What is the effect of blocked exhaust systems on engine performance?

Blocked exhaust systems have a significant negative impact on engine performance due to the essential role they play in allowing exhaust gases to exit the engine efficiently. When the exhaust system is blocked, the natural flow of these gases is obstructed, leading to increased back pressure in the engine.

This back pressure can cause several issues: it prevents the engine from expelling exhaust gases effectively, which in turn restricts the intake of fresh air and fuel during the combustion cycle. As a result, the engine struggles to maintain optimal combustion conditions, leading to a decrease in power output and overall efficiency.

Moreover, the additional stress on the engine components due to this blockage can lead to overheating and additional wear, further exacerbating performance issues. Therefore, decreased power and efficiency arise as direct consequences of a blocked exhaust system, which aligns with the correct answer.