Which operating mode would be more likely to result in a compressional failure of a connecting rod?

The more likely scenario for causing a compressional failure of a connecting rod is related to hydraulic lock caused by head gasket failure. When a head gasket fails, it can allow coolant or oil to enter the combustion chamber, creating a condition where the cylinder fills with liquid instead of a combustible gas-air mixture.

In this situation, when the piston tries to compress the liquid, it can create an immense amount of pressure that the connecting rod is not designed to withstand. Unlike compressible gases, liquids are incompressible, meaning they do not allow for expansion during the compression stroke. This can lead to a failure of the connecting rod, as it may buckle under the excessive force exerted during compression.

Prolonged low idle running, prolonged high idle running, and overspeeding the engine can all cause various types of engine wear or damage, but they do not create the same immediate risk of compressional failure in the connecting rod as a hydraulic lock does. The high pressure and lack of space for compressible fluid in cases of hydraulic lock presents a unique and dangerous scenario specifically leading to the exact type of failure in question.