FAA Powerplant Written Practice Test

What effect does excessive valve clearance have in a piston engine?

1. Increases engine noise

2. Decreases valve overlap

3. Increases power output

4. Leads to valve seat damage

The correct answer is: Decreases valve overlap

Excessive valve clearance in a piston engine primarily affects the timing and operation of the engine's valvetrain. When valve clearance is too large, it results in a delay in the opening of the valve and premature closing. This can lead to a reduced amount of time that the valve is actually open, thereby decreasing valve overlap—the period during which both the intake and exhaust valves are open at the same time. Valve overlap is critical for efficient engine breathing and performance, as it allows for better scavenging of exhaust gases and more effective intake of fresh air-fuel mixture. Reduced valve overlap due to excessive clearance can lead to suboptimal combustion and a decrease in power output, not an increase. Consequently, while excessive valve clearance can create other issues, such as increased wear and tear on components, its primary impact relevant to the question is the decrease in valve overlap.