How does induction system restriction affect volumetric efficiency and power output?

How does induction system restriction affect volumetric efficiency and power output?

• A. Increased restriction has no effect on VE or power.
• B. Increased restriction lowers VE, reduces max power, worsens intake velocity, and can cause richer or lean conditions depending on the system, especially at high RPM.
• C. Increased restriction increases VE and power.
• D. Restriction only affects engine idle.

Answer: (B)

Induction restriction acts like a bottleneck for the incoming air. That bottleneck causes a pressure drop as the piston tries to draw air into the cylinder, so the charge entering the cylinder is less dense. Because volumetric efficiency is about how effectively the engine fills the cylinders with air relative to the theoretical amount, this lower charge density means VE falls. Less air per revolution means less fuel can be burned, so the maximum power the engine can produce drops, and this effect becomes more noticeable at higher RPM when the engine demands more air. Air velocity through the restriction can be higher locally, but the overall filling is worse because the downstream pressure is lower and the cylinder ends up receiving a smaller mass of air per cycle. The fuel system responds to how much air is sensed; with reduced air, the air-fuel ratio can swing toward richer or leaner conditions depending on how the system meters fuel and compensates for the lower manifold pressure, with the swings often more pronounced at high RPM when the demand is greatest.

Induction restriction acts like a bottleneck for the incoming air. That bottleneck causes a pressure drop as the piston tries to draw air into the cylinder, so the charge entering the cylinder is less dense. Because volumetric efficiency is about how effectively the engine fills the cylinders with air relative to the theoretical amount, this lower charge density means VE falls. Less air per revolution means less fuel can be burned, so the maximum power the engine can produce drops, and this effect becomes more noticeable at higher RPM when the engine demands more air.

Air velocity through the restriction can be higher locally, but the overall filling is worse because the downstream pressure is lower and the cylinder ends up receiving a smaller mass of air per cycle. The fuel system responds to how much air is sensed; with reduced air, the air-fuel ratio can swing toward richer or leaner conditions depending on how the system meters fuel and compensates for the lower manifold pressure, with the swings often more pronounced at high RPM when the demand is greatest.