Ahh, the mother of all NEPRT, hydrocarbons and viscosity, pressure and shear stabilized polymerized hydrocarbon chains.
It's funny how some people think a higher octane rating is better, 93 burns slower than 89 and is meant for high compression engines and those with compressors and turbochargers. If an engine is designed for 89 that's what works best. I doubt Yamaha did studies on fuel but used 89 to tune the OEM ECU and cam timing.
1. 93 octane burns slower. Paint a 24" long stripe of 200 octane fuel and ignite one end. It will take 18.3 minutes for the flame to reach the other end.
2. 93 octane burns cooler. Repeat the above stripe of 200 octane fuel and ignite. Because of the very high octane, the fuel is burning so slowly it will be easy to take the flame temperature as it reaches the end of the stripe. It will be around -196°C at that point.
High quality speculation: Yamaha has done extensive studies of fuel for the cylinder head, compression, fuel injection and spark timing design.
[EZ]
Low octane fuel ignites at lower compression temperatures and high octane fuel will resist ignition at higher temperatures caused by higher compression. The fuel must withstand the designed compression and not ignite until the spark plug fires. High octane fuel can withstand much higher compression than the FJR engine produces. That's all folks. Some top tier fuel companies have additive packages in their fuel which may include combustion chamber cleaners, etc but it does nothing for engine performance.
[/EZ]
Light oils like diesel fuel actually has a higher energy content than light fractions like gasoline. Compressed gasses (like air) increase in temperature under compression. In a diesel engine, air is compressed until it is extremely hot, then diesel fuel is injected into the cylinder where the hot compressed gasses ignite the fuel.
An Otto cycle engine compresses the air/fuel in the cylinder to a specific design point, below the ignition temperature, then at the correct mechanical time the spark plug fires and ignites a flame front in the cylinder. If compression temperature ignites the fuel/air mix it will ignite the whole mix at once, across the entire cylinder. Octane is an empirical predictor of where the compression temperature of the fuel/air causes ignition, with the object being to keep the compression from igniting the fuel/air before the spark plug fires.
When either compression temperature or a spark ignites the fuel/air mix the hydrocarbon atom's bonds break and recombine with gasses contained in the inducted air. It is the energy released in breaking the bonds in the hydrocarbons which produces the heat that drives the increasing cylinder pressure which moves the pistons. When a spark plug initiates the ignition it starts a flame front that propagates across the combustion chamber in a controlled progression that has been determined by the engineers that designed the cylinder heads and combustion chamber shape. When there is unintended combustion such as occurs from compression ignition, the fuel/air mix will ignite where ever the temperature is sufficiently high, anyplace in the cylinder and not correctly timed for the position of the piston.
Refining and additives will control how much temperature it takes to initiate the breakdown of the hydrocarbon bonds. A low octane fuel will have hydrocarbon bonds what break down at a lower temperature, a high octane fuel has hydrocarbon bonds which resist higher temperatures before the bonds to break. The energy released by breaking hydrocarbon bonds is fundamental chemistry.
