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weird member
1997 Softail Custom (FXSTC)
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4,127 Posts
A stock Evo (big twin, not Sportster) was designed to run on 87 octane. It won't run better on higher grades of fuel, the only advantage to running 91/93 is that it has higher levels of detergent, but it won't make better power or fuel economy. Quality of fuel is a separate thing... shit gas is shit gas regardless of how high its octane is. It'll run better on 87 octane Shell, Exxon, BP, 76 or whatever top tier detergent fuel brand than it will on 93 octane from a no-name garbage gas station (though not all unbranded gas stations are crap).

Octane rating is strictly the fuel's resistance to spontaneous ignition under heat and pressure (detonation). Has nothing to do with power or efficiency in and of itself.

The only time you see an increase in fuel economy or power by running 93, is some fuel injected cars are designed in such a way that the computer detects what gas is in it and changes the engine's tuning accordingly. Otherwise a vehicle that doesn't have a computer designed to do that will run the same no matter what grade you put in, so long as it's the minimum octane rating or better.
 

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weird member
1997 Softail Custom (FXSTC)
Joined
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4,127 Posts
I was just reading my 1991/1992 Softail service manual, it to use gas of 87 octane or higher . Looks like That's the way to go on these carbed bikes . I also read to avoid ethanol which can cause the fuel lines to deteriate .
That's really only an issue if it sits a long time.

Modern fuel lines can handle it. I run 10% ethanol in my 97 with no problems. The owner's manual (at least for mine) indicates it's fine to use up to 10% ethanol blended gas (but not more).

Ethanol in gas has been a thing in the US since the 80's. Unless you got a bike designed to run on leaded gasoline, 10% ethanol isn't going to hurt anything (as long as you don't let it sit up forever).
 
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