I should apologise for my earlier poor answer due to not reading the question properly. If you do not move the throttle and the RPM drops for some reason, yes the MAP will increase. Shytorque's post explains it simply. It is seen in a mag check or when exercising the prop.
More generally, the MAP does not necessarily increase with reduction in RPM. It could increase or decrease, depending on throttle position and load as well as RPM.
When you close the throttle the MAP drops and vice versa. The manifold pressure will in turn affect the pressure in the cylinder which is effectively your torque. A key concept is that torque x RPM is proportional to power. Hence the power setting table of an aircraft with CS prop gives different combinations of MAP and RPM resulting in the same power - some with higher revs and lower MAPs than others. On the other hand, with a fixed pitch prop if you close the throttle the MAP and RPM will both reduce. Because that also reduces the power, it is still consistent with all of the above. (Power is set by throttle on a fixed prop whereas power is set by throttle and RPM control on a CSU).
Last edited by oggers; 20th Jan 2017 at 13:11.