What should the manifold pressure gauge indicate on a normally aspirated engine operating at full power?

On a normally aspirated engine operating at full power, the manifold pressure gauge typically indicates slightly below the ambient atmospheric pressure. This occurs because a normally aspirated engine does not utilize a supercharger or turbocharger to increase the intake air pressure; it relies solely on atmospheric pressure to fill the engine's cylinders during the intake stroke.

When the engine operates at full power, it is drawing in as much air as possible, which creates a slight vacuum in the manifold compared to the ambient atmospheric pressure. As the engine requires a large amount of air to maintain optimal performance at high power settings, the pressure in the manifold will drop slightly due to this high airflow and the mechanics of the engine's operation.

Understanding the operation of normally aspirated engines is crucial, as it contrasts with turbocharged engines that would operate at manifold pressures above atmospheric levels due to the forced air provided by the turbocharger. This fundamental difference in how the engine draws in air is the key to grasping the behavior of the manifold pressure gauge readings during different operating conditions.