What the main limiter does
The main limiter stops the engine from going past a set speed by cutting fuel or spark once it reaches that rpm. That rpm is set in the End RPM table. On the setup page you choose how the cut happens.
The three settings
| Setting | Options | Our advice |
|---|---|---|
| Cut Method | Injection or Ignition | Ignition cut is sharper and holds rpm tightly. Fuel cut is gentler on the exhaust and turbo. See below |
| Cut Type | Hard Cut or Soft Cut | Soft cut for most street and road-course cars. Hard cut if you want it obvious to the driver |
| Hysteresis | An rpm value (hard cut only) | Small enough that the engine doesn't bounce far below the limit, large enough that it doesn't buzz |
Fuel cut or spark cut?
With Injection selected, the ECU turns off injectors to hold rpm. With Ignition selected, it turns off coils. Spark cut reacts almost instantly, because fuel already in the port or cylinder doesn't matter. But the unburned fuel it leaves behind goes into the exhaust, which is where the pops and bangs come from, and it's hard on turbos and catalytic converters. Fuel cut reacts a little slower, but it's easier on the exhaust side. Whichever you pick, check the result in a log at the limit.
Hard cut or soft cut?
- Hard Cut: below End RPM everything runs normally. Above it, all injectors or coils (depending on your cut method) are switched off. They come back on once rpm drops by the hysteresis amount. The result is the classic on-off "bouncing" limiter.
- Soft Cut: the ECU uses an algorithm to randomly drop individual cylinders, cutting just enough to hold rpm at or below End RPM. It's smoother and holds the limit more tightly.
Hysteresis only applies with hard cut. As an example, with End RPM at 7000 and hysteresis at 100, the cut releases once the engine drops back to 6900.
Reading the limiter channels
Plenty of things besides the main limiter can cut fuel or spark on a Nexus. The limiter channels tell you which one is doing it, so you don't waste time on the wrong feature.
| Channel | What it tells you |
|---|---|
| RPM Limiter Active | Shows Active (1) while any rpm limit is being enforced, Inactive otherwise |
| RPM Limiting Function | Which feature is enforcing the rpm limit |
| RPM Limiting Method | How that rpm limit is being applied, or None |
| Cut Percentage | How much cut is being applied right now. Usually 0% |
| Cut Percentage Function / Method | Which feature is asking for a percentage cut, and how, or None |
| Engine Limiting Function / Method | The summary: shows the rpm limiter's function and method if one is active, otherwise the cut percentage's, otherwise None |
| Engine Limiter Active | Shows whether an rpm limit or a percentage cut is in force, otherwise None |
In practice, Engine Limiting Function is the one to add to your log layout. If the engine stutters or won't rev, it tells you straight away what's responsible. Values you might see include:
- Driver and launch features: launch control, rolling alag, flat shift, traction control, torque reduction, trans_brake, speed limiter, driveshaft rpm ctrl, aux limiter, main limiter.
- Protection: over boost cut, eng protection, limp mode, dtc error.
- Engine management: decel cut, turbo timer, lpg limiter.
- Starting and idle: anti flood, flood clear, rotational idle, start_button, ignition switch, kill switch function.
- Faults and security: immobilizer, map ver error, supervisor error, etc error, etc redundancy error, inj_sys_errors, inj_sys_disable, feature_control_error, torque_model.
"No limiter" means nothing is cutting. If you see something unexpected, go to that feature's settings. For example, over boost cut points you to the overboost cut settings, and eng protection to engine protection.
Troubleshooting
| Symptom | Usually |
|---|---|
| Engine won't rev past a certain rpm, well below End RPM | A different limiter. Check Engine Limiting Function in a log |
| Loud bangs on the limiter | Ignition cut sending fuel into the exhaust. Try injection cut or soft cut |
| Limiter bounces hard | Hard cut with large hysteresis. Reduce it, or switch to soft cut |
| Engine overshoots the limit | Injection cut reacting slowly. Ignition cut holds a limit more tightly |
For launch-specific limits, see launch control, and for a second, switchable limit, see the aux limiter.
Main limiter FAQ
Where do I set the rev limit rpm?
In the End RPM table, not on the main limiter page. The main limiter page only sets how the cut is applied.
Which is safer for the engine, fuel or spark cut?
Both protect the engine from over-revving. Spark cut holds the limit more tightly but is harder on the exhaust and turbo, while fuel cut is gentler but slower to react. We choose based on the setup, and we're happy to advise if you get in touch.
