Is There any Advantage to Higher Octane ? - Pensacola Fishing Forum

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Old 02-02-2016, 10:24 AM   #1
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Default Is There any Advantage to Higher Octane ?

I'm talking about in the smaller engines. I need to get some non-ethanol for my 5500 watt generator (10 hp B&S) for the camp this weekend and any left over I will use in my 4 wheeler (300 cc Kawasaki) and/or my outboard (Honda 35 hp).
The Marathon on Scenic has 91 octane for $2.69. The Beacon has 87 octane for $2.45.
Both used to sell 89 octane which I thought was a good compromise.
Any advantage to 91 octane in the engines I'm speaking of ?
Conversely, can higher octanes hurt smaller engines ?
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Old 02-02-2016, 11:43 AM   #2
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No advantage to the higher octane. All of these engines were designed to run with the lower octane and the higher octane will provide no benefit except to make your wallet lighter. Below is a link that discusses octane ratings for gas.
Higher Octane?
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Old 02-02-2016, 06:52 PM   #3
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Good question.
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Old 02-03-2016, 12:09 AM   #4
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There is one potential benefit to higher than necessary octane. All gasoline gradually loses octane over time, especially if stored in an open or vented system, like boats have.

So the more infrequently you use your boat, the more at risk you are of your fuel losing octane to the point of detriment to your engine. Some of the later model engine electronics can compensate to a degree, (such as retard the ignition timing) but at the expense of reduced performance.

Starting with a higher than necessary octane helps ensure that by the time you burn the fuel that it will be adequate. I think this is one reason that "marine" fuel is usually around 89 to 91 octane ratter than 87. (I think there are a few marine engines designed for a bit higher than 87 octane as well)

This is an unlikely issue if you burn through your fuel within within a month or two.

There are people more knowledgeable than me on this that can correct me if I have misspoken.
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Old 02-03-2016, 08:02 AM   #5
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There is a disadvantage to higher octane in an outboard. Outboards run at 90% of rated horsepower most of the time and CAN burn pistons if you use high octane. Think about the mileage you get with an outboard compared to a vehicle. And look at the RPMs when running at the speed limit and when running on plane in the boat. Stick with the lower octane.
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Old 02-03-2016, 02:14 PM   #6
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That's interesting. I used the 91 octane available at Marathon last time I filled the tank for my Honda 35 hp. I haven't been much and still have about half of it. Maybe I will dilute with 87 octane just to be safe.
That's what I ended up getting yesterday.
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Old 02-03-2016, 06:44 PM   #7
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There is another disadvantage to using higher octane, in that if the engine burns fuel at a lower temperature (i.e., boat engines), a greater percentage of the higher octane fuel goes unburned, leaving more carbon deposits behind. More of a lower octane fuel would be burned, leaving your engine cleaner.
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Old 02-03-2016, 10:19 PM   #8
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So, I wonder why they even carry 91 instead of 87 or 89?
The sign advertises it as "marine fuel."
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Old 02-04-2016, 12:56 AM   #9
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Quote:
Originally Posted by welldoya View Post
So, I wonder why they even carry 91 instead of 87 or 89?
The sign advertises it as "marine fuel."
I think it's because the petroleum engineers that developed marine fuel didn't consult with the PFF.
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Old 06-22-2017, 08:11 AM   #10
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My 2000 model Johnson 115, carb motor.. has a placard on the stem that says 87 octane ethanol fuel is fine... but I still run 89 octane marine fuel.
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