What the Aux RPM Limiter is for
Your main RPM limiter protects the engine all the time. The auxiliary limiter is a second, temporary ceiling that only comes into play when an input tells it to. Flip a switch and the engine is held to a lower rpm, release it and the normal limit returns.
Typical uses we set up on Elite ECUs:
- Basic launch: hold a clutch or button switch and the engine sits at a set rpm on the line. For anything beyond basic, the dedicated launch control function gives you far more control.
- Valet or learner mode: a hidden switch caps revs when someone else is driving.
- Warm-up limit: a lower ceiling you can switch on while the oil comes up to temperature.
Because it only works with an input, the first job is wiring and configuring that switch.
Wiring tab: setting up the switch input
Pick an input for AUX RPM Limiter Switch Input. You can use an SPI or an AVI. Which you choose depends on what's free and what your switch does electrically.
Using an SPI
- Edge Select: choose Falling if the switch connects the input to ground when you turn it on. Choose Rising if it feeds voltage to the input when on.
- Sensor Type: set to Hall Effect for any switch input.
- Pullup: turn this on when the switch only grounds the wire. The pullup gives the input a voltage to sit at while the switch is open, so the ECU can see the change.
Using an AVI
An AVI reads a voltage, so you tell ESP two thresholds: Switch On Voltage and Switch Off Voltage. The gap between them stops the limiter flickering on and off if the signal wanders. Place each threshold well inside the switch's real on and off voltages rather than right at the edge.
A worked example of our own: say your switch puts about 5V on the wire when on and the wire sits near 0V when off. A sensible setup would be on above about 4V and off below about 1V. The input has to climb clearly past 4V before the limiter engages, and drop clearly under 1V before it lets go. Measure your actual switch voltages with a meter or in the ECU's live data before you commit to numbers.
Aux Rev Limit tab: how the cut is applied
Cut Type
| Cut Type | How it limits rpm | Our take |
|---|---|---|
| Hard Cut | Cuts every cylinder at the limit, then waits for rpm to drop by the hysteresis amount before firing again | Simple and firm, but it bounces off the limit |
| Soft Cut | Randomly drops individual fuel or spark events, decided every firing event, to hold rpm at or under the limit | Smooth and quick to respond. Our usual pick for valet and warm-up limits |
| Cut Off | Applies a percentage cut you map, using a cylinder pattern you choose with the Adder | Most tunable. Lets you shape how smooth or rough, and how loud, the limiter feels |
Cut Method
- Ignition: spark is cut. This is the common choice.
- Fuel: injectors are cut.
- Fuel & Ignition: both are cut together.
An ignition cut lets fuel pass through unburned. Held for long periods on a turbo car, that can mean pops in the exhaust and extra heat downstream. For a limiter you'll sit on for a while, keep hold times short and check exhaust temperatures if you log them.
Related tables
You'll find these under ECU Navigator > RPM Limits > Aux RPM Limiter.
- RPM Limit: the rpm where the aux limiter starts cutting. It can be mapped as a 2D table rather than a single number.
- Cut Hysteresis: Hard Cut only. How far rpm must fall below the limit before fuel or spark comes back.
- Cut Percentage: Cut Off only. The percentage of events removed, mappable in an 8x8 3D table.
- Adder: Cut Off only. Sets the pattern of which cylinders get cut, also an 8x8 3D table. Values close to the recommended number for your cylinder count give the most even, smooth cut. The further you move away, the rougher and less even it gets.
| Cylinder count | Recommended Adder |
|---|---|
| 1 | 151 |
| 2 | 151 |
| 3 | 187 |
| 4 | 147 |
| 5 | 198 |
| 6 | 197 |
| 7 | 193 |
| 8 | 179 |
| 9 | 191 |
| 10 | 197 |
| 11 | 201 |
| 12 | 207 |
Troubleshooting
| Symptom | Usually |
|---|---|
| Limiter never activates | Wrong edge selected, pullup off on a ground-switching input, or AVI thresholds outside the real switch voltages |
| Limiter flickers on and off | AVI on and off voltages too close together |
| Very rough, uneven cut | Adder far from the recommended value for your cylinder count |
| Engine bounces hard off the limit | Hard Cut with a large hysteresis. Try Soft Cut |
Running a Nexus instead? See the Nexus aux RPM limit article. For how launch strategies compare, read how launch control works.
