I know that diesel has more energy per volume and that is one reason for increased fuel mileage on diesel vehicles, but do the varying octanes of gasoline have different energy contents? I would figure so on a fuel that is octane boosted with alcohol, since alcohol only has half the energy per volume of gasoline.
JazzMan
Higher octane fuels have lower energy per unit volume. Also RFG (alcohol added) fuels have lower energy per unit volume than regular gasolines of the same octane rating.
1) That wasn't the question. I answered the question, energy per unit volume for the fuel. 2) engine management systems compensate for the O2 in the fuel by adding more fuel. It would depend on the control logic for any particular car if this translates into more energy per piston cycle. But even if it did I would expect it to go directly to waste heat. Much like higher octane fuel than an engine needs is just a waste.
So, it would seem to me that not only does running the lowest octane you can increase your gas mileage, but since it's cheaper to buy you get a double benefit. Cool, that's what I needed to know. Now to figure out why I have to run 93 to prevent pinging on an 8.5:1 CR motor...
JazzMan
The octane difference will not inherently alter the energy content of the fuel. It depends on the way in which the octane was boosted in the higher grades. If the octane was boosted by adding ethanol, for example, the energy content will be lower because of the lower energy content of ethanol. If they boosted it by altering the hydrocarbon mixture of the fuel, the energy content could be higher, lower, or the same.
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