My manual says "at least 89 octane". Should I use 91?

[SeanT]

Hi all. On both boats I've ever owned, the manual and/or engine notes said to use "at least 89 octane fuel". My current manual says exactly this: "Use gasoline with a minimum octane rating of 89."

So, should I use 91? I did some searches here, but most stuff was either about ethanol fuel, or where fuel was only peripherally related to the discussion. I also started to read wikipedia about fuel octane. Once I woke up from that, I posted this question.

 

[nitsuj]

Re: My manual says "at least 89 octane". Should I use 91?

Using any higher than you need to is a waste. If you run 89 and you get no knock or pings, then 89 is fine. If you run 87 with no knocks or pings, then 87 is fine. One of the largest misconceptions in motorsports is that higher octane equals better gas or more power. Octane is about resistance to pre-ignition. Higher octane doesn't "clean it out" or any of the other stuff people think it does.

 

[Knightgang]

Re: My manual says "at least 89 octane". Should I use 91?

Only if you want to pay more for your fuel. In most cases, (At least in my case) it is common to use fuel treatments to help absorb moisture in the fuel tank and most of these products have an octane booster in them. I would not run higher octane fuel unless I had to (89 sold out)... it has happened...

 

[SeanT]

Re: My manual says "at least 89 octane". Should I use 91?

Okay. I wasn't deluding myself into thinking 91 was giving me more power, I was just doing it, well, it said "minimum"!

I don't know what pinging or knocks are, are they funny noises the engine will make?

 

[Pierutrus]

Re: My manual says "at least 89 octane". Should I use 91?

Mis- firing.....
Octane to low.
Use an additive like STP gas treatment.

 

[nitsuj]

Re: My manual says "at least 89 octane". Should I use 91?

Yes, they're noises. Knocks or pings.I don't know how to describe them. I'm sure some one who is better at describing noises can write it and make it understandable.

The short story is this.... If fuel is too volatile, it can be ignited by a hot spot in the cylinder before the spark plug can ignite it. So you get a small explosion, then the plug fires and sets off another. When the 2 flame fronts collide, you get spark knock. Octane is a fuels resistance to this.

 

[Bifflefan]

Re: My manual says "at least 89 octane". Should I use 91?

I don't know what pinging or knocks are, are they funny noises the engine will make?

That is the sound your engine will make if the gas explodes before it is suposed to. It sounds like a little guy inside the motor hitting it with a hammer.
Remember old cars that sounded like they were knocking when you mashed the gas to the floor? thats the sound.

 

[SeanT]

Re: My manual says "at least 89 octane". Should I use 91?

Got it, thanks guys!

 

[Rocky_Road]

Re: My manual says "at least 89 octane". Should I use 91?

I am going to swim against the current...and suggest that you go with 91, if you can only get E-10 mix.

The reason is simple...ethanol laced gasoline loses octane fairly quickly, and unless you are one of the few boat owners that burn a tankful every couple of weeks, you will stand less chance of the octane falling below your minimum requirements to avoid 'knock'. There are numerous articles confirming the octane loss issues with ethanol...if you need one, just holler!

Happy boating!

 

[SeanT]

Re: My manual says "at least 89 octane". Should I use 91?

That boating pattern describes me, but it's good to know anyway. From June to September I'm at least 3 days a week at the lake (an evening or two, and a day or two on the weekend).

Thanks!

 

[Rocky_Road]

Re: My manual says "at least 89 octane". Should I use 91?

That boating pattern describes me, but it's good to know anyway. From June to September I'm at least 3 days a week at the lake (an evening or two, and a day or two on the weekend).

Thanks!

Lucky dog!

Anyway...get in the habit of using Marine Sta-Bil, to fight the negative effects of ethanol. One ounce treats 10 gallons...cheap insurance!

Happy boating!

 

[SeanT]

Re: My manual says "at least 89 octane". Should I use 91?

I'd have been out tonight - but 60kmph winds say otherwise.

So, Stabil should be added with every fill?

 

[Rocky_Road]

Re: My manual says "at least 89 octane". Should I use 91?

I'd have been out tonight - but 60kmph winds say otherwise.

So, Stabil should be added with every fill?

Fights what they term as 'phase seperation'...the ban of ethanol. Also has some good corrosion fighters. One ounce per 10 gallons stabilizes the gas for a year. 2 (or 2.5, can't remember) ounces per 10 gallons will go for several seasons of storage. My main boating magazine tested several brands of stabilizer for ethanol, and liked the Marine Sta-Bil formula best. Be sure that you get the marine blend...!

Happy boating!

 

[Rocky_Road]

Re: My manual says "at least 89 octane". Should I use 91?

P.S. Make sure that you have a decent inline water/fuel seperator installed before the fuel pump. 10 micron filter element is a 'must'!

Happy boating!