What overboost cut does
Turning on Overboost Cut adds an Overboost Cut section to the ECU Navigator in NSP, along with the Cut Pressure table. That table can be mapped in up to 2D, so the limit can vary with a second channel of your choosing instead of being one fixed number.
The logic is simple. If boost stays above the cut pressure for longer than the Delay, the ECU cuts power to protect the engine. It keeps cutting until boost falls below the cut pressure by the Hysteresis amount, then goes back to normal.
NSP quotes all its default values in kPa because there are so many pressure units to choose from. You can display psi if you prefer, but be clear which one you're looking at when you enter numbers.
The settings
| Setting | What it does | Our advice |
|---|---|---|
| Cut Method | How power is removed: Ignition, Fuel, or Fuel & Ignition | Ignition is the more common choice. Fuel cut on a boosted engine can cause lean cylinders, so understand your setup before using it |
| Delay | How long boost must stay over the limit before the cut activates | Just long enough to ignore a momentary spike, short enough that a real overboost is caught quickly |
| Hysteresis | How far below the cut pressure boost must fall before power returns | Big enough that the engine doesn't flicker in and out of cut |
A worked example
Say the cut pressure is 200 kPa and hysteresis is 15 kPa. Boost hits 205 kPa and stays there longer than the delay, so the ignition is cut. Boost drops, and when it falls below 185 kPa (200 minus 15), the engine returns to normal. Without enough hysteresis, boost would drop just below 200, the cut would release, boost would rise again, and the engine would stutter on and off right at the limit.
Where to set the limit
Put the cut pressure above your highest boost target, with enough margin that normal overshoot never trips it, but well below what the engine can survive. It isn't a boost controller. It's the backstop for when boost control fails. The boost control setup is what holds boost on target day to day. Overboost cut is the line of defense for when that control is lost, for example when a wastegate hose splits.
Check the function works by temporarily setting a low cut pressure on a mild pull, confirm it cuts and recovers as expected in the log, then put your real limit back.
Tuning the boost controller itself? See closed loop proportional gain and closed loop integral gain.
