What is normal cylinder compression?
Normal cylinder compression varies by engine type, but for typical gasoline engines a healthy cranking pressure is usually between 120 and 180 psi per cylinder, with little spread between cylinders. Diesel engines operate at much higher pressures, commonly 350 to 500 psi, and should also be uniform across cylinders. Always refer to the manufacturer’s spec for your exact engine.
Normal ranges by engine type
Below are the typical targets for common engine families, expressed as cranking compression per cylinder. Readings can vary with temperature, altitude, and gauge accuracy, so always compare to factory specs for your model.
Gasoline and spark-ignition engines
For gasoline engines, the compression targets are generally within this range:
- Cranking compression roughly 120-180 psi (8.3-12.4 bar) per cylinder.
- Cylinder-to-cylinder variation should be within about 15-20% for a healthy engine, and ideally closer to 10% or less.
- Higher-performance or turbocharged gasoline engines may produce higher numbers but should remain fairly uniform across cylinders.
Concluding paragraph: In practice, consistent readings in this range indicate good seals around piston rings, valves, and gaskets; wide spread or readings outside this window warrant further testing.
Diesel engines
Diesel engines rely on higher compression for ignition; common targets are:
- Cranking compression typically 350-500 psi (24-34.5 bar) per cylinder in a healthy engine.
- Variations across cylinders are ideally within about 10-15%.
- Older or worn diesels may show lower numbers; significant discrepancies suggest possible head gasket or valve issues.
Concluding paragraph: Diesels require tighter variation and higher absolute numbers; readings well outside spec warrant professional diagnostic testing, such as a leak-down test.
High-performance and forced-induction gasoline engines
Some engines tuned for performance or with turbocharging may show different ranges based on design and boost levels. General guidance:
- Cranking pressures in the 150-210 psi range per cylinder are not unusual, depending on configuration and boost.
- Uniformity across cylinders remains the primary diagnostic criterion; large disparities still point to leakage or sealing problems.
Concluding paragraph: Always verify against the engine’s service documentation; tolerances may vary with design and test method.
Interpreting readings and next steps
Reading numbers by themselves is not enough; understanding what they mean and what to do next helps narrow down issues. The following guidance covers common interpretations and actions you can take.
Before listing the steps, here is a quick framework for interpretation and action.
- Healthy gasoline engines typically show cranking compression per cylinder within 120-180 psi, with cylinder-to-cylinder variation under about 20%.
- A single cylinder with a noticeably lower reading can indicate valve leakage, worn seating, piston-ring wear, or a gasket issue;
- Multiple low readings suggest a more widespread sealing problem or a head-gasket issue;
- A leak-down test pinpoints the source of leakage (valves, rings, or head gasket) and helps guide repairs;
- A wet compression test—repeating the test after adding a small amount of oil to the cylinder—can help distinguish ring wear (oil raises readings) from valve leakage (oil has little effect).
- Tests should be performed with the engine at normal operating temperature and using a calibrated gauge for reliability.
Concluding paragraph: Proper interpretation relies on multiple data points, temperature, engine state, and tool accuracy. When readings stray from spec, consult a qualified technician for precise diagnosis and repair.
What to do next
Based on the readings, consider these next steps to diagnose and address potential issues:
- If all cylinders are within specification and show little variance, no immediate action is usually required beyond routine maintenance.
- If one or two cylinders are weak, perform a leak-down test and evaluate valve timing, head gasket, and possible ring wear.
- If readings are consistently low across all cylinders, inspect piston rings and cylinder walls for wear or damage; major wear may require engine overhaul.
- If readings differ between cold and hot states, re-test after the engine reaches operating temperature to account for temperature effects.
Concluding paragraph: Accurate interpretation depends on corroborating tests, temperature conditions, and tool accuracy; rely on professional guidance for repairs when necessary.
Summary
Normal cylinder compression is not a single universal number. It depends on engine design and type. For gasoline engines, aim for roughly 120-180 psi per cylinder with minimal variation across cylinders; for diesels, target about 350-500 psi with tight cylinder-to-cylinder consistency. Use compression tests in conjunction with leak-down testing and manufacturer specifications to judge engine health. Consistent, in-spec readings indicate a well-sealed engine; out-of-spec readings signal the need for diagnostic follow-up and potential repairs.
Note: Always consult your vehicle’s service manual or a qualified technician for specs specific to your engine model and application.
