Author Topic: octane denotation  (Read 248 times)

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Offline Cut4fun .

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Re: octane denotation
« on: March 07, 2018, 10:18:35 am »
 Why can I use 91 octane when my manual says I need at least 95?


There are three different rating systems used to find the octane number of a fuel. The oldest is the Research Method. This method uses a special test engine with a variable compression ratio to compare the relative detonation resistance of fuels with equivalent heptane/octane mixes.


A newer method called the Motor Octane method also uses a test engine, but runs at 900 RPM instead of 600 as in the Research Method, and uses higher temperatures and variable timing to compare fuels. It is considered a more accurate gauge of how gasoline will perform in modern engines than is the Research Method, but it’s rarely used in any kind of advertising because the rating numbers tend to run from about 8 to 12 points lower than the ratings arrived at with the Research engine. A fuel rated 100 Research Octane Number (RON) will only post up a best of 90-92 Motor Octane Number (MON), in spite of the fact that they have very close to the same real detonation resistance regardless of the test method. But oil companies are much more likely to promote their products by quoting RON than MON, if you let them, because it comports with all those marketing myths they’ve been selling all these years. This is where the third rating method comes in.


In an effort to reduce consumer confusion and promote some level of consistency, the US Government requires that the average octane number achieved by both methods be posted on gas pumps and be called the “Anti-Knock Index”. You see it as “R+M/2” on the pump. So when your manual says you need 95 octane, and your bike is from Europe or Japan, you’re being quoted Research Octane Number. The equivalent Motor Octane number would be about 86, and the average would be 90-91, so that’s what you would look for at the gas pump.


Up Next: So, do I need race fuel?

 So, do I need race fuel?


If you can buy pump gasoline that meets the minimum octane requirements of your engine, you don’t need race gas or octane boosters to raise the octane number any higher. Your engine will run detonation-free on any gas that rises to that level, and paying any money out to run the octane rating up any higher than that is just a pure waste.


There are, or may be, several other reasons to improve on the pump gas you find in your particular area. A lot of what goes into commercial automotive pump gas is there to do things other than create power, and those ingredients may be partially or completely inert as far as their contribution to the amount of power the engine can produce from it (referred to as “energy content”). Ethanol fuels are a good example. By itself, ethanol has an RON of 108, but its MON is only 88. E85 fuel is 104 RON, and only 85 MON. Furthermore, to get the same power as non ethanol gasoline, you have to burn 15-25% more of it.


Oxygenating agents are added to pump fuels to aid in the more complete burning of fuel for the purpose of reducing emissions. Oxygenates are added to race fuels as accelerants, and there is often a fairly big difference in the chemicals chosen for that job. Ethanol is an oxygenate, but it produces much less energy per volume in and of itself than most gasoline components, so it reduces the energy content. MTBE (methyl tertiary butyl ether) is an oxygenate that produces more energy when burned than ethanol, and releases more oxygen in the process, so it’s more often used in race fuels.


You can generally gain power through using race gas, but rather than a gift that keeps on giving, it’s a modification that you have to keep on paying for for as long as you want to use it, and it often requires rejetting to make the switch, so you kind of have to stay with it. The extra power IS NOT a result of the usually higher octane number, but comes from the specific blend of hydrocarbon compounds used in the formula. A number of octane increasing components such as xylene and toluene also increase the energy content since they actively contribute to the combustion event, as opposed to tetraethyl lead, with is essentially inert as a fuel component.


As far as vaporization rates, burn rates, etc., etc., differently configured engines require fuels with different attributes. Factors are carbs vs. fuel injection, long vs. short intake tracts, high vs. low RPM operation, and steady state vs. non-steady state operation (like boats and airplanes vs. dirt bikes). Race fuels come in such diverse varieties for this reason.


Incidentally, octane boosters are mostly snake oil. There are a few good ones that are available from automotive speed shops, but most of the ones you see on the shelf at the auto parts store are useless. They say they raise octane by one or two or three points, but that’s a change of 0.1 to 0.3, not 1 to 3 octane. Injector cleaner might actually be more effective.


Next: So, will more octane benefit me?

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