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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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