ARC-TUNING — Forced Induction

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Scarecrow Briefing — Forced Induction

Above-atmospheric tuning. Boost target, wastegate duty cycle, spark vs psi, richer PE under boost. Gated on a 2-bar or 3-bar MAP sensor — without one, the PCM can't read pressures above 105 kPa and the entire FI workflow refuses to open.

(1) MAP sensor + OS variant gate (spec §5.8). If pcm.map_sensor_bar === 1, page refuses to render. Above-atmospheric VE/PE/spark cells are locked out. Need 2-bar (positive) or 3-bar (high-boost) sensor first, plus an OS variant that recognizes it. (2) AFR target shifts richer under boost. KB band: 11.0–11.8 AFR at peak boost (vs 12.5–13.0 NA-PE). Charge cooling from richer mixture is the primary detonation defense. (3) Spark vs psi is a pull table. Typical: 0.5–1.0° pulled per psi above ~6 psi. Aggressive turbo builds may pull 1.5°/psi. KB: hpacademy-boost-response, wastegate-duty-cycle-formula. (4) Wastegate duty cycle is closed-loop boost control. Formula-driven: target boost / current boost → duty cycle correction. Spring pressure is the floor; WGDC raises above the spring. (5) IAT (charge temp) compensation matters more under boost. Higher pressure = higher temperature. Intercooler quality determines how much the comp table needs to do.

KB: ls-boost-tuning-overview.md, vasttuning-boost-control-wastegate-preload.md, hpacademy-boost-response-turbo-spool.md, hpacademy-wastegate-duty-cycle-formula.md. Spec §5.8.

4.1 MAP Sensor Gate 📍 LOCATE

HPTEngine → Sensors → MAP Sensor Bar / Range

⚠ Section gate per spec §5.8

Gate condition: pcm.map_sensor_bar >= 2 AND mods.forced_induction.type !== 'NA'. If either fails, the whole FI page refuses. Why: a 1-bar sensor saturates at ambient — the PCM cannot know how much boost is happening. VE / PE / spark cells above 105 kPa are written but never visited. The 2-bar swap is mechanical (sensor) and software (OS variant change to recognize the new sensor's voltage curve).

Forced induction tuning needs a sensor that can read above ambient. Without one, this whole page is locked.

⛔ SCARECROW HALTS HERE IF…

1-BAR MAP + FI declared Page refuses. Path forward: 2-bar or 3-bar MAP sensor + OS variant update. KB: ls-boost-tuning-overview.md.

4.2 Boost Target by RPM (Closed-Loop WGDC) 📍 LOCATE

HPTEngine → Boost → Boost Target vs RPM

— Target curve, WGDC trims to it

Target boost as a function of RPM. Typical streetable curve: ramp from spool RPM (~3000) at ~6 psi, peak by 4500 at full target (10–14 psi for stock LS1 internals, 18+ for forged), hold flat to redline. Closed-loop WGDC corrects pressure error to hit this curve. Spring pressure sets the FLOOR (what you get with WGDC at 0%). Anything above spring = WGDC working.

How much boost you want at each RPM. The wastegate adjusts to hit the target.

4.3 Wastegate Duty Cycle (WGDC) 📍 LOCATE

HPTEngine → Boost → Wastegate Duty Cycle

— Closed-loop control authority

Duty cycle commanded to the boost solenoid. 0% = wastegate fully open (spring pressure only). 100% = solenoid fully closed (max boost the turbo can deliver). The closed-loop logic compares target vs actual boost and adjusts WGDC. Tuning the WGDC table = giving the closed-loop a good starting point so it doesn't have to make big corrections. KB formula: WGDC = (target_psi − spring_psi) / (max_psi_capability − spring_psi) × 100, then trim from there.

How much the wastegate solenoid is "closed." 0% = wastegate flapping open at spring pressure. 100% = pinned shut for max boost.

4.4 Spark vs Boost (Timing Pull) 📍 LOCATE

HPTEngine → Spark → Spark Advance → Boost Pull Table

— Pull timing as boost rises

A negative-additive table. Above ~6 psi, pull spark advance progressively. Typical: −0.5°/psi for stock-internal builds (conservative), −1.0°/psi for built engines on pump gas, −1.5°/psi for race-fuel boost builds. Charge density rises faster than burn time can compensate, so the timing window for MBT (max brake torque) shrinks. Pull more than you think you need — detonation under boost = bent rods.

As boost rises, pull spark timing back. The combustion chamber gets more crowded with charge, so timing has to come back to keep it from knocking.

4.5 PE Under Boost (Richer AFR Target) 📍 LOCATE

HPTEngine → Fuel → PE → Boost-Active Cells

— Richer than NA-PE

AFR target shifts richer when boost is present. KB band: 11.0–11.8 AFR at peak boost (vs 12.5–13.0 NA-PE). The extra fuel charge-cools the cylinder — cooler charge resists detonation. Don't chase WOT AFR with STFT — STFT does not apply at WOT (per spec §5.4). Read AFR Actual from wideband and adjust the PE table cells directly.

Run the engine richer when on boost — the extra fuel cools the cylinder and prevents knock.

4.6 IAT Compensation under Boost 📍 LOCATE

HPTEngine → Fuel → IAT Compensation

— Charge temp comp

Higher boost = higher charge temp = more fuel needed to maintain target AFR (cool the charge) AND less spark to avoid detonation. The IAT comp tables apply across all driving — but their values are most aggressive in the boost cells. Intercooler quality determines how steep the comp curve needs to be. Air-to-air with good frontal area: gentle slope. No intercooler / heat-soaked: steep slope. Log IAT during sustained boost runs.

When intake air gets hot from being compressed, the engine needs more fuel and less spark. A good intercooler keeps this manageable.

Forced Induction (Boost)

Scarecrow Briefing — Forced Induction