ARC-TUNING — Drivability Pack

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Scarecrow Briefing — Drivability Pack

The "everyday driveability" cluster: cold start, transient enrichment, AFM/DOD disable (Gen IV), VATS pass-through, fan setpoints, skip-shift disable. Each is a small fix on its own. Together they make the difference between "tuned car" and "tuned car you'd want to actually drive."

(1) Cold start = startup AE + warmup enrichment + idle airflow ramp. Three things working together: extra fuel during the first few seconds (startup AE), enriched AFR until ECT > ~140°F (warmup), and airflow tables that give the cold engine enough air to stay running. If idle (this section's neighbor) isn't dialed, cold start won't be either. (2) AE (Acceleration Enrichment) is transient fuel. When you stab the throttle, the wall film in the manifold momentarily strips. AE adds fuel to compensate so the engine doesn't bog. Too little = bog. Too much = black smoke + stumble. (3) AFM/DOD disable (Gen IV). AFM cycles between V8 and V4 modes for fuel economy. Bigger cams break AFM — the smaller cylinders see overlap they can't tolerate. Delete decision required (KB: afm-dod-disable-gen4.md): mechanical (lifter swap + cam removes feature) OR software-only (PCM stays in V8 always). (4) Skip Shift (manual T56). Forces 1→4 shift at low load. Annoying. Tuners disable. Setting in the cal. (5) VATS pass-through (anti-theft). Most LS swap cars need VATS off so the engine fires without the original key. Standalone scalar, must be set or engine cranks but won't start. (6) Fan setpoints. Low-speed fan on temp, high-speed fan on temp, both off temps (with hysteresis). Stock setpoints are too cold for built engines that run higher target ECT — bumping them up keeps the fans from cycling constantly.

KB: afm-dod-disable-gen4.md, vvt-cam-phaser-gen4.md, hpacademy-webinar-330-vats-afm-delete.md. Spec §4.2 (Drivability Pack).

5.1 Cold Start Fueling 📍 LOCATE

HPTEngine → Fuel → Cold Start → Startup Enrichment

— Critical for first 30s of operation

Three layers: (a) Startup Pulse — extra injector pulse at first crank to wet the manifold, (b) Warmup AFR — enriched AFR target until ECT > ~140°F (typically 12.5–13.5 vs stoich 14.7), (c) Idle airflow ramp — covered in the Idle section's 1.3 Cold-Start Percentage Max. All three must agree. Symptom of mismatch: starts and dies, needs 2–3 cranks, runs rough until warm, or floods on first crank.

The first 30 seconds after a cold start. Extra fuel, a richer AFR target, and more airflow until the engine is warm.

5.2 Acceleration Enrichment (AE) 📍 LOCATE

HPTEngine → Fuel → AE → AE PW vs TPS Rate

— Transient throttle-tip-in fuel

When TPS rate-of-change exceeds threshold, add a momentary fuel pulse. Compensates for fuel film stripping off manifold walls during transient — the wall-wetting effect. Stock LS values are reasonable. Bigger cam + bigger throttle body changes the rate sensitivity — usually need to raise AE PW slightly. Symptom: bog on tip-in (AE too low), or black smoke/stumble (AE too high).

Extra fuel when you stab the throttle. Stops the engine from bogging when you suddenly ask for power.

5.3 AFM / DOD Disable (Gen IV) 📍 LOCATE

HPTEngine → Fuel → AFM → Disable

⚠ Cam-aggressive forces decision

AFM cycles 8 cylinders down to 4 under cruise to save fuel. Stock works fine; bigger cams break it (overlap intolerance in V4 mode). KB: hard rule on AFM-equipped Gen IV — delete decision required before tuning. Three options: (a) Mechanical delete — non-AFM lifters + non-AFM cam. Strongest. (b) Software-only delete — PCM forced to stay V8 always. Lifters still AFM but never get told to switch. Risk: a stuck-collapsed lifter is silently fatal. (c) Leave stock — only valid on stock-cam Gen IV.

AFM lets the engine run on 4 cylinders to save gas. Doesn't work with a big cam. Two ways to remove it: replace the lifters (best) or just tell the PCM to never use it (cheap but riskier).

⛔ SCARECROW HALTS HERE IF…

AFM-EQUIPPED + AGGRESSIVE CAM + STOCK LIFTERS Software-only AFM disable on a built cam with stock AFM lifters is a known time-bomb. Recommend mechanical delete. Won't proceed silently.

5.4 Skip Shift Disable (T56 / TR6060 manual) 📍 LOCATE

HPTEngine → Transmission → Skip Shift → Disable

— Manual-trans only

CAGS (Computer-Aided Gear Selection / "Skip Shift") forces 1→4 shift at light load to satisfy CAFE fuel economy. Almost universally disabled on tuned manuals — the on-the-road behavior is annoying. Single scalar in the cal. Mechanical bypass also exists (skip-shift eliminator harness) for cars without re-tunes.

The thing on manuals that forces you from 1st to 4th gear. Universally hated. Turn it off here.

5.5 VATS / Anti-Theft Disable 📍 LOCATE

HPTEngine → Vehicle → VATS / PassLock → Disable

— Required for swap engines

Most LS-swap chassis lack the original VATS module/key. PCM expects a VATS handshake before allowing fuel + spark on first crank. If VATS is enabled in the cal but the chassis doesn't provide the handshake, the engine cranks but won't fire. Single scalar to disable. Be aware: if the chassis DOES have VATS and it's disabled in the cal, theft protection is gone.

Anti-theft system. If you swapped an LS into a different car without the original key, this needs to be off or the engine won't start.

5.6 Fan Setpoints (Low / High Speed) 📍 LOCATE

HPTEngine → Cooling → Fan Setpoints

— Higher ECT target = higher setpoints

Stock LS thermostat is 195°F; fans typically come on at ~210°F (low-speed) and ~225°F (high-speed). Built engines often run higher target ECT (especially with deletes / catless / FI) — bumping setpoints up 5–10°F prevents constant fan cycling on warm days. Hysteresis between on/off setpoints (~5°F gap) is critical to avoid hunting.

When the cooling fans turn on. Stock setpoints are too cold for built engines — fans run constantly. Raise them a bit.