Why rotational idle exists
Many lift-off anti-lag setups keep extra air flowing into the engine when you come off the pedal, often by cracking the throttle open or using a bypass. That airflow keeps the turbo lit, but it has a side effect: at idle the engine is suddenly getting far more air than it needs, so it wants to rev well above a normal idle.
Rotational idle solves that without taking the air away. The Nexus cuts cylinders to keep rpm down, and it rotates which cylinders are cut so no single one sits cold. The closer rpm gets to the limit you set, the more cylinders it cuts. The result is a steady idle and that lumpy, rally-car sound. Some people run it for the sound alone.
You only need it when the extra airflow can't be switched off. If your anti-lag setup closes its air source at idle, rotational idle isn't needed.
Main settings
| Setting | What it does | Our advice |
|---|---|---|
| Enable Rotational Idle | Turns the function on | Wire and test the activation switch first |
| Activating Engine Demand | Rotational idle is active below this throttle position (cable throttle) or pedal position (drive-by-wire) | Just above your closed-pedal reading, so it releases as soon as you step on it |
| Idle Valve Position | How far the idle valve is opened while the function is active | Match how much air your anti-lag strategy needs |
| End RPM | Above this rpm, every cylinder is cut | A little above your normal non-anti-lag idle speed, adjusted until idle is stable |
| Cut Method | Chooses whether ignition or injection is cut | See below |
Ignition cut or injection cut?
- Injection: the injectors for the cut cylinders stop firing. This is the most common choice for rotational idle.
- Ignition: spark is cut but fuel still flows. People choose this when that fuel is wanted for cooling, but unburned fuel reaching a hot exhaust is part of how anti-lag makes heat, so think about what your turbo and exhaust can take.
Conditional activation
You'll rarely want rotational idle on all the time, so it can be tied to a switch and extra conditions.
- Condition Enable: must be on if you've set up a switch in the wiring tab. Wiring is covered in our rotational idle wiring article.
- When Rotational Idle Switch is On, And/Or: lets you use the switch alone, or combine it with another condition.
- Select: the ECU channel to test, for example coolant temperature or vehicle speed.
- Maths: the comparison operator. You can test whether the channel matches the value exactly, differs from it, sits above or below it, or is at least or at most that value.
- Value: the number the channel is compared with.
For example, you might require the switch to be on and coolant temperature to be above normal operating temperature, so it can never run on a cold engine. The same condition logic appears across NSP, and our condition settings article explains it in detail.
Setting it up, step by step
- Set up your anti-lag air source and confirm how high the engine idles without rotational idle.
- Set Activating Engine Demand just above closed throttle or pedal.
- Choose your cut method and a conservative End RPM a little above normal idle.
- Set the idle valve position your strategy calls for.
- Enable it, turn on the switch, and watch rpm. Adjust End RPM until idle is steady without surging.
- Check that it drops out cleanly as soon as you press the pedal.
Rotational idle is usually paired with anti-lag, and anti-lag puts a lot of heat into the manifold and turbo. Log exhaust temperatures and don't leave the car idling in this mode longer than you need to.
Troubleshooting
| Symptom | Usually |
|---|---|
| Idle surges up and down | End RPM too close to the natural idle speed, or too much air for the cut to control |
| Engine stalls when it activates | End RPM too low, or idle valve not opened enough |
| Cut stays on while driving off | Activating Engine Demand set too high |
| Nothing happens | Condition Enable off with a switch configured, or condition not being met |
FAQ
Do I need rotational idle for anti-lag?
Only when your extra anti-lag airflow is fixed rather than switched. See rolling anti-lag for related setups.
Is it the same on Elite ECUs?
The idea is the same, but the Elite has its own page. See rotational idle for Elite.
