How much does engine horsepower decrease for each 1-inch drop in barometric pressure?

Engine horsepower decreases by approximately 3% for each 1-inch drop in barometric pressure because lower barometric pressure means that the air density is reduced. Air density is crucial for combustion because a lower density means that there are fewer oxygen molecules available for mixing with fuel in the combustion chamber. This reduction in available oxygen results in less efficient combustion, leading to a decrease in engine power output.

Understanding that the relationship between barometric pressure and horsepower is tied to the principles of fluid mechanics and combustion efficiency helps clarify why the correct answer is this specific percentage. Recognizing that other factors, such as temperature and humidity, also influence engine performance is important, but this specific drop is well-established in automotive and small engine mechanics.