* Engine Design: The specifics of the combustion chamber design, fuel delivery system (carburetor, fuel injection), ignition system, and even the type of fuel significantly impact the minimum compression needed for reliable operation.
* Fuel Type: Higher octane fuels can tolerate higher compression ratios without detonation (pre-ignition). Lower octane fuels require lower compression ratios to avoid knocking.
* Engine Condition: Worn piston rings, valve leaks, or other mechanical issues will significantly reduce effective compression, requiring a higher *measured* compression ratio to still function. An engine with some wear might still run with a compression ratio lower than a brand new engine's design spec, but below a certain point, it'll fail to start or run reliably.
* Altitude: Engines at higher altitudes have less air density, reducing the effective compression even if the compression ratio itself is unchanged.
While you might find some general guidelines (e.g., some sources suggest a minimum of around 100 PSI for older gasoline engines), these are very rough estimations and aren't reliable across all engines. A proper assessment requires considering the specific engine type and its overall condition. A compression test with a gauge is the only accurate way to determine if the compression is sufficient for an engine's reliable operation.
