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Fuel Octane and Older Engines


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11-09-09, 04:27 PM   #1  
Fuel Octane and Older Engines

Is there good reason to use 89 octane gas over 87 octane in older 4 stroke engines? I have a 10 yo Briggs Twin II 17hp and a 13 yo Tecumseh 4.5hp TVS115. I heard somewhere that 89 octane would help older engines run cooler?

 
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11-10-09, 12:13 AM   #2  
No, run regular gas. Higher octane fuel actually burns slower than regular gas, which is necessary for higher compression engines to avoid spark knock (pre-detonation or what some people incorrectly call "rattling valves"). If the engine was designed to run on regular fuel, it will perform best with regular fuel, and worse with higher octane fuels, although the difference will be negligible. Modern Briggs engines (the past 20 years or so) are not high compression, and the timing and other aspects of the engine are designed around the use of regular 87 octane fuel.


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11-10-09, 05:57 AM   #3  
So carbon buildup in the cylinder(s) after long service time does not raise compression enough to warrant higher octane?

 
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11-10-09, 05:57 AM   #4  
I use Shell 91 octane and notice that I can cut thicker grass and throw heavier snow. lol I'm j/k.. I use it cause there's no ethanol in it and other than price per litre there's no difference when compared to the lower grades.

 
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11-10-09, 06:43 AM   #5  
Posted By: H0Td0g I use Shell 91 octane and notice that I can cut thicker grass and throw heavier snow. lol I'm j/k.. I use it cause there's no ethanol in it and other than price per litre there's no difference when compared to the lower grades.
Here in the U.S. now ALL gasoline has ethanol in it. Shell now has nitrogen added to its gas. I use Shell gas only now since I noticed 3 years ago that I get better gas mileage using Shell than any other gas. The nitrogen also keeps the valves & other internal parts clean.

 
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11-10-09, 06:59 AM   #6  
Hmm, how does Shell get nitrogen into gasoline? Nitrogen is a real gas right? I'm familliar with using nitrogen gas in "head" spaces of reactors processing chemicals to retain a desired color but not familliar with its use in gasoline?

 
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11-10-09, 08:03 AM   #7  
Here is just one of many reviews on the subject...Shell announces new nitrogen-enhanced gasoline | The Car Tech blog - CNET Reviews

Starting today, all Shell gas stations will be pumping a new fuel: Shell's nitrogen-enhanced gasoline.

Before you start speculating about the wonder fuel of tomorrow, understand that this nitrogen-enriched gasoline differs from the gas Shell was pumping yesterday in its engine-cleaning detergents and additives.

Shell claims that its nitrogen-enriched gasoline cleans better than before and protects better against engine buildup. Of note, all three grades of gasoline will contain the nitrogen-enriched detergents, with its V-Power premium grade containing five times the government-mandated amount. The results are fewer intake deposits, cleaner combustion chambers, and less fuel injector fouling and intake valve sticking.

Citing the rising complexity of new direct-injected and hybridized engines, Shell also claims that this new fuel is better suited to the rigors of modern drive trains.

Considering that detergents and additives are federally mandated in the United States, and just about every brand of gasoline being pumped today makes some mention of the power of their detergents, we don't see Shell's gas as being very revolutionary. We're also inclined to believe that the "nitrogen enhanced" title is more of a gimmick than anything else. However, any step toward cleaner, better-running engines is a good one in our book.

Shell stated that it didn't expect this new nitrogen-enhanced gasoline to result in an increase in fuel price. But with gas prices being set by individual stations and changing daily, that will be nearly impossible to measure.
I would be inclined to take the same view. It doesn't hurt for sure but it is not a new wonder fuel.

 
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11-10-09, 10:36 AM   #8  
If you have so much carbon buildup that it has a significant effect on the compression ratio, you have other problems that need to be addressed. A good running engine that uses little oil should not accumulate much carbon. If it uses a lot of oil or runs for short periods of time, at low speeds, in extreme cold (never reaching operating temp), uses bad fuel, and/or runs too rich, it may accumulate carbon. The common small engine in the common homeowners garage doesn't have any use for high grade gas.


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11-10-09, 07:52 PM   #9  
Correct me if i'm wrong,, but Nitrogeon is an inert gas that makes up most of our atmosphere This gas can be absorbed into a liquid under pressure just as carbon dioxide in soda pop.. If you dive you know that Nitrogeon narcosis (the bends) can be a serious problem,, Thats because the pressure under water causes nitrogeon to dissolve into your blood,, If you release the pressure too fast (surface in a hurry) the nitrogeon creates bubbles in your blood, as does the carbon dioxide in the soda pop when you open it... What are they doing to keep an inert gas in suspension in the fuel @ atmospheric pressure??? I kinda think this is an advertising excersise as I think it's not possible to keep nitrogeon enriched in the fuel unless they (& your fuel tank) keep it under pressure @ all times,,, It's been a while since I've been to school,, but I don't think the laws of physics have changed,, Have they???... Now you have some food for thought... Roger

 
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11-10-09, 08:17 PM   #10  
Thks, I'll stick to a 87 octane top tier gas for my machines.

 
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11-10-09, 10:06 PM   #11  
Posted By: hopkinsr2 Correct me if i'm wrong,, but Nitrogeon is an inert gas that makes up most of our atmosphere This gas can be absorbed into a liquid under pressure just as carbon dioxide in soda pop.. If you dive you know that Nitrogeon narcosis (the bends) can be a serious problem,, Thats because the pressure under water causes nitrogeon to dissolve into your blood,, If you release the pressure too fast (surface in a hurry) the nitrogeon creates bubbles in your blood, as does the carbon dioxide in the soda pop when you open it... What are they doing to keep an inert gas in suspension in the fuel @ atmospheric pressure??? I kinda think this is an advertising excersise as I think it's not possible to keep nitrogeon enriched in the fuel unless they (& your fuel tank) keep it under pressure @ all times,,, It's been a while since I've been to school,, but I don't think the laws of physics have changed,, Have they???... Now you have some food for thought... Roger
I heard Shell bought some of their own laws of physics with the billions they're raking in. ha! I'm guessing the nitrogen enrichment that they use is similar to what's in fertilizers. Some kind of nitrate or ammonium additive. It's probably a Caramilk secret kinda deal so who knows. Makes sense to me though cause my fertilizer is in solid non-pressurized form and the bag says it has nitrogen in it.

 
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