Fixed geometry turbochargers that lack controls are described as designed to provide maximum boost pressure at which condition?

• A. Rated speed or peak horsepower
• B. Idle speed
• C. Low engine load
• D. Maximum engine torque at low speed

Answer: (A)

Fixed geometry turbochargers without any boost control rely on the exhaust energy to drive the turbine, so boost rises as engine speed increases. Since there’s nothing to bleed off excess boost, the pressure climbs with RPM until the compressor reaches its flow limit. This means the highest boost occurs at the engine operating point where exhaust flow and energy are greatest—typically around the rated speed when horsepower peaks. At idle or light load, exhaust flow is too small to generate much boost, and at other points the boost won’t reach the maximum because the compressor can’t deliver more air. So the design aims for maximum boost when the engine is producing its peak power.