What Does Compression Testing Tell You?
It measures one thing well. That thing is not the whole story of the engine.
What the numbers prove
A compression test measures the peak pressure each cylinder builds while the engine is turned over. The comparison between cylinders is the informative part. A cylinder well below its neighbours indicates a sealing problem: rings, valves, a head gasket or, in the worst case, damage to the bore.
Consistent figures across all cylinders suggest the basic mechanical health of the engine is sound, at least in terms of sealing. That is genuinely useful information, and it is more than you get from no test at all.
What it does not tell you
A clean set of compression figures is compatible with all of the following.
- Worn bearings. Compression does not measure the bottom end.
- Oil consumption through valve stem seals. An engine can hold pressure and still burn oil.
- Cooling system problems or a history of overheating that has not yet caused a sealing failure.
- Timing component condition: belts, tensioners and water pumps.
- Whether the accessories, sensors and harness are present and working.
- Corrosion or damage from poor storage.
What to ask for instead of the phrase
- The actual figures, cylinder by cylinder.
- Whether a leak-down test was also done. It localises where pressure is escaping, which a compression test does not.
- Whether the test was performed cold or warm, and with the throttle open.
- Who performed the test and whether there is a dated record.
- How the engine has been stored since it was removed.
Common questions
Is a compression test enough to trust a used engine?
On its own, no. It says nothing about bearings, oil consumption, timing components, storage condition or accessories. Treat it as one input alongside the actual numbers, a leak-down test if available, the donor vehicle's history and the supplier's inspection process.
What is a leak-down test?
A leak-down test pressurises each cylinder with compressed air and measures how much pressure escapes. Unlike a compression test, it can localise where the leak is: past the rings, through an intake valve, through an exhaust valve or into the cooling system. It is the more informative test when compression figures are borderline.
Related pages
- Engine FinderTell us the vehicle and we identify the engine before quoting it.
- JDM Engine QuestionsAnswers to the questions Phoenix JDM engine buyers actually ask.
- What "Compression Tested" Actually MeansThe test sellers advertise, what its numbers prove, and what to ask for.
- How to Buy a Used JDM EngineThe order to do things in, and what to settle before money moves.
Find the right engine for your vehicle
Tell us the year, model and trim. We identify the engine your vehicle takes and come back with current availability and pricing.