In a normal operating engine, what should a vacuum gauge read?

In a normal operating engine, a vacuum gauge should typically read between 16-20 inches of mercury. This range indicates that the engine is drawing a good amount of air into the cylinders, which is essential for proper combustion. A reading within this range suggests that the engine is operating efficiently and that there are no major leaks in the intake system or problems with the valves.

A gauge reading lower than this range, such as 8-11 inches or even lower, may indicate issues like internal engine wear, intake leaks, or exhaust system restrictions. Higher readings, while they might seem desirable, can also indicate problems such as valve train issues or improper tuning that can affect engine performance. Thus, the focus should be on readings that align with the standard operational metrics for most gasoline engines, which is why the range of 16-20 inches of mercury is considered the correct answer in this context.