When the throttle is partially closed on a supercharged engine, why does MAP decrease?

• A. Less mass of mixture is drawn into the supercharger, so the pressure developed decreases.
• B. MAP increases due to backflow.
• C. Throttle position has no effect.
• D. Temperature drops causing reduced pressure.

Answer: (A)

The main idea is that MAP (manifold absolute pressure) is the pressure in the intake manifold, which depends on how much air the supercharger can push into the engine. When the throttle is partially closed, the amount of air that can enter the supercharger is reduced. With less air flowing through the compressor, it cannot develop as much pressure rise (the compressor operates at a point on its map with a lower discharge pressure). That lower discharge pressure translates to a lower pressure in the intake manifold, so MAP falls.

In other words, throttle closure restricts mass flow through the supercharger, limiting the boost the system can produce. The other notions (backflow, no effect from throttle, or temperature alone causing a pressure drop) don’t explain why the manifold pressure actually decreases in this situation.