Jay,
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If you run a lower octane fuel than recommended, and the engine begins to detonate, the "knock sensor" will retard the timing and stop the knock.
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I agree.
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Octane slows the burn rate in the combustion chamber, and doesn't "give" you anything.
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I respectfully disagree.
It is true that one is trying to keep the air/fuel mix to self-ignite to avoid knocking. Wikipedia (I know, I know, don't trust anything you read on the internet...) states that Octane rating:
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is used as a reference standard to describe the tendency of gasoline, petrol, or benzin fuels to self ignite during compression prior to the desired position of the piston in the cylinder as appropriate for valve and ignition timing.
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As far as I know it does not affect the actual speed at which the gasoline will burn when it does ignite. (Please correct me if I'm wrong - THAT would be fascinating!)
Hm... how can I re-phrase what I am thinking. What about Diesel engines?
They operate without spark plugs based on the principle that the air/fuel mix self-ignites because of the high pressures within the cylinder. Compression ratios are higher (around 16:1 if I recall). Because temperatures are higher, and also because diesel contains more energy per unit of volume, the explosions are more efficient. Same amount of fuel + bigger bang = more efficiency.
By extension a higher compression ratio in a gasoline engine should yield higher efficiency. The only way I know of how to achieve this is to have a air/fuel mix that can be compressed more without self-igniting. And since the engine has a fixed displacement this can only be achieved by allowing more air without allowing more fuel to be injected. Therefore the valve timing adjusts to allow maximum air within the cylinder. This increases the air temperature as the air is compressed more. Now you have a lot of really hot air that needs a fuel that does not self-ignite - ergo - a higher octane rated fuel.
And since all cars I know have excess cooling capacity the excess heat is negligible in the life-expectancy-of-components scheme of things.
But please, (no sarcasm) correct me where I am wrong. I am here for the same reason as all of us: to save fuel and coin and improve efficiency.
