Rotational idle

Rotational idle is the drift-car cousin of anti-lag. With the clutch in and the throttle shut, the ECU holds a high idle (typically 2 500–4 500 rpm) with spark cut so the engine is spinning, the turbo is in, and a clutch-kick actually does something. It is not a 2-step (that is launch, wheels stationary) and it is not full ALS (that is off-throttle in gear with extra fuel and retard).

When it should be on

• Clutch switch in (digital input, grounded when the pedal is down).
• TPS below a small threshold — the driver is not asking for throttle.
• Vehicle speed optional: some tuners enable it only below a speed so it cannot sit on in a long straight with the clutch slipped.
• CLT above a minimum. A cold engine at 4 000 rpm spark-cut idle washes the bores.

How it holds RPM

The usual implementation is a spark-cut rev limiter at the rotational-idle target, with enough fuel to keep the engine alive and the turbo from stalling. Air comes from the idle valve, a cracked ETB, or the throttle stop. If the engine cannot physically make that RPM with the throttle shut, the limiter has nothing to cut — lower the target or add air.

Tune

1. 01Clutch switch

   Log it. Pressed = on, released = off. Invert in software if the hardware is the other way. A floating input will randomise this.

2. 02Target RPM

   Start around 3 000 on a small turbo, 3 500–4 000 on a big one. The number that keeps boost from falling over between corners is the right number, not a forum screenshot.

3. 03Fuel

   Slightly rich of idle VE. Rotational idle that goes lean pops in the intake and eventually holes a piston.

4. 04Handoff

   Clutch out must return you to the normal spark table in tens of milliseconds. If it hangs, the arming TPS or clutch hysteresis is too wide.