Should the Mercedes-Benz S400 use 95 or 98 octane fuel?

[DeHarrison]

According to the official vehicle manual recommendation, the Mercedes-Benz S400 should use 95 octane fuel. In addition to checking the appropriate fuel grade in the vehicle manual, the Mercedes-Benz S400 can also refer to the label on the fuel tank cap, which will indicate the recommended fuel type. Typically, the fuel grade can also be determined based on the engine's compression ratio. Vehicles with an engine compression ratio between 8.6-9.9 should use 92 octane fuel, while those with a compression ratio between 10.0-11.5 should use 95 octane fuel. However, with the application of new technologies, the compression ratio alone cannot determine the required fuel grade. High-compression engines can also be tuned to use lower-octane fuel, as factors such as ignition timing, turbocharging technology, and Atkinson cycle technology also play a role. Generally, the higher the fuel octane rating, the higher the octane number and the better the anti-knock performance. 92 octane fuel contains 92% isooctane and 8% n-heptane, while 95 octane fuel contains 95% isooctane and 5% n-heptane. If the Mercedes-Benz S400 occasionally uses the wrong fuel grade, simply switch back to the correct grade after consumption. However, long-term use of the wrong fuel grade can have the following effects: For vehicles recommended to use lower-octane fuel, using higher-octane fuel will not cause damage, but the increased octane number may 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 recommended to use higher-octane fuel, using lower-octane fuel 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 will develop 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 conditions, affecting not only driving stability but also causing abnormal wear on pistons and cylinders, potentially leading to cylinder scoring.