
sound with the tooling we have (byte-diff, not semantic table-diff). Either:
1. Decode the fuel tables to actual VE values (`table_map.py`/`TABLES.md`
   already locate them) and do the composition arithmetically, cell by cell,
   rather than at the byte level — a real project, not a script tweak.
2. Or just use a real matched commercial tune (DNK, since TTP's storefront is
   effectively defunct) rather than DIY-composing one.