What indicates satisfactory compression in an engine?

Satisfactory compression in an engine is indicated by pressure build-up in the combustion chamber during the compression stroke. When you perform a compression test by placing your thumb over the spark plug hole, if the compression is adequate, the force of the air-fuel mixture being compressed will push your thumb off the spark plug hole.

This force signifies that the engine is creating enough pressure to initiate combustion, essential for the engine to run efficiently. Adequate compression is crucial for proper engine performance, as it ensures the fuel will ignite effectively under the conditions present in the cylinder.

The other options do not accurately measure the compression. For instance, simply having your thumb stay over the spark plug hole indicates insufficient pressure, while a flywheel spinning without resistance does not provide information regarding cylinder pressure. Lastly, a spark plug that sparks continuously may indicate electrical system integrity but does not directly reflect the health of the compression in the engine cylinder. Therefore, the forced removal of your thumb signifies healthy compression.