It really depends on what direction you want to go regarding engine monitoring and management. For example, performance monitoring requires a different set of gauges as opposed to engine health monitoring. A whole slew of sensor data is available at the ECU which can pulled via any OBD-II reader (or feature-rich engine management system) when necessary. This helps if you want to get some static values to aid in troubleshooting or with a project, etc. So the more likely question is, what type of monitoring do you need to perform while sitting in the drivers seat underway(?).
If performance monitoring of a turbo motor setup is your objective, then my opinion is as follows:
- Boost Gauge: Absolutely most important. Allows the driver to constantly monitor boost values for optimal boost.
- EGT (Exhaust Gas Temperature): Essential for tuning and properly monitored can allow the driver to save a motor if things go wrong under high RPM.
- AFR-WB (Wideband Air/Fuel Ratio): Again, essential for proper performance monitoring. Allows the driver and tuner to push the motor to the edge without going over. Alerts driver to dangerously lean conditions.
The more important question here is where are you going to mount the gauges you choose to go with? The Legacy interior and dash is not the most gauge friendly setup Subaru came out with…