Which indicator mechanism is used to measure manifold pressure?

• A. A bourdon tube gauge
• B. A capacitance sensor
• C. A hydraulic indicator
• D. A differential bellows

Answer: (D)

Manifold pressure is a relatively low, variable pressure inside the intake, so the sensing element needs to be highly sensitive to small pressure changes. A differential bellows setup does just that: one bellows is exposed to the manifold pressure and the other to a reference pressure, and the difference between them moves the bellows. That movement is transferred through linkages to the pointer, giving a direct, accurate read of the actual manifold pressure.

This arrangement provides good sensitivity and linear response in the low-pressure range typical of manifold pressure. A bourdon tube gauge, while reliable for higher pressures, isn’t as responsive or linear at these low levels. Capacitance sensors are more modern and uncommon in traditional MP indicators, and hydraulic indicators are not used for this purpose.