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RON = Research Octane Number
MON = Motor Octane Number
AKI = Antiknock index = (RON+MON)/2
The balance between RON & MON is actually set by the appreciation of the fleet(all cars in use) pool requirement. As new engine designs enter the market they are evaluated to determine their requirements and influence on the gasoline standards. So generally that says that today, unleaded cars appreciate a fuel that has a 50/50 balance of RON & MON.
In leaded fuel days the appreciation was weighted to (RON+3xMON)/4
Regardless of whether it is regular premium or midgrade, the refineries now tend to blend standard readily available fuels to the AKI standard of (RON+MON)/2.
In North America we tend to sell our fuels described by AKI. While in Europe they tend to sell described by RON, however the AKI relationships still apply.
These are characteristics of the fuel.
Engines also have an octane requirement in terms of both RON & MON which is usually less than the recommended fuel in order that the recommended fuel will satisfy. However as miles are accumulated that octane requirement can rise slightly and historically stabilized after 25000-35000 miles. That is why sometimes vehicles that use regular when new have been known to appreciate mid-grade later.
Knock sensors do influence the impact of the octane used as they can adjust timing within a specified range to cope with AKI of the fuel relative to the octane requirement of the engine.
Since 91 AKI appears to be the maximum in some locations I would be surprised if SCION did not take that seriously in the calibration of the engine for the North American market. Afterall, I am sure they wouldn't want to have deal with a lot of disatisfaction on this subject.
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