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MNBIGBEAR

Is there a big difference with gas??

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MNBIGBEAR

I know I have read some of the previous posts but does anyone know if there really is a big difference between say 87 octane and the 91 octane that you can buy? I have a 4 stroke yamaha and I have heard both sides of the coin. Some say the cheaper stuff actually burns better and then some say that the 91 burns cleaner. Anyone know for sure?

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mechanictim

The higher the octane of the fuel the harder it is to ignite, when operating a high compresion engine the high octane prevents preignition(ignition of the fuel air mix caused by the combined heat of the combustion chamber and the heat of compression) which causes knock. If your engine has a compresion ratio of 9.5 to 1 or less you will get better performance with the 87 octane. For a compresion ratio of 10.5 to 1 or greater you need the 92 octane, for engines in between you can use the 89 to 91 octanes.

The lower octane fuel will run cleaner in a low compresion engine because it will burn more completely and because it burns more completely it also will get better mileage.

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New Yankee

Bigbear - gas choices can be the same Ford v Chev arguements. The difference in octane ratings is the measure of the resistance of the particular hydrocarbon blend to pre-ignition knock, or pinging. The additives that a particular refiner or brand uses can differ, but the formulation of higher octane fuel does not inherently create a better or cleaner fuel, just the temperature at which unaided or spontaneous ignition occurs - pinging (remember temps increase under compression - the old muscle cars running 12 to 1 needed high test to avoid pre-ignition).

I think we often assume if it costs more it has to be better, and often that assumption is correct. What works for you may not work for me. As someone else posted, and I wholeheartedly agree with, is my truck/boat/etc runs better when it's clean smile.gif Not pulling your leg, but cause and effect and the perception of same often overules physics, at least in our own worlds.

I'll not try to dissaude you from using high octane if you feel it works for you, just as I wouldn't try to tell you your Makers Mark is no better than well bourbon since they may be the same proof. I think a compelling arguement can be made that using what your owners manual suggests is likely the best solution.

There really is a wealth of info on the web if you simply search 'gasoline octane'. You will find opinions on either side of the spectrum but the majority opinion will suggest the same. I'm a 87 or 88 guy, since I don't own anything that needs the high test - DARN! Theoretically, high test in a lower compression engine could cause the carbon build up (due to incomplete combustion at lower temps) that one seeks to avoid and create a self fulfilling need after carbon builds to a point of increased compression. From a practical standpoint, I suspect it would take quite a few years of use to create this, and for outboards would often be negated/eliminated by the use of ethanol, seafoam or the annual de-carb that many do routinely.

My two... and worth what you paid for it!

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