Cranking vacuum should be?

Cranking vacuum is a measurement that indicates the efficiency of the engine's ability to create a vacuum within the intake system during the cranking process. A reading of 2.5 inches of mercury (in. Hg) or higher is considered acceptable as it suggests that the engine has a good amount of vacuum, which is necessary for proper fuel atomization and air-fuel mixture, especially during starting when the engine is under load.

When the cranking vacuum is at or above this level, it typically indicates that the valves and piston rings are in good working condition, allowing for effective compression and a proper seal within the combustion chamber. If the cranking vacuum is lower than 2.5 in. Hg, it may indicate potential issues such as valve leakage, worn piston rings, or other air intake restrictions, which could lead to poor engine performance or starting difficulties.

Therefore, the correct choice reflects a healthy engine state where cranking vacuum is sufficient to support effective operation during cranking.