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Old 11-28-2014, 02:50 PM   #21
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...As far as the 89 octane goes, it can be hard to find in some areas and it was never refined at the refinery I where I worked. I will not swear to it, but I would bet that most 89 is "made" the by the same method. It is splash blended. Meaning a certain amount of 87 octane is loaded into the compartment and then the amount of higher octane, 93.5 in the case of the refinery I worked at, is added to raise the 87 to 89 which here is called mid-grade.
In some (many? all?) gas stations the 89 is created on-site by mixing the low and high octane. You'll see two fill ports for the underground tanks, with the markings for 87 and 91 but not 89.

Around here, the base gasoline is the same for all brands, all filling up at the same depot with only the additive being different.
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Old 11-28-2014, 09:13 PM   #22
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I am with B47, go with my vehicle owners manual.
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Old 11-28-2014, 10:05 PM   #23
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When I had a gasser I always ran the 87 that was not ethanol. We used to have 87 which was no ethanol, 89 that had 10% ethanol, and premium which was 91. Now they are mixing 85 with 91 to create 87 with no ethanol which makes it about 30 cents higher and the ethanol blend is now 87. On the other side of our state they sell 85. Steer way clear of that stuff! I inadvertantly filled with it once while pulling our 5er and my 5.7 Ram hated me for it!
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Old 12-01-2014, 10:11 AM   #24
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Some cars need higher octane and say so in the owners manual, otherwise per the industry use regular gas and all is ok and cheaper. Modern fuel injection cars are set to use whatever fuel is listed in the owners manual, so the things we saw with carburetors and chokes decades ago are long gone.

US Gov site on octane.



Test by independent company on effect of octane on modern cars:

CBC Marketplace test on fuel.
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Old 12-01-2014, 01:43 PM   #25
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When I had a gasser I always ran the 87 that was not ethanol. We used to have 87 which was no ethanol, 89 that had 10% ethanol, and premium which was 91. Now they are mixing 85 with 91 to create 87 with no ethanol which makes it about 30 cents higher and the ethanol blend is now 87. On the other side of our state they sell 85. Steer way clear of that stuff! I inadvertantly filled with it once while pulling our 5er and my 5.7 Ram hated me for it!
The 85 octane should only be sold in areas above 3000ft (approx) elevation - where it is sold as "regular". For an engine without a tubo- or super-charger, the reduced ambient air pressure reduces maximum engine power anyway, so that the full 87 octane is not needed. BTW, you also get better mileage at altitude due to both reduced air resistance and reduced power from the engine.

As was pointed out, the 85 octane will often cause performance issues down at sea level if you are using anything close to full power.

Turbo- and superchargers restore air pressure (and then some), and so need their normal octane gas while driving in the mountains.

Ethanol has an inherently higher octane rating than gas, and so is used as an additive (where allowed) to boost the octane rating. But ethanol has less BTUs/gal, and so mileage suffers slightly. Before electronic variable ignition timing, engines (such as in dragsters) would use alcohol for maximum output in short bursts because of the higher octane.

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