So after reading about the new Miata makes more horsepower using 91 verses 87 octane fuel, I decided to put 89 in my CX5 for 2 full tank fills and it did feel more responsive than normal.
There are four possible explanations, one of which is the placebo effect. Here's the other three:
1) The 87 octane fuel is under spec. Unless you or another entity sends the 87 octane to a lab for octane testing, it could be under spec. There is at least one oil industry person on this forum that swears up/down this is not possible but his view is not supported by the results of random spot checks for proper octane that is performed by various states department of weights measures.
2) The fuel meets the minimum octane spec but it achieves it with more than 10% ethanol. This will lower the amount of energy embodied in a gallon of fuel. Again, random sampling by various agencies around the country shows this does happen occasionally.
3) It's possible that the higher octane fuel you used could have less than 10% ethanol, especially now with oil trading under $30/barrel. It takes more oil to make higher octane fuel without adding ethanol but with oil at record low prices I imagine refineries may be adjusting the processes used to achieve the various octane. If so, the higher grades could have lower ethanol content than 87 octane. But, it could also go the other way. There is no legal requirement that fuel sellers (wholesale or retail) disclose the actual amount of ethanol (other than to say it may be as high as 10%).
Before doing this kind of seat of the pants comparison, it's important to fill up with the tank as empty as possible. I recommend driving at least until your trip computer reports "0 miles until empty". And, for the reasons alluded to above, just because it performs better for one or two tanks of a different octane, doesn't mean it will continue to do so. Refineries have a number of tools up their sleeves to achieve their goal of creating a standardized product that meets the required specifications but, even though it generally meets those minimum specifications, it's not always the same stuff. Sometimes it exceeds a minimum spec by a bigger margin and sometimes it meets the minimum spec using refining/blending techniques that result in a superior product (more embodied energy would be one example).
Yes, it's complicated. As a consumer you don't have access to the details. But if you find a fuel that seems to work better, keep filling up at that station, with that same fuel. But don't make the blanket statement that it runs better on all 89 octane unless you do extensive testing at a number of different stations and, at each station, you can always tell the difference between the two octanes. For reasons outlined above, and the fact that the North American CX-5 is designed to not detonate on 87 octane, that's very unlikely. Yes, it adjusts various parameters (including ignition and injection timing) on the fly to avoid detonation but it should never adjust these parameter past that point (even if you put 100 octane in there). The detonation sensor is merely a failsafe that will kick in if the fuel is under spec or some component (like an oxygen or MAF sensor) is providing faulty values.
