From the FAA's Pilot's Handbook of Aeronautical Knowledge (much is crap but it does show the FAA party line on things):
For any given rpm, there is a manifold pressure that should not be exceeded. If manifold pressure is excessive for a given rpm, the pressure within the cylinders could be exceeded, placing undue stress on the cylinders. If repeated too frequently, this stress can weaken the cylinder components and eventually cause engine failure. As a general rule, manifold pressure (inches) should be less than the rpm.
A pilot can avoid conditions that overstress the cylinders by being constantly aware of the rpm, especially when increasing the manifold pressure. Conform to the manufacturer’s recommendations for power settings of a particular engine to maintain the proper relationship between manifold pressure and rpm.
When both manifold pressure and rpm need to be changed, avoid engine overstress by making power adjustments in the proper order:
• When power settings are being decreased, reduce manifold pressure before reducing rpm. If rpm is reduced before manifold pressure, manifold pressure will automatically increase, possibly exceeding the manufacturer’s tolerances.
• When power settings are being increased, reverse the order—increase rpm first, then manifold pressure.
• To prevent damage to radial engines, minimize operating time at maximum rpm and manifold pressure, and avoid operation at maximum rpm and low manifold pressure.
The engine and/or airframe manufacturer’s recommendations should be followed to prevent severe wear, fatigue, and damage to high-performance reciprocating engines.
