What Is a Compression Test?
A test that measures each cylinder's compression pressure - the clearest way to assess an engine's internal health without taking it apart.
A compression test measures, with a gauge, the pressure each cylinder develops on the compression stroke. The condition of the rings, cylinder bore and valves cannot be seen from the outside; compression is the combined health check for all three.
In the end, two things matter: the figures should fall within the manufacturer's range, and the difference between cylinders should be small. The table below helps interpret the findings.
Compression findings
Finding | What it indicates |
|---|---|
Difference between cylinders | Local fault: valve or rings |
Overall low compression | General wear, overhaul may be approaching |
Blue smoke + oil consumption + low compression | Points to rings |
Weak initial start-up | Points to valves |
An engine on a purchase survey may well 'start and sound fine'; low or uneven compression lays bare the wear beneath that smooth note on one sheet of paper. Together with the sea trial, it is the core of an engine assessment.
Keep the results page; if the test is repeated years later, the comparison is often more meaningful than the number on its own.