Should the Outback use 92 or 95 octane gasoline?

[VonSawyer]

According to the official vehicle manual recommendation, the Subaru Outback should use 92 octane gasoline. In addition to checking the appropriate gasoline grade in the vehicle manual, the Outback's fuel filler cap also indicates the recommended octane rating. Typically, the gasoline grade can also be determined by the engine's compression ratio: vehicles with compression ratios between 8.6-9.9 should use 92 octane, while those between 10.0-11.5 should use 95 octane. However, with the application of new technologies, the compression ratio alone cannot determine the required gasoline grade, as high-compression engines can still be tuned to use lower-octane fuel. Other factors, such as ignition timing, turbocharging technology, and Atkinson cycle technology, also play a role. Generally, higher-octane gasoline has a higher octane rating and better anti-knock properties. 92 octane gasoline contains 92% isooctane and 8% n-heptane, while 95 octane gasoline contains 95% isooctane and 5% n-heptane. If the Outback occasionally uses the wrong gasoline grade, simply switch back to the correct grade after consumption. However, prolonged use of incorrect gasoline may have the following effects: For vehicles requiring lower-octane fuel, using higher-octane gasoline will not cause damage, but the increased octane rating alters the fuel's ignition point, leading to delayed combustion. This reduces the engine's power output and thermal efficiency, resulting in poorer performance. For vehicles requiring higher-octane fuel, using lower-octane gasoline can cause engine knocking. The significantly lower octane rating reduces the fuel's ignition point, causing premature ignition during the compression stroke. If combustion occurs before the spark plug fires, resistance builds up during the upward stroke. This resistance makes the engine run very unstably. Mild knocking only increases noise without significant engine damage, but severe knocking indicates critical engine conditions, affecting driving stability and causing abnormal wear on pistons and cylinders, potentially leading to cylinder scoring.