Does anyone here realized that you get less miliage on Octane 94 than 91?

And I honestly can't tell the different between the two except for my pocket.

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?

Thank you for the detailed explanation. It makes perfect sense. I don't suppose you have any video clips of this dyno test or even just the graphs, do you?

Yes, USNA94, please prove what the optimal octane is for the 370z.

Well, to be fair, the question of what the optimal octane for the 370Z is, and whether or not higher octane produces additional power, are two separate questions. Given the level of detail in USNA94's explanation of how octane works, I have no reason to doubt the information. It'd just be cool to see some definitive evidence is all. After all, unless he's making up stories, he was privy to such evidence by virtue of his lab test. It'd be nice if he has the graphs from that test or something to share with us.

You are absolutely correct and explain it well.Unfortunetly there are those that when they hear the word "race gas" they right away believe it will make the car run faster.
As you said the only reason for race gas which is higher grades of octane is only needed for high compression to avoid detonation.It is hard for someone that isn't familiar with building engines or the mechanics of a motor to understand.
So all they are doing is wasting money.Hopefully what you stated will sink in and some will believe.There are of course those that no matter what will believe what they want to believe regardless!

Well, it will if it's oxygenated, won't it? Or am I missing something? I understand the higher octane won't do anything, but the oxygenation helps doesn't it? (Not that I'd spend the money on that stuff to run on a regular basis, mind you.)

Optical would be whatever octane you tune it for before it starts knocking and pinging ... its hard to say what its tuned for stock because the ECU is the learning type so it can adjust itself to whatever premium gas is avaliable to it. You could say that its tuned to 91 and will increase performance with 93 as it "learns" it or that its tuned to 93 and decreases performance with 91. Whatever works best for you I assume.

I wish I did but as I took this class over 15 yrs ago (damn I'm getting old) all that stuff is long gone. But maybe if I poke around this interweb thingie some more I can find some data/graphs posted some place. Honestly, this is a pretty common lab experiment to run in engineering classes.

If you would like to provide me your engine, a test stand, and a dyno (I'll supply the gas of various octane ratings because i'm generous like that) I would gladly provide this information with some pretty graphs and everything. If not, I suggest going with what the owners manual states. I hear those Japanese engineers are pretty sharp.

USNA94, do you by chance know what 98 RON means in relation to, say, 93 octane here in the US? I'm curious because of the post below:

98RON equates to about 92 octane here in the states.

Here in the states we post the average of the RON and the MON. Other countries sometimes only post the higher number.
The below explanation is copied from: How Much Octane Do I Need?

What is octane? How can it help my engine? two laboratory octane numbers determine the overall octane quality of a gasoline. Both numbers are determined in single-cylinder, variable-compression-ratio engines. The first is the research octane number (RON), where operating conditions are mild. This is the number that gas stations in the 1960s put on their advertising signs out in front, numbers that ranged from a little over 100 to as high as 104. If an engine is detonating (pinging) at part throttle, it usually needs more RON.

The second laboratory octane number is also determined in a single-cylinder, variable-compression-ratio engine and is called motor octane number (MON). The MON test is conducted under more severe operating conditions (higher rpm and higher inlet temperature) than the RON test, and as a result, the number is lower. If an engine is detonating at wide-open throttle, a higher MON will usually satisfy it.

The number shown on the black and yellow sticker on the service station gas pump is the average of the RON and MON, or antiknock index (AKI). If the sticker shows an AKI of 92, RON is typically between 96 and 97, and MON is typically between 87 and 88. The RON and MON can vary slightly but still must average a minimum of 92 to have a pump posting of 92. Higher octane unleaded fuels will have a greater difference between RON and MON. For instance, 76 Unleaded Racing Gasoline has a RON of 106 to 107 and a MON of 94 to 95, which gives an AKI of 100 to 102.

Well, I might just spend some time and money on dyno'ing with 91, 93, 94 and share the results with you all. One of these days...

I agree that with an older engine/ECU, given a fixed set of engine parameters from the factory, running a higher-than-necessary octane rating is total waste in every respect. However, we know that even on a traditional engine, higher octane fuels allow tuning headroom. When tuning the A:F ratios and the spark timing, you can account for higher octane fuel and get more power that way. The flipside is by tuning for say 110 octane race fuel, you've now made a car that will knock on regular pump gas.

The big question mark with our cars is: given a stock ECU, is the stock ECU smart enough to tune itself upwards for a higher octane fuel automatically? We already know that the stock ECU knows how to de-tune itself when presented with lower-octane fuel by using sensors to detect the onset of knocking and making the appropriate adjustments.

There seems to be some evidence that, when given a much higher octane fuel, our ECU will slow retune itself for higher power using those same sensors as a guide. I find it a bit hard to believe myself, but someone did do a dyno with some 110 race fuel and showed some statistically significant gains a while back.

Edited to add: Even without "oxygenated" fuel, just plain higher octane does allow a higher horsepower tune. By making the fuel more knock resistant, the timing can be advanced further without knocking, which results in more power.