What should be avoided when operating an engine with a constant-speed propeller?

When operating an engine with a constant-speed propeller, avoiding high manifold pressure settings with low RPM is crucial for maintaining engine health and efficiency. High manifold pressure at low RPM can lead to a condition known as "detonation," where the fuel-air mixture in the combustion chamber ignites prematurely due to excessive pressure. This situation can cause excessive and uneven forces on the engine components, potentially leading to severe engine damage or failure.

At low RPM, the engine is not operating at a sufficiently high speed to effectively utilize the high manifold pressure, which can also create a situation where the engine is not able to expel exhaust gases properly, further contributing to overheating and inefficient operation. Maintaining the appropriate balance between manifold pressure and RPM is key to ensuring smooth engine operation and longevity.