Should the Mistra use 92 or 95 octane gasoline?

[StRiley]

The Mistra should use 92 octane gasoline, as this is the recommended fuel grade specified in the vehicle's official user manual. In addition to checking the appropriate gasoline grade in the user manual, the fuel cap of the Mistra will also indicate the recommended fuel type. Typically, the fuel grade can also be determined based on the engine's compression ratio. Vehicles with a compression ratio between 8.6-9.9 should use 92 octane gasoline, while those with a compression ratio between 10.0-11.5 should use 95 octane gasoline. For higher compression ratios, 98 octane gasoline is recommended. However, with the application of new technologies, the compression ratio alone cannot determine the appropriate fuel grade, as high-compression engines can also be tuned to use lower-octane gasoline. Other factors, such as ignition timing, turbocharging technology, and Atkinson cycle technology, also play a role. Generally, the higher the gasoline octane rating, the higher the octane number and the better the anti-knock performance. 92 octane gasoline contains 92% isooctane and 8% n-heptane, while 95 octane gasoline contains 95% isooctane and 5% n-heptane. If the Mistra occasionally uses the wrong gasoline grade, simply switching back to the correct grade after consumption is sufficient. However, long-term use of the wrong gasoline grade can have the following effects: For vehicles designed for lower octane fuel, using a higher octane gasoline will not cause damage, but the increased octane number can alter the fuel's ignition point, leading to delayed combustion in the engine. This reduces the engine's power output and thermal efficiency, resulting in poorer performance. For vehicles designed for higher octane fuel, using a lower octane gasoline can cause engine knocking. Due to the significantly lower octane number, the fuel's ignition point decreases, causing premature ignition during the compression stroke. If combustion occurs before the spark plug fires, resistance is created during the upward stroke. This resistance makes the engine run very unstably. Mild knocking may only increase noise without significant engine damage, but severe knocking indicates serious engine issues, affecting driving stability and causing abnormal wear on pistons and cylinders, potentially leading to cylinder scoring.

[NoraRose]

My Mistra is the 2018 1.8L model, and the manual clearly recommends 92-octane gasoline. At first, I hesitated whether to try 95-octane, since I heard higher-octane fuel might offer better performance? So I specifically tested two full tanks, but there was absolutely no difference in power or fuel consumption—just that 95-octane was noticeably more expensive. An experienced gas station attendant told me that the Mistra’s engine compression ratio is designed for 92-octane, and forcing 95-octane is just a waste of money. Now, after three years and 50,000 kilometers, I’ve always stuck with 92-octane, and the engine runs perfectly. One piece of advice, though: avoid mixing fuels from different gas station brands. It’s fine for emergencies, but long-term mixing could affect the fuel system.