Originally Posted by USNA94

This is spot on and 100% correct. Operating your car with anything higher than required to prevent knocking is just wasting your money. Octane rating does not relate to the energy content of the fuel. It is only a measure of the fuel's tendency to burn in a controlled manner, rather than exploding in an uncontrolled manner. Octane is not something you add to a fuel, it is just a rating on a fuel's resistance to preignition (knocking). Certain additives are added to the fuel to increase the octane rating. Until the 1970's, Tetra-ethyl lead was that additive (hence leaded gas) but has been since replaced by MTBE as the additive used for health and environmental reasons.

A fuel with a higher octane rating can be run at a higher compression ratio without detonating. Compression is directly related to power, so engines that require higher octane usually deliver more motive power. Engine power is a function of the fuel, as well as the engine design, and is related to octane rating of the fuel. Power is limited by the maximum amount of fuel-air mixture that can be forced into the combustion chamber. When the throttle is partially open, only a small fraction of the total available power is produced because the manifold is operating at pressures far below atmospheric. In this case, the octane requirement is far lower than when the throttle is opened fully and the manifold pressure increases to atmospheric pressure, or higher in the case of supercharged or turbocharged engines. Many high-performance engines like the Z's are designed to operate with a high maximum compression, and thus demand high-octane premium gasoline. A common misconception is that power output or fuel mileage can be improved by burning higher octane fuel than a particular engine was designed for. The power output of an engine depends in part on the energy density of its fuel, but similar fuels with different octane ratings have similar density. Since switching to a higher octane fuel does not add any more hydrocarbon content or oxygen, the engine cannot produce more power.

However, burning fuel with a lower octane rating than required by the engine often reduces power output and efficiency one way or another. If the engine begins to detonate (knock), that reduces power and efficiency for the reasons stated above. Many modern car engines feature a knock sensor which detects knock, and then sends a signal to the engine control unit to retard the ignition timing. Retarding the ignition timing reduces the tendency to detonate, but also reduces power output and fuel efficiency.

While working on my undergraduate degree (mech eng) I took a course on internal combustion engines and my professor was an expert in this field. I clearly recall him telling us that using a higher octane rating than required was a waste of money. He suggested that if your car did not ping or knock while going up an incline or hill (where this usually presents itself first) then you were fine with your current octane rating. We even had a lab where there was a Ford V-8 on a test stand hooked up to a dyno that we ran tests on comparing power versus different variable such as fuel mixture and octane rating. Our tests showed that there was no appreciable increase in power when using a higher octane rated fuel but there was definitely a decrease in power when using a lower octane rated fuel at the higher end of the RPM spectrum.

Now, would you like me to prove that Force=mass x acceleration or are you going to take my word for it?