Well the problem is marketing numbers versus engineering numbers. I don't know the forumula off hand to give you the answer you want, but in general, engines with higher compression tend to do better with higher octane fuels in both power and mpg (in the case of atkinson cycle engines, power is sacrificied for efficiency so high mpg but not more power).
There are cars that are more tailered towards lower octane gas and get better power/mpg by using the recommended fuel octane rating such as the HCH.
The atkinson cycle engine (like found in the prius) tends to be on the higher side of compression ratios, which to me means in order to really take advantage of this engine, you need to use higher octane fuel. But the problem is higher octane fuel tends to be more expensive, so marketing kicks in and says you can use regular because you end up saving more even though you might get better mpg using higher octane fuel, but from an engineering point of view, the recommended octane level will be higher than what is listed.
So there is going to be an ideal octane rating for every engine for efficiency and one for power. The problem is those numbers are rarely published.
There have been many posts/threads on this subject. However, premium fuel has less energy than regular 87 octane so the FE may be slightly lower with premium. Use premium only if the engine requires it otherwise you are just wasting your money.
ok I searched a little bit for an answer to my question, but didn't really find anything. A simple question:
will my car (a FEH, so it does not "require" premium gas), get better gas mileage using premium gas (91+ octane) instead of regular gas (87 octane)?
I'm not asking about engine performance in terms of power. I know some cars require premium gas to get peak HP. I'm asking strictly about effect on mpg.
My testing in Huntsville AL revealed 87 octane, Shell, Exxon and Chevron had the highest energy content per gram. All other brands and octanes had about 11% less energy than these. I did see some evidence that at nearly maximum ICE power, the 93 octanes were less bad but not as much energy as the 87 octane Shell, Exxon and Chevron.
The higher octane gas ( these days ) is being cut with ethanol to give the higher octane number... which will give lower MPG... but at the same time it is cleaner burning, and cleaner for your car.
So it's back to a case of pay now, or pay later.
It should be noted that OCTANE is NOT an ingredient. It is only a NUMBER. The number does not tell you what is in the gas. The number tells you how the gas behaves. Ethanol contains zero octane molecules, but has an octane rating of 110. E85 has an octane rating of 105.
ok I searched a little bit for an answer to my question, but didn't really find anything.
What about all the advice you can find through a Google search? Wherever I look (outside this forum), the only reason to go uptown on octane rating is if your engine is experiencing knock -- premature combustion of the fuel. And if you're getting engine knock at the recommended fuel grade, then you should be taking your vehicle in for servicing. Engines with higher compression ratios sometimes get away with lesser-grade octane if they have working knock-sensors and/or you're driving gently. Higher octane inhibits gasoline from combusting too easily under higher compression; It has no fuel efficiency or power benefits.
I'm not sure if gpsman1 can qualify his statements about ethanol. It's the opposite here, where regular grade gets the highest average percentage of ethanol additive while premium has none.
I know for sure ethanol is used to raise the octane in the states of Colorado, Nebraska, Kansas, Iowa, South Dakota, Nevada, Minnesota, ( except super premium which can only be used for off-highway use, such as snowmobiles, and boats ).
I suspect it is also used for this in Illinois, and Indiana, but I don't visit those areas. I visit all the others above frequently.
With the 110 octane rating, and costs of well under $2 per U.S. gallon, it is the cheapest booster for the fuel blenders/distributers.
Some states mandate ethanol must be in all blends.
Some mid-west states ( NE, IA, KS ) do not put it in the lowest grade.
And in Minnesota, the ethanol capitol of the world, I cannot even find 87 octane anymore... 89 is the lowest sold. My car gets a jolt every time I'm in CO and UT when I fill up with 85 octane. 85 always boosts my MPG. But you have to be over 3500 feet elevation to find/buy 85 octane.
Use what the manufacturer recommends. They are telling you what the engine was designed to run on. Using anything else will likely have a negative effect on both your performance and fuel economy.
(I'm sure someone somewhere will post that in THEIR car THEY got better performance using a different fuel rating than recommended. In reality, your car will run best on what is recommended unless there is a problem with your vehicle...in which case the right action is to get it fixed not change fuel rating.)
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