--- title: What Should My BluePrint Engine Compression Test Read? description: compression test a BluePrint engine, compare SBC, BBC, SBF, LS and Hemi results, and identify low or inconsistent cylinder readings. --- [Skip to content](https://knowledgebase.blueprintengines.com/knowledge/what-should-my-blueprint-engine-compression-test-read#main-content) English Show submenu for translations ![IN CRATE ENGINES..png\]](https://knowledgebase.blueprintengines.com/hs-fs/hubfs/IN%20CRATE%20ENGINES..png?height=40&name=IN%20CRATE%20ENGINES..png) Open main navigation Close main navigation - English Show submenu for translations - Go to www.blueprintengines.com Go to www.blueprintengines.com How can we help you? - There are no suggestions because the search field is empty. 1. [Knowledge Base](https://knowledgebase.blueprintengines.com/knowledge?hsLang=en) 2. [Technical Support](https://knowledgebase.blueprintengines.com/knowledge/technical-support?hsLang=en) # What Should My BluePrint Engine Compression Test Read? ### ***What Compression Should My BluePrint Engine Have?*** **Do not diagnose a BluePrint engine based only on a generic PSI number.** Cranking compression can vary considerably depending on: - Engine family - Compression ratio - Camshaft profile and valve timing - Starter cranking speed - Battery condition - Engine temperature - Throttle position - Altitude - Compression gauge - Engine condition A performance engine with a larger camshaft can produce a different cranking compression reading than a milder engine even when both engines are mechanically healthy. **Cylinder-to-cylinder consistency is more important than comparing the engine to an arbitrary PSI specification.** What Should I Expect From Each Engine Family? ### Small Block Chevy (SBC) BluePrint Small Block Chevy engines are available in many combinations including **350, 383, 400, 406, and 427 cubic-inch engines**. Because compression ratios and camshaft profiles vary significantly between these combinations, there is **not one correct cranking PSI specification for every BluePrint SBC**. When testing an SBC: - Test all cylinders - Hold the throttle fully open - Use the same number of compression strokes - Compare all readings - Investigate cylinders that are significantly different from the rest A performance SBC with a larger hydraulic roller cam may produce different cranking pressure than a milder street engine. ### Big Block Chevy (BBC) BluePrint Big Block Chevy combinations include engines with significantly different displacement, compression ratios, cylinder heads, and camshaft profiles. For example, large-displacement Pro Series engines can use aggressive camshaft profiles that affect cranking cylinder pressure. Do not assume that a BBC should produce a particular PSI simply because another BBC produced that number. **Compare the cylinders within the engine being tested.** ### Small Block Ford (SBF) BluePrint Small Block Ford engines include combinations such as **302, 306, 347, 351W, 408W, and 427W**. Camshaft profile can have a substantial effect on cranking pressure in these engines. When evaluating an SBF, pay close attention to: - Consistency between cylinders - Battery and starter cranking speed - Throttle position - Engine temperature - Whether one cylinder is noticeably lower than the others Do not compare a performance 408W or 427W directly to the expected cranking compression of a stock-style 302. ### LS Engines BluePrint LS engines are available in naturally aspirated and power-adder configurations with different compression ratios and camshaft profiles. A lower static compression power-adder LS combination can behave differently during a compression test than a higher-compression naturally aspirated engine. When testing an LS engine: - Make sure the battery is fully charged - Disable fuel and ignition - Remove all spark plugs - Keep the throttle fully open - Test every cylinder - Compare the complete set of readings Do not use an OEM production-engine PSI specification to automatically determine whether a performance BluePrint LS engine has a problem. ### Gen III Hemi BluePrint Gen III Hemi combinations also use performance camshaft profiles that can affect cranking compression. When compression testing a Hemi: - Disable fuel and ignition - Remove all spark plugs - Fully open the throttle - Maintain consistent cranking speed - Test all cylinders - Compare the readings as a complete set One cylinder that is considerably different from the others is more diagnostically important than whether every cylinder matches a generic internet PSI specification. How Do I Perform a Compression Test? ### 1. Warm the Engine When possible, bring the engine to normal operating temperature before testing. A warm engine generally provides the most representative results. If the engine cannot safely be run, document that the test was performed cold. ### 2. Disable Fuel and Ignition Disable both systems so the engine cannot start while testing. Depending on the installation, this may include: - Disabling the fuel pump - Disabling EFI fuel delivery - Disconnecting ignition power ### 3. Remove All Spark Plugs Remove **all spark plugs** before performing the test. This allows the starter to crank the engine consistently. Keep the plugs organized by cylinder so their condition can also be inspected. ### 4. Make Sure the Battery Is Fully Charged **Consistent cranking speed is critical.** A weak battery can cause the engine to crank progressively slower and make later cylinders appear to have lower compression. Charge or support the battery if necessary. ### 5. Install the Compression Gauge Thread the compression tester into the spark-plug hole. Hand-tighten the adapter and make sure it seals correctly. ### 6. Open the Throttle Fully Hold the throttle blades **fully open** during the test. A closed throttle restricts incoming air and can produce misleadingly low compression readings. ### 7. Crank the Engine Crank the engine through approximately **4–6 compression strokes**, or until the gauge stops increasing. Use the same procedure for every cylinder. ### 8. Record Every Cylinder Do not test only the cylinder you suspect has a problem. Record every reading so the cylinders can be compared. What Does One Low Cylinder Mean? One cylinder significantly lower than the others may indicate a sealing concern involving: - Intake valve - Exhaust valve - Piston - Piston rings - Cylinder - Head gasket A compression test identifies that the cylinder behaves differently, but it does not always identify the exact cause. A **leak-down test** should be considered when the compression test identifies a questionable cylinder. What Do Two Adjacent Low Cylinders Mean? When two neighboring cylinders are significantly lower than the remaining cylinders, additional diagnosis is required. A sealing problem involving the cylinder head or head gasket is one possible cause. Do not replace components based solely on the compression readings. Additional testing should be performed to locate the source of the pressure loss. What Is a Wet Compression Test? If one cylinder produces a low reading, a wet compression test can provide additional diagnostic information. 1. Record the original compression reading. 2. Add a **small amount of clean engine oil** to the cylinder. 3. Repeat the compression test using the same procedure. 4. Compare the new result with the original reading. ### Compression Increases Significantly If compression increases substantially during the wet test, additional investigation of the **piston rings and cylinder sealing** may be necessary. ### Compression Does Not Change Significantly If the reading remains approximately the same, additional investigation may be needed in areas such as: - Intake valve sealing - Exhaust valve sealing - Cylinder head sealing - Head gasket A leak-down test can help isolate the source. Why Are My Compression Readings Lower Than Someone Else's Engine? Even two engines from the same general engine family can produce different compression readings. Differences can result from: - Different camshafts - Different compression ratios - Different cylinder heads - Different starter speeds - Battery voltage - Engine temperature - Altitude - Throttle position - Compression gauge accuracy This is why a customer's SBC, BBC, SBF, LS, or Hemi should **not automatically be diagnosed by comparing its PSI reading with someone else's engine**. ### ***What Information Should I Record Before Contacting BluePrint?*** Record: - Engine part number - Engine serial number - Compression reading for **every cylinder** - Whether the engine was hot or cold - Whether the throttle was fully open - Approximate number of compression strokes - Battery condition - Any wet-test results - Relevant engine symptoms For example: - Cylinder 1: \_\_\_ PSI - Cylinder 2: \_\_\_ PSI - Cylinder 3: \_\_\_ PSI - Cylinder 4: \_\_\_ PSI - Cylinder 5: \_\_\_ PSI - Cylinder 6: \_\_\_ PSI - Cylinder 7: \_\_\_ PSI - Cylinder 8: \_\_\_ PSI Providing the complete set of readings makes the results much more useful for diagnosis. - [General Support](https://knowledgebase.blueprintengines.com/knowledge/general-support?hsLang=en) - [Technical Support](https://knowledgebase.blueprintengines.com/knowledge/technical-support?hsLang=en#main-content) - [General Engine FAQ](https://knowledgebase.blueprintengines.com/knowledge/technical-support?hsLang=en#general-engine-faq) - [BluePrint GM Style Engines](https://knowledgebase.blueprintengines.com/knowledge/technical-support?hsLang=en#blueprint-gm-style-engines) - [BluePrint Ford Style Engines](https://knowledgebase.blueprintengines.com/knowledge/technical-support?hsLang=en#blueprint-ford-style-engines) - [Transmissions](https://knowledgebase.blueprintengines.com/knowledge/technical-support?hsLang=en#transmissions) - [Fuel Systems](https://knowledgebase.blueprintengines.com/knowledge/technical-support?hsLang=en#fuel-systems) - [Parts and Accessories](https://knowledgebase.blueprintengines.com/knowledge/technical-support?hsLang=en#parts-and-accessories) - [Carburetors](https://knowledgebase.blueprintengines.com/knowledge/technical-support?hsLang=en#carburetors) - [Alternator](https://knowledgebase.blueprintengines.com/knowledge/technical-support?hsLang=en#alternator) - [Term X](https://knowledgebase.blueprintengines.com/knowledge/technical-support?hsLang=en#term-x) - [Valve Train](https://knowledgebase.blueprintengines.com/knowledge/technical-support?hsLang=en#valve-train) - [Factory Five FAQ](https://knowledgebase.blueprintengines.com/knowledge/factory-five-faq?hsLang=en) - [Engine Specs](https://knowledgebase.blueprintengines.com/knowledge/engine-specs?hsLang=en#main-content) - [Bare Block](https://knowledgebase.blueprintengines.com/knowledge/engine-specs?hsLang=en#bare-block) - [Discontinued Engine Specs](https://knowledgebase.blueprintengines.com/knowledge/discontinued-engine-specs?hsLang=en#main-content) - [Engine Support](https://knowledgebase.blueprintengines.com/knowledge/discontinued-engine-specs?hsLang=en#engine-support) [![Chill listening crop-3](https://knowledgebase.blueprintengines.com/hs-fs/hubfs/IN%20CRATE%20ENGINES..png?width=92&height=24&name=IN%20CRATE%20ENGINES..png "Chill listening crop-3")](http://navy-blueprint-logo) www.blueprintengines.com Help Center Copyright © 2026, BluePrint Engines