A naturally aspirated diesel engine at full throttle will have an intake manifold pressure that is:

In a naturally aspirated diesel engine, the air drawn into the combustion chamber comes solely from atmospheric pressure, without any assistance from a supercharger or turbocharger. At full throttle, the engine is operating at maximum intake air flow, but because it relies on atmospheric pressure to fill the cylinders, the manifold pressure typically remains slightly less than atmospheric pressure. This is primarily due to the factors such as intake resistance, air ducting, and the vacuum created by the downward motion of the pistons during the intake stroke.

When the engine is at full throttle, it works hard to pull in as much air as possible to support the combustion process; however, even at that maximum effort, there are still losses and restrictions that cause the intake manifold pressure not to reach atmospheric levels. Thus, while the engine is receiving a substantial amount of air, the pressure will indeed be slightly lower due to these dynamics.

Understanding this principle is crucial for marine engineers and those working with diesel engines, as it guides them in troubleshooting and optimizing engine performance.