Quote:
Originally Posted by masraum
87 octane has more energy than 91-92-93 octane, but then ethanol also, I believe, reduces octane (so is it 87 with the ethanol or is it 87 octane w/10% ethanol).
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Higher octane fuels actually have more *potential* energy, but they require higher pressures (CR) to extract that energy. So, it really depends on what your CR is, and the limits of your car's factory "tune". If your CR is high enough, and your ECU allows for agressive-ish timing, then running higher octane
can result in better performance and mileage. It typically does in fact on many newer/modern engines, thought not by huge margins. If your user manual calls for a "minimum" octane as well as a higher "recommended" octane, it's probably a safe bet that the latter will give you a slight performance and/or MPG boost.
Also, ethanol increases octane, but also reduces energy/efficiency. It's also not as stable, so the "shelf life" of E-blends is not as long. As such, regular lower-octane fuel can be "proofed up" with ethanol, but that octane rating will diminish much quicker than a non-ethanol fuel. Also, when you see an octane rating at the pump, that is the minimum value with or without ethanol. So, an 87 octane E10 is a blend of 90% 85-octane gasoline and 10% ethanol.