rotheno
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High Octane fuel burns hotter
Temperature and pressure are directly related which means as pressure is increased, temperature increases also.
Let's assume we have a high performance / high compression engine (the Apex is not high compression). When compression is raised, it increases the pressure at top dead center and consequently the temperature also.
87 octane fuel burns at a lower temperature so it would ignite before the piston reaches top dead center (pre-ignition) which is why high compression engines run poorly on low octane fuel. It takes a higher temperature/pressure to ignite 93 octane so it won't burn until the plug ignites it at TDC. It burns hotter as a result.
Temperature and pressure are directly related which means as pressure is increased, temperature increases also.
Let's assume we have a high performance / high compression engine (the Apex is not high compression). When compression is raised, it increases the pressure at top dead center and consequently the temperature also.
87 octane fuel burns at a lower temperature so it would ignite before the piston reaches top dead center (pre-ignition) which is why high compression engines run poorly on low octane fuel. It takes a higher temperature/pressure to ignite 93 octane so it won't burn until the plug ignites it at TDC. It burns hotter as a result.
Mighty
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That makes sense, thanks.
snowbeast
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I have had 5 yammi 4 - strokes and all ran better with 87 octane,it dont matter what octane it is,if 10% alcohol is added,its in all octanes offered at that pump,91 is no differant,now there are still a few stations around here that dont have 10% alcohol in there stuff,but they are far and few between.
Terminator
Expert
I run 87 octane in all my Yammi 4-strokes and htey run awesome. Felt no difference w/ 93 so stopped paying the extra $$$$.
Snomad
Extreme
There are a few other considerations using 87 octane fuel assuming that it is 10% ethanol like it is around here. Even though the 87 will burn or ignite easier the actual "power" or energy derived is lower than an 87 without ethanol; thus potentially reducing mileage. (Makes you wonder just how this would truly help with emissions/global warming if you need to burn more of it to cover the same ground in a vehicle. Not to mention the energy consumed growing the crops to ferment into ethanol.) This fuel I am told will not store well past 3 months, and notice that there are now "special" stabilizers out there for ethanol blended fuel.
The other consideration is that fuel injection systems are much more forgiving of this fuel as the ethanol cleans out the fuel system. Have fun cleaning out those tiny orifices in the carbs on quads, boat motors, etc. after all that crud winds up there. For this reason alone, never mind the octane, if you have a carbed two stroke use 91 with a little bit of isopropyl.
Lastly, ethanol blended fuel has the characteristic of sucking moisture out of the atmosphere. You would think that the ethanol would act as a de-icer and handle the moisture but not so. All it takes is 4 teaspoons of water added to a gallon of ethanol blended gas for the ethanol to separate from the "gas". The ethanol and a layer of other components wind up at the bottom part of your tank and the "gas" in the top layer. This is particularly bad in a two stroke and especially a marine application where there is usually more humidity. Do a search on ethanol fuel on the web and see what happened in Hawaii.
The other consideration is that fuel injection systems are much more forgiving of this fuel as the ethanol cleans out the fuel system. Have fun cleaning out those tiny orifices in the carbs on quads, boat motors, etc. after all that crud winds up there. For this reason alone, never mind the octane, if you have a carbed two stroke use 91 with a little bit of isopropyl.
Lastly, ethanol blended fuel has the characteristic of sucking moisture out of the atmosphere. You would think that the ethanol would act as a de-icer and handle the moisture but not so. All it takes is 4 teaspoons of water added to a gallon of ethanol blended gas for the ethanol to separate from the "gas". The ethanol and a layer of other components wind up at the bottom part of your tank and the "gas" in the top layer. This is particularly bad in a two stroke and especially a marine application where there is usually more humidity. Do a search on ethanol fuel on the web and see what happened in Hawaii.
copo427ss
TY 4 Stroke Junkie
was doing a search and came across this older thread. im a firm believer of using only the octane the engine needs. but todays new gasoline just plain sucks. in smaller quantanties i have personally witnessed it turn bad in just 3 weeks(87oct). oil refineries are cutting back on the quality of their products. some states ive even heard of them running 15% ethenol now. i feel sorry for people in those areas. i have been stepping up a little and using 89oct instead of 87. it burns cleaner, doesnt cost that much more and doesnt go bad as fast as 87 does.
Snomad
Extreme
Let me clarify what our gas is here. 87 octane is 10% ethanol, 89 octane is 5% ethanol and 91 octane is no ethanol here. One of the biggest issues around here is the corrosion the ethanol fuel creates in the float needle and seat in carbs. I've done 3 yammi quads this year where the needle is jammed and the bowl can't fill. The replacement needle and seats appear to be stainless now where the originals were an aluminum needle and a brass barrel. Another issue is that for older boats with onboard gas tanks the fuel lines weren't alcohol resistant. The problems are too big and potentially costly to not run premium in two stokes and older carbureted four strokes. I think that the lower octane fuels would be better for fuel injected four strokes rated for it when it comes to cold weather starting.
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Snomad said:
copo427ss
TY 4 Stroke Junkie
I think that the lower octane fuels would be better for fuel injected four strokes rated for it when it comes to cold weather starting
true the lower the octane, the hotter and faster it burns. but lower octane also burns irregular and not as clean. you also have to remember that all fuel(minus race fuel?) has summer and winter blends to help with cold/ hot starting.
true the lower the octane, the hotter and faster it burns. but lower octane also burns irregular and not as clean. you also have to remember that all fuel(minus race fuel?) has summer and winter blends to help with cold/ hot starting.
MADDOG24
Expert
87 octane. The exception would be at the beginning of the season I will mix some 93 octane in there to bring the overall octane level up on the few gallons of old fuel I have in the tank. Its my understanding that fuel, as it ages, will lose its octane level.
Octane ratings and flame speed are not directly related. That has been a long standing myth. 87 octane does not burn faster or slower than 93 unless its formulated to do so. You can ask a refinery to make you 93 octane fuel that burns at whatever speed you want (generally speaking). Could be faster than somebodies 87 octane, could be slower. It's all in how they mix the different hydrocarbons to make that fuel.
Octane ratings are a measure of a given fuels resistance to detonation or autoignition. If your fuel has an octane rating high enough to prevent autoignition of the end gasses (detonation), you will gain no additional power by running a fuel with a higher octane rating.
Flame speed, or how fast a given fuel burns is dictated by the hydrocarbon components. Faster flame speed is obviously better as the more fuel you can oxidize (burn), the more energy is extracted from it during a combustion cycle.
Octane ratings are a measure of a given fuels resistance to detonation or autoignition. If your fuel has an octane rating high enough to prevent autoignition of the end gasses (detonation), you will gain no additional power by running a fuel with a higher octane rating.
Flame speed, or how fast a given fuel burns is dictated by the hydrocarbon components. Faster flame speed is obviously better as the more fuel you can oxidize (burn), the more energy is extracted from it during a combustion cycle.
pharsyd
Newbie
picked up wifes 2012 GT and it has on the cowling by the gas cap, on the tunnel and in the manual, all state use a minimum of 91. Hope that will be right.... dealer said the same thing as well... use 91 or more if possible...
Sorry, i found this thread from the general yamaha questions section and didnt realize this was a apex thread.
Sorry, i found this thread from the general yamaha questions section and didnt realize this was a apex thread.
Malibu
Expert
In norway the worst we have is 95 okt.testing proves you use less with 98 okt.and more hp
My attak says 91 RON. This is true, RON means research octane number. There is also motor octane which is much lower, if you look at the gas pump it says R+M/2 octane. The number displayed on the pump is the average of research and motor, this gives you the "Road" octane number. Yamaha uses the RON number because that is the lab tested way to determine necessary octane requirement for a particular engine. The owners manual will sometimes show the R+M number, but never on the emissions plate.
pharsyd
Newbie
Ok.... So which do you use? Living here in Canada, Ontario more exactly...... Do I use higher than 91 when available if the pump/ motor octane of 91 is actually lower than the machine stating Ron 91?
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