This was one of the questions in my online ground school quiz (PPL), detail as follows:
To set climb power, you reduce the throttle (manifold pressure) and then the props (RPM lever). After setting the props you expect the manifold pressure to:
A) not change B) temporarily rises than fall again to where it was C) fall slightly D) rise slightly
My original answer was there is no net change in the MP reading after performing those two reduction actions.
My understanding of MP reading is that a reduction in throttle would mean more suction/vacuum as a result so the MP reading would decrease. On the other hand a reduction in RPM would mean less suction/vacuum so the MP reading would increase.
Because of the above, and that there is no other detail from the question as to how much changes each action had, wouldn't it be correct to expect MP reading to first decrease (from the throttle reduction) and then rises back again (from the RPM reduction). The net change therefore should be 0.
The correct answer however was d) slightly rises. The explanation that was given is because the airplane was transitioning from take off power to climb power.
Why does MP rises slightly in this case?