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Hello - I'm here in Upstate NY where the majority of our gas has the "Contains 10% Ethanol" stickers on them. When you find non-Ethanol pumps, the octane is usually 90 or 91. When I first bought my Spider Abarth (it still only has around 2500 miles on it!) I used 93 octane fuel with ethanol. The car was great! I did something stupid.... on a long drive I thought "why not try lower octane fuel??" because I didn't think I'd be using the benefits of the octane on the highway. Well, when I got to my destination, the vehicle definitely felt less responsive... less GET UP AND GO. So I decided to put in some octane booster and switched to non-ethanol gas. But I don't think (maybe it's just in my head) that using a 1/2 bottle of octane booster and running 90/91 octane non ethanol fuel is better than the 93 with ethanol I started with. I honestly haven't a clue. I'm not at all technical on any of this -- so I'm not sure what the best course of action here is. If you ask me, the vehicle today isn't as "peppy" as when I bought it and was using 93 ethanol fuel. What's the best for performance? What's the best for the vehicle? Is there such a thing as "too much octane"? I mean if I put 93 octane and then add 1/2 bottle of octane booster, is that too much? Just trying to figure out what the best thing to do is!!! Thanks for helping out a clueless new guy!!
 

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Use the higher octane.
 

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I would just run 93 and not stress about it.
 

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Ethanol boosts octane levels which is why that gas was rated at 93 and the other without ethanol was 91. It is why some people tune their cars to use 85% ethanol. More power-less mpg.
 

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Ethanol contains about 30% less energy than gasoline (76,100 BTU/US gal vs 114,100 BTU/US gal), so a 10% ethanol mix reduces fuel economy (mpg or l/100km) by about 3%. On the other hand, it raises octane so you get more octane per (dollar, pound, Euro). Those with more knowledge please correct me if I'm wrong but higher octane, all things being equal, allows higher compression ratio / higher boost for more volumetric efficiency and thus better fuel economy for a given level of power output. I think it also allows for more spark advance which also improves power and efficiency. I believe there's also a thermal effect...I don't think ethanol is as effective in cooling the cylinder and heads, which is one reason why high concentrations of ethanol are not recommended for air-cooled small engines.
 

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The manufacturer says 91, so it gets 91. It doesn't call for 93, so I don't waste money on 93 (unless the station only has 89 and 93). I don't pay much attention beyond that.
 

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91 and 93 depend on your location most of the time. Back east 93 is more common, out west its 91. I even saw some 90 in Texas on my way to Oklahoma in June. With my water/methanol injection I believe I am at about 111-114 effective octane rating when its spraying and running it hard. Allows for more timing advance, cooler AIT and EGT, and knock protection.
 

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Does E10 burn hotter than pure gasoline?
 

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The owner’s manual says 91 or above. It says ethanol up to 15% is fine. It recommends oxygenated fuel.
 
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Does E10 burn hotter than pure gasoline?
You actually answered this question in your post above. The more ethanol, the lower the BTU of the mix. E10 is ~110K vs pure gas at 114K for the same volume.
 

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Alcohol has more latent evaporation...it will combust more completely and remove heat from the combustion chamber. Alcohol is good.

water/meth doesn’t have an equivalent “octane” rating because you aren’t adding octane molecules, you’re adding “effective Knock protection”.

93 isn’t wasting money especially for those with tunes. The ECU is highly adaptive and predictive, any amount of knock activity counts in the ECUs predictions for knock. If you dont Race send it on 91.
 

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water/meth doesn’t have an equivalent “octane” rating because you aren’t adding octane molecules, you’re adding “effective Knock protection”.
You are technically correct, the best kind.

I did use the word effective also. Octane is a molecule, no dispute. Octane Rating is the ability to resist knocking. Water/Methanol injection adds knock resistance also along with cooling the charge air. So while it isn't the actual Octane molecule (which has a max rating of 100, have to add Ethanol or Methanol to the blend to get higher), it is doing the same thing in a different way and in simple terms, adds to the effective Octane Rating.
 
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