# Is There any Advantage to Higher Octane ?



## welldoya (Oct 5, 2007)

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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## BubbaFett (Mar 17, 2012)

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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## Dive1 (May 16, 2015)

Good question.


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## RMS (Dec 9, 2008)

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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## sealark (Sep 27, 2007)

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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## welldoya (Oct 5, 2007)

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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## Yo-Zuri (May 31, 2015)

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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## welldoya (Oct 5, 2007)

So, I wonder why they even carry 91 instead of 87 or 89?
The sign advertises it as "marine fuel."


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## RMS (Dec 9, 2008)

welldoya said:


> 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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## Burnt Drag (Jun 3, 2008)

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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## Wharf Rat (Sep 27, 2007)

If my choice is lower octane ethanol fuel vs higher octane non-ethanol fuel I'm going with the higher octane every time.


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## dockmaster (Sep 27, 2007)

Mabey the Marathon station is really 87....how do you know....lol

Sent from my VS990 using Tapatalk


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## fishnfool (Oct 2, 2007)

Octane doesn’t cause fuel to burn hotter or give better performance to an average engine. Gasoline is designed to burn at a pretty specific btu rate. Octane actually slows the burn of fuel down in combustion chamber/top of the piston so that the flame pattern more completely covers the top of the piston. This tends to avoid detonation, which is an instant, or uncontrolled burn all in one area and often cause what you hear and is called “valve rattle”. What you’re actually hearing is the piston usually rattling in the bore because all the load of the “bang” is focused to try and cock the piston sideways down the bore. This is not to be confused with pre-ignition by the way. Pre-ignition is an event where a “hot-spot” become illuminescent and will light the fuel/air mix off at any point in the stroke(uncontrolled ignition event)One of my mentors growing up building race motors etc, always told me to run the lowest octane fuel that will consistently avoid detonation(which destroys pistons) because the more violent and quick the fuel burns, the more consistent power it will make. Just some engine physics............from an old guy.


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## Dmoney (Feb 19, 2017)

IMHO, there is a great advantage in burning non- ethanol gas in all small engines and boats. I am assuming the prevalent idea in this strand is ethanol vs. non- ethanol. To that discussion I can demonstrate from personal experience, there is this great advantage. Ethanol in gas turns to water if stored in a static state . I have found that eliminating the water and other distillates in the fuel systems of the above mentioned engines greatly offsets the issues I am reading here. I will not put ethanol fuels in any of mine. Since I began this practice my motors have spent far less time being repaired. I do believe that if all of these engines were being ran everyday, year round these advantages would be diminished. The problem with these types of engines is they are not typically ran regularly. Again, all of this is speculative based on my experience. I had a tiller that I had trouble getting it to run each spring until I switched to non- ethanol gas, the last 2 years it has cranked and ran quite well within 2-3 pulls of the rope. My outboards do the same.


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## jim t (Sep 30, 2007)

Along these lines... I used to run 87 octane 10% ethanol in my boat, Honda generator. lawnmower, edger.

After not using my Honda EU 2000 generator for awhile, it was hard to start, once I got it started (with a squirt of Seafoam) it would surge at idle.

Since switching to non-ethanol gas it runs GREAT and starts quickly. Same for my mower. My trimmer powerhead is in the shop though cause it won't start.

I had NO trouble with my Evinrude 225 ETEC with 89 octane ethanol gas, but 89 Octane non-ethanol gas is available at the same pump. I use non-ethanol instead, just in case, but it is $0.50 more expensive.

Jim


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## Boat-Dude (Sep 24, 2011)

Pay me now or pay me later.


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## fishnfool (Oct 2, 2007)

There are multiple issues with ANY ethanol additive in a gasoline product. 
First and foremost, ANY ethanol based fuel will require more fuel to be burned than straight gasoline due to the BTU rate of any alcohol being less than a traditional gasoline. Alcohol of any type generally burns at about 1/2 the temperature of gasoline, so to make the same amount of horsepower from an alcohol-based fuel(10% is generally what they mix ethanol with gasoline) it will take more of the mixed fuel to make the same temperature during the combustion cycle in your engine than it will a straight gasoline, regardless of octane(which once again, has nothing to due with temperature of the burn) to explain it simply as far as the burn temperature goes, it takes a large explosion in the cylinder to drive the pistons back down the bore. The larger and hotter the explosion, the faster the pistons are driven down(or the more force they are driven down with) and this is how power is made. Controlling the explosion is a rather interesting science and I could really get deep in to it but won’t do it here. 
Secondly, as in any alcohol-based fuel, you have to have the proper fuel system to use that type of fuel. While it might seem insignificant, ethanol/alcohol will attack many older types of fuel lines and either cause leaks(under pressure) or the lines will start to fall apart internally and clog filters and/or carburetors and injectors for the fuel injected crowd. On a two-stroke, clogged fuel systems lead to lean fuel mixes, which lead to detonation or high cylinder temps....which burns/breaks pistons. If you haven’t burnt a piston in a two-stroke you’re in for a treat on that one!
Third and also important in marine use, the fact that ANY alcohol additive or mixture in fuel ALSO causes water to accumulate in your fuel. This is due to the condensation associated with the evaporative rate of alcohol. Any temperature inversion or difference will cause condensation to occur depending on dew point, temperature and pressure altitude(usually whatever sea-level is here) it varies from minute to minute with the sometimes drastic temp changes we get in thunderstorms, big north winds, etc. This water accumulation generally isn’t really harmful on something that is run daily/weekly. Here’s the catch, the separation can occur within hours, once again depending on temps and pressures. If this occurs through several weeks or months and you haven’t run your boat/car/motorcycle that water can accumulate in float bowls in your carbs(corrodes aluminum carbs at a very rapid rate) in filters, once again corrosion, and fuel injectors causing them to stick(open and closed) I’ve had this issue with my bike and one of my hotrods that sat for a few months. My car actually lost one whole side of the carb going down highway 29 and when I finally got it home, the carb was junk(irreparable) and it cost me a $400 carb. The car had been at a body shop in Jackson Ms. for about three months....that’s it.
In short, if you can stay away from an ethanol mixture you’ll be better in the long term. Between parts failures and fuel system upgrades you might save yourself a big wad of cash by not hunking a motor 25 miles off the beach(or in grande lagoon) run your motor weekly to keep fuel from sitting and getting stagnant, use a fuel stabilizer if possible because it doesn’t just stabilize fuel, it also lubes fuel system components and can help to condition rubber parts. Sorry for the length but I haven’t seen anyone actually try to take the time to explain this stuff...........and I had a few extra minutes after taking my old 69 Dart out for a ride tonight! Tomorrow, the 70 Camaro gets its first run in a month!


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## X-Shark (Oct 3, 2007)

I've been doing some experimenting with fuel in my vehicles.


The truck is a '15 F150 3,5 Ecoboost. 4 X 4.....The last load of fuel I got was from the Shell/CircleK on the NE corner of Jackson and New Warrington Rd. They have 87 Non Ethanol. First I could not get a full load of fuel. It shut off @ $80 and would not let me restart it or the attendant override it. So I went back the next day to top it off and then start my fuel mileage check. It Sucked! 16mpg The fact that it shut off at $80 means it will suck for a boat. My truck has a 36gal tank. The low fuel light comes on at 30gal burnt. 



Just got the next load of fuel. Lil BP station on Michigan or Beverly Parkway.....What ever it's called right there. This is where I get fuel for my small engines. It's also where I fill cans to dump in the boat....The place is way to small to get a rig in there. It's 91 Octane Non Ethanol. 



The engine in this truck is designed to run on 87.....But says under severe conditions....Towing is considered Severe. To run Premium. Now what happening is the computers sense spark knock. It will put more advance in the motor and the motor makes more power and gets better fuel mileage. Computer senses spark knock and it retards the timing. The higher Octane allows for more timing. I run that same fuel in My '55 Ford.


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## welldoya (Oct 5, 2007)

Just re-noticed this thread that I started a while ago.
This is kinda related just not for outboards.
I bought a Stihl Combi system recently. I was reading the manual and it said to use at least 89 octane. Anything less will cause the engine to run hot.
Sooo, since the non-ethanol that the station up the road carries is only 87 octane, I took my gas jug with me when I went to Pensacola and filled up at the Marathon on Scenic. They have 91 octane. Didn't want to chance using the 87.


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