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Octane Questions

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Old 10-02-2002, 06:28 PM
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Question Octane Questions

My HP500EFI say right on the motor to use 87 octane fuel. That's great. Now, I usually buy 87 or 89 (tried 91 but the transom was really dirty). I've heard that the fuel loses some of it's octane when sitting.

So my question are are:

1: If my tank is 1/2 empty (or for the optimists....half full), how much does it lose and how fast?

2: Does heat have anything to do with it?

3. Does how much fuel in the tank have anything to do with it?

4: Does the whole tank lose the octane, or just the top 3 inches, or what?

I'm just wondering since my boat has been sitting for a month, then I got to wondering about those poor LOTO people that have to store their boat for months on end.

Looking for real answers.
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Old 10-02-2002, 06:41 PM
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I've never heard of fuel losing octane. Fuel gets harder to burn as it ages, up to the point at which it doesn't burn (I poured it on the floor and couldn't light it with a match!). Very roughly put, the higher the octane, the harder it is to burn. Thats why you got the black transom, not all the fuel was getting burned, and you lost a couple horsepower by spending more on gas! A 502 mpi loses 2 hp going from 87 to 93 octane! But I digress, if anything the octane would be rising, although its really the ried vapor pressure thats changing.
Now forget all that stuff, put fuel stabilizer in your tank and top it off if its going to sit longer than a couple of weeks. Then your good for up to a year or more!
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Old 10-02-2002, 06:57 PM
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Ok I'm bored so I'll give this a shot.

Octane rating is nothing more than the stability rating of the fuel.
If your fuel is full of oxygenates like MTBE or ethanol, fuel has a tendancy to sour more quickly. It becomes so stable it will never burn. Also these oxygenates also attract water.
So at the end of the season I fill up the tank and add stabilizer.

I believe that once fuel starts to go bad that all of it is bad. I dont know if temp has anything to do with it.

When you run 91 the computer is set for a less stable fuel (89)
so it doesn't burn completetly.

Hows that for some BS

Last edited by hot water; 10-02-2002 at 07:11 PM.
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Old 10-02-2002, 07:08 PM
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Higher octane fuel burns slower which makes it burn cooler. So, higher octane fuel will cause a motor to produce less hp. Also, higher octane fuel will "absorb" water faster.
 
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Old 10-02-2002, 10:01 PM
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Interesting thoughts. Some I've never thought of. I was thinking that it was more like alcohol evaperating from from a drink or something (gezzzz.......what a horrible example) where the alcohol evaporates before the water (Jack and water example).

I wish there were some additional thoughts on this subject.
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Old 10-02-2002, 11:21 PM
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Nort,
Did I miss something here?? Did you say your boat has gone unused for a month!!! For a moment I thought you were talking about someone at LOTO--not someone who lives in sunny CA !!!!
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Old 10-02-2002, 11:38 PM
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octane and stability are not related, but higher octane fuels usually have a better "additive pack" that is in them which DOES enhance the stability of the gas. If the additives in them were similar, then there would be no difference in stability or shelf life (strictly speaking about pump gasolines).

Higher octane ratings burn slower - yes
Lower octane ratings burn faster - yes
Lower octanes have a higher BtU content for a given volume - yes

87 octane gas is by far the "cheapest" gas, both in cost to the consumer and even more so to the producer. As has been mentioned, the additive pack is typically just about worthless in these cheap gasolines.

StaBil will make even a tankfull of cheap gas last thru layup. AND, yes, evaporation is the means by which the volatiles "exit" the fuel and leave behind the turpentine-like liquid that you pour out of your old pushmower after it sits for 4 yrs...
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Old 10-02-2002, 11:44 PM
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I recall a post that said one octain point per month. My Gas sat in my boat for a year to the day while I did my paint job. The boat burned up that NO problem. If the one octain point per month is correct then I had about 74 octain fuel in there.

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Old 10-02-2002, 11:54 PM
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Actually it settles down after a while so you were probably at about 82-84
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Old 10-02-2002, 11:58 PM
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Bigwavz-

That was pretty funny! It's been "all work and no play" for Nort this last month, and a good part of this month too.

BUT........ I'm leaving for Havasu at midnight tomorrow for the AHBA poke-her run!!!!

Thanks for the input guys. Very interesting stuff.
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