What does a massive drop in manifold pressure after reducing RPM indicate?

A massive drop in manifold pressure after reducing RPM typically indicates a problem in the intake system. Manifold pressure is an important measure of the air pressure within the intake manifold of a reciprocating engine. When RPM is reduced, one would generally expect to see a corresponding adjustment in manifold pressure based on the engine’s operational demands.

If there is a significant drop in manifold pressure beyond what is normal, it can suggest that there is an issue with how air is entering the engine. This could be due to a restriction, a leak, or other abnormalities in the intake system that prevent adequate air from reaching the manifold. An effective intake system is crucial for maintaining appropriate manifold pressure, especially when engine power levels are adjusted.

In contrast, a malfunctioning propeller control and throttle control would likely result in different types of pressure changes or engine performance issues that are not specifically characterized by a massive drop in manifold pressure alone. The intake system's integrity directly affects the pressure available to the cylinders, thus making it the most logical cause for such a significant pressure drop after RPM reduction.