What is the impact of too high a gasoline octane rating on engines?

High octane gasoline requires a higher compression ratio in engines; using high-octane gasoline with engines that have a low compression ratio is a waste.

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During the design phase of an automobile engine, the octane rating of the fuel to be used is determined based on the compression ratio. The compression ratio is a very important structural parameter of an engine; it represents the ratio of the volume of gas at the start of compression when the piston is at its bottom dead center to the volume of gas at the end of compression when the piston is at its top dead center. In terms of power and efficiency, the compression ratio should be as high as possible. With a high compression ratio, it offers good power and high thermal efficiency, resulting in improved vehicle acceleration and top speed. However, due to the limitations of the cylinder material properties and gasoline combustion knock, the compression ratio of gasoline engines cannot be too high. The compression ratio of an engine has no inherent connection to whether a car is high-end or luxurious. Simply put, a higher compression ratio allows the use of fuel with a higher octane rating. The higher the fuel octane rating, the slower the oil burns, the lower the combustion knock, and the engine requires a higher compression ratio ； Conversely, lower-grade fuel burns more quickly, results in greater combustion knocking, and the engine compression is lower. The fuel rating is also related to the engine’s ignition timing. Low-octane gasoline burns quickly, requiring a delayed ignition timing ； High-grade fuel burns slowly, so the ignition timing needs to be advanced. For example, if an engine is supposed to use 93-octane gasoline as specified in the manual, but 90-octane gasoline is used instead, it is necessary to use a timing light to adjust the ignition advance by one to two degrees. If one ignores the recommendations of the vehicle manufacturer, uses fuel with a lower octane rating, and does not adjust the ignition timing, it may result in difficulty starting the engine ； When accelerating, there is a clear sound of metal colliding inside the engine ； After a long drive, the engine shakes or fails to stop when the ignition switch is turned off. So, when choosing gasoline, is it true that the higher the octane rating, the better? Not either. The main factor in choosing the gasoline octane rating is the engine’s compression ratio. Parameters such as the compression ratio and ignition timing have already been set in the engine computer; drivers simply need to choose the appropriate gasoline by following the instructions carefully, and there will be no problems at all. In the engine computer programs of modern cars, there are adaptive programs designed to make fine adjustments for gasoline with lower anti-knock properties, but no such programs exist for higher-octane gasoline. Therefore, the blind use of high-octane gasoline not only results in weak acceleration while driving, but also prevents the advantages of its high anti-knock properties from being utilized, leading to a waste of money.

It can be said that engines with a high compression ratio require gasoline with a high octane rating, whereas engines with a low compression ratio can use gasoline with a lower octane rating. Using high-octane gasoline in engines with a low compression ratio is a waste. Engines with a high compression ratio play a decisive role in reducing fuel consumption; from this perspective, automobile manufacturers, or rather engine manufacturers, determine what grade of gasoline should be used, while the petroleum processing industry plays a supporting role.

Gasoline with a high octane rating is good for the corresponding engines. Because the higher the aromatic content in gasoline, the slower its combustion rate, and the lower the combustion knock. Engines require a high compression ratio; in other words, high-quality engines (with high compression ratios) need gasoline of a higher octane rating. A high compression ratio plays a decisive role in reducing fuel consumption. From this perspective, it is the automobile manufacturers, or rather engine manufacturers, who decide what octane rating the gasoline should have, while the petroleum processing industry serves to produce such gasoline.

A car designed to use 93# fuel going to fill up with 97# gasoline – either they have too much money or they’re stupid.

This depends on the car’s compression ratio; a higher octane rating requires a higher compression ratio. If it is too high, incomplete combustion can occur, causing damage to the engine.