Why measure turbo speed?
Boost pressure tells you what the engine sees, not what the turbo is going through to make it. At altitude, or with a small turbo pushed hard, the shaft can be spinning far faster than the boost number suggests. A speed sensor gives you the real number, which is useful two ways:
- Matching: with shaft speed and airflow you can place your operating points on the compressor and turbine maps and see how efficiently the turbo is working.
- Protection: the ECU can watch for overspeed and bring in an engine limit before the turbo is damaged.
Calibration: number of blades
The sensor counts compressor blades as they pass. Enter the number of physical blades on your compressor wheel, counting every blade you can see at the inducer. This setting assumes a sensor with a built-in divide-by-eight circuit, the type commonly sold with Garrett and BorgWarner turbos. If your sensor works differently, the reading will be off by a fixed factor, so check it before trusting it.
Display and diagnostics
Warning Maximum sets the shaft RPM at which turbo speed gauges, bars and number displays switch to their warning state. Set it at or a little below the turbo maker's recommended maximum.
The Diagnostics node can enable a high frequency DTC. Give it a severity and it's linked to an engine protection level, so an overspeed can do more than light up a gauge.
Wiring
Turbo speed sensors use an SPI, and any free SPI will do. The recommended settings:
| Setting | Value |
|---|---|
| Edge Select | Falling |
| Sensor Type | Hall Effect |
| Pull Up | Enable |
The SPI Glitch Filter sets how long, in microseconds, a pulse has to hold its new state to count. Anything shorter is thrown out as noise. If your reading has random spikes, the filter and the wiring route are the first things we check. Keep the sensor wire well away from ignition leads.
Log turbo speed alongside boost and RPM for a few pulls before you set any protection level. Seeing the real curve makes it much easier to pick a sensible warning point. For the Elite version, see turbo speed sensor 1 and 2, and for the boost side, boost control.
