How to Read an Engine Compression Test Report: What the Numbers Really Mean

Understanding Engine Compression Testing

When you're shopping for a used engine, one of the most important quality indicators is the compression test report. At Used Engines Store, every engine we sell undergoes professional compression testing—but what do those numbers actually mean? This guide will help you understand compression test results and make confident buying decisions.

What is a Compression Test?

A compression test measures the pressure (in PSI - pounds per square inch) that each cylinder can generate during the compression stroke. This simple test reveals the internal health of an engine without taking it apart, showing you the condition of pistons, rings, valves, and head gaskets.

Think of it as a health checkup for your engine—the numbers tell you if the engine's vital organs are working properly.

What PSI Numbers Are Good?

Compression readings vary by engine type, but here are general guidelines:

• Gasoline engines: 125-160 PSI is typical for a healthy engine
• High-performance engines: 150-200+ PSI
• Diesel engines: 275-400+ PSI (much higher due to compression ignition)
• Older engines: 100-125 PSI can still be acceptable depending on the application

Important: Always compare your readings to the manufacturer's specifications for your specific engine. A 2015 Ford EcoBoost will have different ideal numbers than a 2005 Chevy 5.3L V8.

The 10% Rule: Cylinder-to-Cylinder Variance

Here's the critical part most buyers miss: consistency matters more than the absolute number.

All cylinders should read within 10% of each other. For example:

Good compression test:
Cylinder 1: 145 PSI
Cylinder 2: 148 PSI
Cylinder 3: 142 PSI
Cylinder 4: 147 PSI
Variance: 4% - Excellent!

Problematic compression test:
Cylinder 1: 145 PSI
Cylinder 2: 148 PSI
Cylinder 3: 110 PSI
Cylinder 4: 147 PSI
Variance: 25% on Cylinder 3 - Red flag!

A cylinder reading 15-20% lower than the others indicates worn piston rings, valve problems, or head gasket issues—all expensive repairs.

Wet vs. Dry Compression Tests

Professional compression reports often include both dry and wet tests:

Dry test: Standard compression test with no additives

Wet test: A small amount of oil is added to the cylinder, then retested

How to interpret the difference:

• Pressure increases significantly (20+ PSI): Worn piston rings (oil temporarily seals the rings)
• Pressure stays the same: Valve or head gasket issue (oil doesn't help)
• Pressure increases slightly (5-10 PSI): Normal wear, generally acceptable

What Different Readings Indicate

All Cylinders Low (but consistent)

Possible causes: Worn piston rings, valve timing issues, or normal wear on high-mileage engines
Verdict: May still be usable depending on how low and the intended application

One or Two Cylinders Low

Possible causes: Burnt valve, broken piston ring, or head gasket leak on those cylinders
Verdict: Avoid—this engine needs internal repairs

Two Adjacent Cylinders Low

Possible causes: Blown head gasket between those cylinders
Verdict: Major red flag—expensive repair needed

All Cylinders High and Consistent

Verdict: Excellent! This is what you want to see

Red Flags in Compression Reports

Walk away if you see:

• Any cylinder below 90 PSI (gasoline) or 250 PSI (diesel)
• Variance greater than 15% between cylinders
• Two adjacent cylinders both reading low
• Wet test shows 30+ PSI increase (severe ring wear)
• Missing data or incomplete test results
• No compression test provided at all

How Compression Relates to Engine Longevity

Good compression numbers mean:

• Better fuel efficiency: Proper combustion chamber sealing
• More power: Maximum pressure for combustion
• Longer lifespan: Less internal wear means more miles ahead
• Easier starting: Especially in cold weather
• Lower oil consumption: Healthy rings keep oil where it belongs

An engine with excellent compression can easily deliver another 100,000+ miles with proper maintenance.

Real-World Example: Good vs. Problematic

Example 1: Excellent Used Engine (2018 Toyota 2.5L 4-Cylinder)

Cylinder 1: 185 PSI
Cylinder 2: 188 PSI
Cylinder 3: 186 PSI
Cylinder 4: 187 PSI
Analysis: Perfect consistency, high readings, 1.6% variance. This engine is in excellent condition.

Example 2: Avoid This Engine (2012 Ford 5.4L V8)

Cylinder 1: 145 PSI
Cylinder 2: 148 PSI
Cylinder 3: 95 PSI
Cylinder 4: 142 PSI
Cylinder 5: 146 PSI
Cylinder 6: 98 PSI
Cylinder 7: 144 PSI
Cylinder 8: 147 PSI
Analysis: Cylinders 3 and 6 are 35% low. Likely blown head gasket or valve issues. Needs major repairs.

Questions to Ask Your Supplier

When reviewing a compression test report, ask:

1. Was the test performed with the engine warm or cold? (Warm is standard)
2. Were all spark plugs removed during testing? (Required for accurate results)
3. What was the throttle position? (Should be wide open)
4. How many cranking revolutions per cylinder? (4-6 is standard)
5. Can I see the actual test report, not just verbal numbers?

Why Used Engines Store Tests Every Engine

At Used Engines Store, we don't just test engines—we document everything. Every engine comes with:

• Complete compression test report with all cylinder readings
• VIN-matched verification
• Mileage documentation
• Visual inspection photos
• Warranty coverage for your peace of mind

We believe transparency builds trust. When you know how to read these numbers, you can buy with confidence.

Final Takeaway

A compression test report is your window into an engine's internal health. Look for:

• ✓ Readings within manufacturer specs
• ✓ Less than 10% variance between cylinders
• ✓ No individual cylinders below 100 PSI (gasoline) or 275 PSI (diesel)
• ✓ Complete documentation from a trusted supplier

Armed with this knowledge, you can confidently evaluate any used engine and avoid costly mistakes. And remember—if a seller won't provide compression test results, that's the biggest red flag of all.

Ready to find a quality-tested used engine? Browse our inventory of compression-tested, VIN-matched engines with full documentation and warranty coverage.