* Engine condition: A perfectly rebuilt engine will have higher compression than one with worn rings, valves, or a damaged head gasket.
* Camshaft: A performance camshaft will often produce lower cylinder pressures than a stock cam.
* Engine speed: Cylinder pressure increases with RPM. Measurements are typically taken at a specific RPM (often near peak torque).
* Crankshaft position: Cylinder pressure readings will vary slightly depending on where the crankshaft is positioned during the test.
* Fuel mixture: An excessively rich or lean fuel mixture can affect cylinder pressure.
A compression test measures *relative* compression between cylinders. The goal is to have consistent compression across all cylinders. A significant difference between cylinders points to a problem in that cylinder (like worn rings, a burnt valve, or a head gasket leak). The absolute pressure reading is less critical than the consistency.
Generally:
* Acceptable range: A typical range for a relatively healthy 400 small-block Chevy might be between 130-150 PSI (pounds per square inch). Some might see slightly higher or lower within the acceptable range depending on the aforementioned factors.
* Concerningly low: Seeing readings consistently below 100 PSI or a significant drop (e.g., 30 PSI or more) between cylinders usually indicates a need for investigation. This could mean a rebuild is necessary, but further diagnostics are required to pinpoint the problem (leakdown test recommended).
Before concluding a rebuild is necessary:
A compression test alone is insufficient. A leak-down test is crucial to *locate* the source of low compression. This helps determine if you need a full rebuild, or if a less extensive repair, such as replacing rings or valve seals, will suffice.
In short: Don't focus solely on a magic pressure number. Focus on consistent readings across cylinders. Low and/or inconsistent readings warrant further investigation with a leak-down test to diagnose the actual problem before deciding on a rebuild. Consult a qualified engine mechanic for a proper diagnosis.
