Wall-Film Compensation¶

The ECU models the fuel puddle on the intake-port walls and corrects what it injects, so the cylinder gets the fuel you asked for — through cold cranks and fast throttle changes.

Demo build: this panel is locked

The enrichment column is available in the full game. See Demo vs Full Version.

What it is¶

In a port-injected engine some of every injection lands on the port wall as a liquid film instead of reaching the cylinder, and the film evaporates back into the airflow over time. Two numbers describe it — the classic X–τ (x-tau) model:

X — the fraction of injected fuel that goes into the film.
τ (tau) — the film's evaporation time constant: how slowly it gives fuel back.

When fuel demand rises, the film grows and steals fuel (a lean dip); when demand falls, it releases extra (a rich tail). Cold ports make X bigger and τ longer, which is why cold tip-ins stumble and cold decel runs rich.

Why it matters¶

With WALL-FILM COMP enabled, the ECU runs its own film model, fed with the fuel that was actually delivered (after DFCO and limiter cuts), and injects

\[\frac{\text{fuel the cylinder should get} - \text{film evaporation}}{1 - X}\]

The correction is clamped to 0.1× – 8× of the requested fuel, and the film "dries" at 8×τ when it isn't being fed. It runs last in the chain, after the other enrichment layers, and is included in the TOTAL multiplier and the left-hand ENRICH readout. Closed loop treats it as deliberate enrichment, so it stays out while the film is correcting.

What changes when it is on (important):

The crank table stays active until the engine has caught (not just below about 550 RPM) and becomes simply the cranking mixture — about 1.03–1.10× instead of the classic 3.5× at −40 °C. The puddle is no longer hidden inside a huge multiplier.
Warmup enrichment returns 1.0× until the engine has caught, then applies (tapered by load).
Acceleration enrichment should be zero. Wall-film compensation already covers the transient; leaving AE on double-fuels tip-ins and pumped throttles (a rich bog).

Stock tunes. All three shipped engines have it on, with AE at zero.

How to tune it¶

You normally don't need to: the stock curves match the film model. If you change a curve, judge it by λ during transients — warm first, then cold:

A lean dip on tip-in → raise X for that coolant temperature.
A rich bump → lower X.
τ sets how long the correction lasts (longer τ = slower release).

Keep the three curves smooth.

Rule of thumb

Film comp on → AE at zero, crank about 1.0–1.1×. Film comp off → you do the job by hand with big crank and AE curves.

Watch out

Turning it off on a stock tune leaves crank at about 1.05× and AE at zero, so cold starts will likely fail and tip-ins stumble. If you disable it, rebuild the crank curve (back toward 2–3.5× when cold) and the AE curves first.

In the game¶

FUEL page → right column → WALL-FILM COMP card.

A header status chip, OFF / ON.
The X-TAU MODEL toggle: DISABLED / ENABLED.
Three coolant-temperature curves you can edit: ECT → FILM FRACTION X, ECT → EVAPORATION TAU and ECT → CRANK PUDDLE (the extra share of fuel that a dry, cold port soaks up while cranking at low speed; it fades as RPM rises).
Live readouts: FILM: n mg (the ECU's film estimate) and the correction multiplier (1.00x means no correction).