ALT-2 The value of using High Octane Fuel
Octane is a fuel's resistance to preignition or detonation, the tendency of the fuel/air mixture to start burning before the spark fires. Since that causes an explosion that occurs while the piston is still rising on the compression stroke of the Otto cycle, it can be very damaging to your engine.
The Octane Number is a percentage of resistance to preignition in comparison to a standard fuel, called "octane." If the fuel resists preignition 10% better (fires at 10% higher pressure) than octane, it has an octane number of 110. If it's 10% worse (fires at 10% lower pressure), its octane number is 90.
One of the ways of improving the efficiency of an internal combustion gasoline engine is to raise the compression ratio. That's because the "expansion ratio" - the difference between the volume in the cylinder between the initiation of the power stroke and the opening of the exhaust valve - is higher with higher compression. That's easy to understand, since obviously the pressure would be higher in the cylinder, which is what pushes the car.
Raising the compression ratio not only improves power, but since it takes the same amount of gas to cause a higher cylinder pressure, it also takes less gas to give the *same* performance. So higher compression engines give better fuel economy than low compression engines of the same power.
But you can't do that if the octane of the fuel won't support the added pressure.
The result of all this is that while a modern engine will protect itself from damage caused by using a lower octane gas than the manufacturer recommends, it does so by advancing the spark (cause it to occur later in the power stroke) and richening the fuel mixture (more gasoline in the incoming fuel mixture). Both of these strategies reduce efficiency, burn more gas, and lower performance.
So don't think you'll necessarily save money by using cheaper (lower octane) gas. It can actually lower your gas mileage. To know for sure, you'd have to do instrumented tests to figure out whether the lower cost of regular (typically less than 5% here) justifies the loss in fuel mileage. If you lost even 1 mpg, the math doesn't support the switch to the lower grade.
The other half of the equation, whether using higher octane gas in a car that is designed to run on regular will make it perform better, is that it won't. That's because higher octane is useful only in solving a problem this car doesn't have. That is, it's not in danger of damaging itself from preignition. To look at it another way, it's like a couch potato drinking a performance beverage. It won't hurt him any more than any other sweetened drink, but it won't turn him into an Olympicmarathoner, either. That's because he doesn't have the problem (in this case, electrolyte depletion) that the drink is supposed to cure. A lower compression engine does not have the only problem (susceptibility to preignition) that higher octane solves.
