Engine
Understanding Cylinder Compression Readings
A differential compression reading describes how a cylinder held pressure during a particular test. A value such as 74/80 generally records 74 psi on the cylinder side with 80 psi supplied through the test equipment. It is not an engine horsepower measurement or a prediction of remaining engine life.
To interpret a logbook entry, identify the engine, the test procedure, each cylinder's result and any associated findings. The applicable manufacturer's guidance and a qualified mechanic's assessment determine what the results mean for that engine.
What the test measures
The differential test compares pressure across a calibrated restriction while air is supplied to the cylinder. Leakage through the cylinder's sealing surfaces affects the indicated result. Lycoming's published instructions specify equipment characteristics and emphasize test conditions and the examiner's judgment. Lycoming Service Instruction 1191A
This is a maintenance test involving compressed air and a propeller that can move unexpectedly. The discussion here is for reading records, not performing the procedure.
Why there is no universal passing number
Different engine guidance must not be collapsed into a single rule such as “anything below 70 is bad.” Lycoming's SI 1191A discusses absolute readings, differences between cylinders and changes over time. Continental-engine guidance also uses a master-orifice reference when evaluating allowable leakage; the FAA's Continental maintenance course highlights that distinction. FAA Safety Team, Continental Engine Maintenance
A recorded master-orifice value provides context for the test setup under the applicable procedure. It is not a blanket approval of cylinder condition. Nor should a reviewer apply that reference to a different engine manufacturer's test without checking the applicable instructions.
For a purchase, the practical question is: “Which current manufacturer procedure was used, and what did the mechanic conclude from the complete examination?” That is more useful than debating a number in isolation. For the wider records review, see what to look for in aircraft logbooks before you buy.
Read the entire entry, not just the lowest value
Extract the following information into your review notes:
- Test date and engine identity.
- Engine time and the clock used.
- Each cylinder's number and pressure reading.
- Master-orifice or equipment-reference value, if applicable and recorded.
- Notes about test conditions or a repeat test.
- Leakage observations, borescope findings and other diagnostic results.
- Corrective action or a stated follow-up plan.
Leakage location helps direct the investigation. Lycoming associates intake leakage with the intake valve, exhaust leakage with the exhaust valve, and breather or oil-filler leakage with the piston-and-ring area. Preserve that observation without turning it into an independent diagnosis. Lycoming SI 1191A
If the log contains only six numbers, report that limitation. Do not fill in an unrecorded borescope examination or assume that the engine was tested hot.
Build a cylinder-by-cylinder history
Averages hide the issue you are trying to understand. Compare each cylinder with its own prior results and mark any intervening replacement or repair.
The following is an illustrative records-review example, not a diagnostic threshold:
| Entry | Cylinder No. 3 | Other documentation | What the reviewer should do |
|---|---|---|---|
| First inspection | 74/80 | No additional finding in the supplied entry | Preserve as the recorded baseline |
| Next inspection | 59/80 | Follow-up inspection referenced | Request that inspection's findings |
| Subsequent work | Cylinder replaced | Installation entry available | Separate the removed cylinder's history from the replacement |
| Later inspection | 76/80 | New cylinder now installed | Do not describe this as the old cylinder “recovering” |
A change of engine or cylinder makes a continuous-looking spreadsheet potentially misleading. Keep serial numbers where available, and never compare two different installed engines as though they were one.
Add the evidence that the numbers cannot provide
Ask for referenced borescope findings, photographs, shop reports and follow-up entries. Read those alongside documented oil consumption concerns, filter findings and relevant maintenance work. Each contributes a different observation; none should be invented when absent. Servicing history is covered in oil-change intervals in aircraft records.
An entry that records a concerning result followed by a documented evaluation and corrective action tells a different story from an isolated result with no follow-up in the package. Conversely, high readings do not answer questions about every other engine component.
When test conditions or supporting records are missing, the useful conclusion is narrow: “The log records 59/80 for No. 3; the referenced follow-up report was not supplied.” That wording gives the shop a clear task and avoids implying either failure or clearance.
Questions to take to the mechanic
- Which procedure and reference values apply to this exact engine?
- Were there leakage or borescope findings requiring action?
- Do the readings compare the same installed cylinders under comparable conditions?
- Was a repeat test or corrective action completed, and where is it documented?
- What current examination is appropriate before the transaction or planned operation?
PlaneFacts can organize the recorded readings and link them to the underlying entries. The value is in making the evidence and its gaps easy to follow, while preserving the distinction between a historical test result and a current maintenance decision.
Common questions
Does 60/80 mean the cylinder has 75 percent of its power?
No. Dividing the two indicated pressures does not give a cylinder's power output. The reading comes from a leakage test through specified equipment.
Should one low reading automatically trigger a cylinder replacement?
A records reviewer should not prescribe replacement from the number alone. The mechanic must apply the engine manufacturer's procedure, associated findings and applicable maintenance requirements.
Can I compare the seller's readings with a new pre-buy test?
Yes, as evidence, while accounting for engine identity, cylinder changes, equipment, conditions and procedure. Ask the inspection provider to explain meaningful differences rather than assuming every numerical change represents wear.
Frequently asked questions
Does 60/80 mean the cylinder has 75 percent of its power?
No. Dividing the two indicated pressures does not give a cylinder's power output. The reading comes from a leakage test through specified equipment.
Should one low reading automatically trigger a cylinder replacement?
A records reviewer should not prescribe replacement from the number alone. The mechanic must apply the engine manufacturer's procedure, associated findings and applicable maintenance requirements.
Can I compare the seller's readings with a new pre-buy test?
Yes, as evidence, while accounting for engine identity, cylinder changes, equipment, conditions and procedure. Ask the inspection provider to explain meaningful differences rather than assuming every numerical change represents wear.
PlaneFacts reports support diligence. They do not establish current airworthiness or replace an aircraft inspection or a qualified maintenance review.