Originally Posted By: bigt61
Remember, that octane is an explosion inhibitor - lower octane is more volatile and has higher energy content (BTU). High octane is needed in high performance engines so they can squeeze the air-fuel tighter(high compression) for a bigger bang which equals more horsepower. Low octane fuel will explode too easily causing pre-ignition (it ignites all by itself from being squeezed - like a diesel) This pre-ignition causes power loss and engine damage in a gasoline engine. Octane additives are expensive - therefor the price difference.
Not exactly, but you have the basic idea. Octane is the name of a molecule. The more of these molecules, the greater the gasoline's ability to resist detonation caused by high compression ratios and too much ignition advance. That is, it raises the temperature at which the fuel begins to combust. Pre-ignition is usually caused by hot spots, like carbon build up on the exhaust valve and dirty spark plugs. Either way, its bad news for engines.
Ethanol has no octane molecules, but it has an octane equivalency based on what temperature it will combust — and it is higher than gasoline.
But yes, you have identified why I am asking the question! I have E0 91 octane and E10 91 octane available locally to me (no reliable 93 octane sources). So what do I pick? Where is the power? Pure gasoline has 3% more energy (BTU's), but ethanol can withstand more compression and spark advance for more power. If E10 91 starts life as E0 91 and ethanol is added to it, then it is higher than 91 octane, and I can use more spark advance to make more power. On the other hand, if it is E0 87 and ethanol is added to it, then I am still at 91 octane, but with less BTU's, so even worse.
Yes, I have two vehicles with high compression engines (9.9:1 and 10.7:1) and I do tune them for power, so this is not just a factory motor and factory tune.
I have heard that higher octane = faster burn by uninformed people, and I have heard it is slower burn from more educated people. Personally, I am not positive there is a correlation between octane and flame speed. I could be wrong though.