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There are several properties of gasoline that can effect how the fuel is burned in a real engine. Of primary concern is Octane, which is simply the resistance of the fuel/air mixture to knocking. You can find the Octane requirement of your engine in your owner’s manual. In general, it is just fine to use higher Octane than required because high Octane gasoline’s burning properties are the same as lower Octane fuels. There is a popular myth that high-Octane gasoline burns slower than regular grade, but that is not true.
The rate at which fuel burns in a real engine is dependent on many factors like temperature, engine design, speed and load and ignition timing. The way that the burning speed of a fuel is measured is in a laboratory “bomb” and is called the laminar burn velocity (LBV). The higher the measured LBV, the faster the fuel will burn in a real engine (all other factors equal). If we look at surveys of LBV, we can see that there is no correlation between LBV and Octane of real commercial gasoline. There are small variations during the year and based on location, but on the average, the LBV of high-Octane is the same as regular grade.
There is a small difference between the LBV of fuels with Ethanol and without. In general, adding Ethanol speeds up the LBV, but even 25% ethanol raises the LBV less than 1%.
Other fuel properties that are important are vapor pressure (how readily it evaporates) and the heating value (how much energy per gram of fuel). These vary a little bit between winter blended and summer blended fuels. High Octane winter-blend has slightly lower vapor pressure (on the average) than regular grade in some surveys.
references: SAE 2003-01-3265, AIP Conference Proceedings 2191, 020063 (2019)
I just use whatever the manual recommends now, lol.
Btw, are the manuals written by the engineers themselves? I didn’t want to ask that in a new thread.
My experience is that they review any year-to-year changes with the appropriate engineers, but the manuals are written by a publication department. Maybe the first manual was written by the engineers!
A lot of folks don’t really understand what “calibrated for 87 Octane” really means. Of course it does mean that the engine can be expected to make its rated power and torque on that gas spec, but it doesn’t mean the engine will not be knock-limited in some normal driving conditions. For one thing, compression ratios are relatively high these days to optimize fuel economy in normal driving. The result of this is that the CR is a little bit too high to not be knock limited at full load. Every manufacturer does this, and they can do a god job because they all have knock detection systems to prevent knock damage and still allow the engine to run close to its knock limit at full load.
Also realize that high ambient temp, low humidity, high barometric pressure, and carbon deposits all hurt the actual knock resistance of an engine in the real world. The normal calibration process doesn’t set the spark advance to run on the knock limit (letting the knock detection system to stay active) under normal high-load conditions. Instead, the spark advance is calibrated to run really close to the knock limit but not to incipient knock. What all of this means regarding Octane is that you might expect using higher than specified Octane to offer a very slight performance and efficiency benefit when you are running at high loads, especially if you are running in hot/dry/ high Baro ambient conditions. Note that some ECU’s have adaptive calibrations that can infer what Octane you have put in your car and adjust the calibration map accordingly.
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