What instrument provides information about engine performance in a complex airplane?

The manifold pressure gauge is crucial for providing information about the engine performance in a complex airplane. In such aircraft, particularly those equipped with a constant-speed propeller and a retractable landing gear, the manifold pressure gauge indicates the absolute pressure of the fuel-air mixture entering the engine's cylinders. It helps pilots assess the engine's performance during various phases of flight, such as climb, cruise, and descent.

Monitoring manifold pressure is essential for managing engine output because it directly correlates with the engine's power setting and efficiency. When the manifold pressure is lower, it generally indicates that the engine is producing less power, and this can be vital information for optimizing fuel consumption and maintaining the desired climb rate or cruise speed.

Other instrument options serve different functions: the attitude indicator displays the aircraft's orientation relative to the horizon; the vertical speed indicator measures the rate of ascent or descent; and the heading indicator shows the aircraft's current heading. None of these instruments provide direct information about engine performance like the manifold pressure gauge does.