Howdy , all. I'm reading through the posts so some answers may be repetitive again some more...
If the OEM PCM is designed for 91 octane (like Gen 1, 2... when knock sensors came in I don't know) then running 93 will not produce more power. Higher and higher octane may lose power because the rate of combustion (and therefore optimal timing) or volatility (and therefore in-cylinder air-fuel mixing) changes go beyond what the OEM PCM can adjust for.
Ethanol blended gasoline is from 0% to 10% EtOH, so has up to 3% ish percent oxygen in the fuel. The OEM PCM should easily adjust for this to produce the same power. Fuel economy will be worse on a per-gallon basis because 3% if that gallon is oxygen.
If there are knock sensors, then the PCM should adjust to max best torque if it can, until it thinks it's running too hot. Then depending on the algorithm, it'll do something to keep you from overheating the heads. If there are no knock sensors, over 91 wont' hurt, but won't help either.
Depending on what model year, I think some engines now look at O2 signals even at WOT. Those clever EPA scientists have been forcing OEMs to include more and more "off-cycle" emissions (i.e. making them part of the cycle.) But let's pretend they didn't... and have a beer... then the WOT fixed air-fuel ratio would be rich and the O2 would bring it towards stoichiometric. I don't believe oxygenated fuel would make a WOT rich condition turn into lean. The OEMs would have a lot of melted pistons as warranty returns.
While the octane rating is indeed the fuel's resistance to knock, the different octane ratings have slightly different BTU contents. In the good old days things like butane were added to the premium. It was cheap, high octane, volatile, and low BTU content. But regulations on tank vapors stopped that. Then heavy hydrocarbons were used in premium (you probably heard of toluene; there are many cousins) but again, the heavy HC cause emissions problems, so again they were limited. So these days I think all grades are pretty close to the same BTU/gallon, not considering the oxygenate addition.
If you are not knocking, a lower and lower octane would not change the power output because the timing isn't changing. A higher and higher octane would not change the power output either, because the timing isn't changing. The engine needs knock sensors to make a power difference.
I'm trying to think why a very high speed motorcycle engine would benefit from lower octane (in the absence of knock.) I think there is something else (oxygenate or not, volatility?) going on there.
Back in my college days, premium would have been lower MPG because of the volatile components added to make higher octane. (See butane above.) They were high octane, but low BTU. If you think about propane, it's the same - high octane, poor BTU, poor MPG.
George, you are on the right track. If the cam-in-cam system includes knock sensors, then you are right. Just go to an online auto parts site and see if you can order a knock sensor for a Gen IV. Then you'll know.
Chuck... thermal conductivity is like pink insulation (poor, it's a good insulator) and copper metal is good (heat transfer.) Anyway, if you've read this far, you'll see that while the gasolines of 20 years ago were different between premium and regular, today they are far less so because the allowed boiling range is limited. The volatile components are gone (similar reason why the gas cap has to be tight or the CEL comes on) and the heavy components are gone (because they cause HC emissions.)
Dunno if the '08 has a knock sensor and what would happen. 93 is not always available, so I would think it would adjust to allow 91 octane.
I guess I should visit the Gen III-IV side more often.
Octane rating - Wikipedia, the free encyclopedia
