What are acceptable compression readings?
In most gasoline engines, acceptable dry compression per cylinder typically falls in the 120–180 psi range, with cylinder-to-cylinder variation generally staying within about 10–15%. Diesel engines run at higher pressures (roughly 400–600+ psi) and have different specifications. Always consult the specific manufacturer’s service data for your engine to confirm the exact targets.
Compression readings are a key diagnostic tool used to gauge the health of an engine’s cylinders. This article explains typical ranges, how to measure them, and how to interpret the results. It also covers how readings differ between gasoline and diesel engines and what steps to take if numbers fall outside the expected range.
Typical ranges and what they mean
Because compression specs vary by engine family, these guidelines provide a practical baseline for common passenger cars. Always reference your owner’s manual or service data for precise targets.
- Dry compression target for gasoline engines: healthy cylinders commonly show roughly 120–180 psi per cylinder when measured with a cold or warm engine and the throttle fully opened.
- Inter-cylinder variation: the difference between the highest and lowest cylinder readings should usually be within 10–15% of the highest reading; some manufacturers allow up to 20% as a maximum variation.
- Wet compression test: if a cylinder reads low on a dry test, adding a small amount of engine oil into the cylinder and retesting can indicate whether ring sealing is the issue. A noticeable rise after the oil is added suggests worn rings or piston sealing; little or no change points toward valve leakage or a head gasket issue.
- Diesel engines: compression readings are much higher, commonly in the 400–600+ psi range, and the acceptable variation is engine-specific. Always follow diesel OEM specifications.
In practice, these figures serve as a starting point. The key is consistent readings across cylinders and alignment with the manufacturer’s specifications. When in doubt, a follow‑up leak‑down test provides a more precise diagnosis.
How to perform a compression test
To obtain reliable readings, follow a standard procedure and reference your vehicle’s manual for any model-specific steps or precautions.
- Prepare the engine: ensure the engine is at operating temperature or gently warm as recommended; remove all spark plugs or glow plugs; disable the ignition system and, if needed, the fuel system to prevent misfiring or starting during the test.
- Set up the gauge: use a calibrated compression gauge and connect it to one spark plug hole with the throttle held fully open to maximize air intake during the test.
- Crank and record: with the throttle wide open, crank the engine for 4–6 revolutions and record the highest stable reading for that cylinder. Repeat for each cylinder.
- Wet check (optional): if a cylinder reads low, add about a squirt (1–2 tablespoons) of clean engine oil into the cylinder and retest to compare dry versus wet readings.
- Compare and interpret: review the readings against the manufacturer’s spec and look for consistency across cylinders. Plan follow-up diagnostics if any cylinder is out of range.
These steps form a solid foundation for assessing engine health. If results appear abnormal, consult a technician and pursue additional tests such as a leak-down test to pinpoint the source of the issue.
Interpreting results
Understanding what the numbers mean helps you decide on the next steps. The following quick guide translates readings into likely causes, with caveats about engine type and condition.
- Normal/healthy: All cylinders read within the specified range and show minimal variation (commonly within 10–15%).
- Low reading in one cylinder: Could indicate worn piston rings, leaky valves (intake or exhaust), a head gasket issue, or scoring on the cylinder wall. A leak-down test can help differentiate between these causes.
- Low readings in several adjacent cylinders: Might point to a shared problem such as timing issues, a seized camshaft lobe, or a head gasket leak between cylinders.
- High reading: Relatively uncommon and often indicates measurement error or a timing-related problem; rare in healthy engines. Recheck gauge setup and repeat the test if readings are unexpectedly high.
- Wet test interpretation: If the dry reading is low but the wet test shows a significant improvement, ring or piston sealing is likely the culprit; if there’s little to no improvement, valve seals or a head gasket issue are more probable.
Results should always be interpreted in the context of the engine design and manufacturer specifications. When readings deviate from the norm, follow up with a leak-down test and review related systems such as timing, valves, and head gaskets.
Engine type considerations
Gasoline vs. diesel and other nuances
Gasoline engines typically rely on compression readings in the 120–180 psi range per cylinder, with careful attention to even cylinder-to-cylinder performance. Diesel engines operate at substantially higher pressures and have different diagnostic approaches; a compression test on diesels is often followed by a leak-down test, and exact targets come from the OEM manual. Modern engines with variable valve timing or direct injection may introduce additional tolerances; always check the specific service data for accurate targets.
Summary
Acceptable compression readings depend on the engine type and the manufacturer’s specifications. For most gasoline engines, a dry compression per cylinder of about 120–180 psi with 10–15% maximum variation across cylinders is typical, while a wet test can help distinguish rings from valves. Diesel engines require higher baseline pressures and their own set of tolerances. Always perform the test with proper procedure, compare all cylinders to the spec, and use follow-up tests such as leak-down to pinpoint issues. When in doubt, consult a qualified technician and the vehicle’s service documentation for precise targets.
