Should I use 92 or 95 octane gasoline for my car?

[LaAdriel]

The octane rating of gasoline to be used for a car should be based on the recommendation in the official vehicle user manual. In addition to checking the appropriate gasoline grade in the car's user manual, it will also be indicated on the fuel tank cap. Usually, the gasoline grade can also be determined based on the engine's compression ratio. Cars with an engine 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. If the compression ratio is higher, 98 octane gasoline should be selected. However, with the use of some new technologies nowadays, the octane rating cannot be solely determined by the compression ratio. A high compression ratio can also be adjusted to use lower octane gasoline because, apart from the compression ratio, other factors such as ignition timing, turbocharging technology, and Atkinson cycle technology also play a role. Generally, the higher the 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. For a 2007 Passat, occasionally using the wrong gasoline grade only requires switching back to the correct grade after use. However, long-term use of the wrong gasoline grade can have the following effects: For vehicles recommended to use lower octane gasoline, mistakenly using higher octane gasoline will not cause damage, but the increase in octane number will change the fuel's ignition point, leading to delayed combustion in the engine. This means both the engine's power output and thermal efficiency will decrease, and the actual feedback experience is reduced performance. For vehicles recommended to use higher octane gasoline, using lower octane gasoline can cause engine knocking. Because the octane number is too low, the lowered ignition point of the gasoline will cause it to ignite prematurely during the compression stroke. If the gasoline ignites before the spark plug fires during the compression stroke, resistance will occur during the upward stroke. This resistance will make the engine run very unstably. If it's mild knocking, it will only increase noise without significant damage to the engine. However, if there is noticeable knocking, it indicates severe engine conditions, affecting not only driving stability but also causing abnormal wear on pistons and cylinders, and in severe cases, cylinder scoring.

[ArthurFitz]

Let me briefly explain this issue. Having driven for decades, I'm used to deciding which fuel to add based on the vehicle manual. Usually, there's a label on the fuel tank cap indicating the recommended octane number. My old car has always used 92-octane fuel, saving a lot of money, while 95-octane, though better at anti-knocking, is significantly more expensive. Some people use 95-octane for so-called engine protection, but it's completely unnecessary for ordinary family cars unless it's a performance vehicle that specifically requires it. The key is not to use the wrong octane number, as engine knocking can damage the cylinders, leading to costly and time-consuming repairs. I've seen a neighbor who used the wrong fuel and ended up spending thousands on major repairs. Seasonal changes also matter; fuel quality can degrade in high summer temperatures, so using 95-octane can be a precaution. Overall, 92-octane is sufficient and cost-effective for 90% of cars. New drivers should always check the manual before fueling and not make assumptions.