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Thread: Octane rating

  1. #1
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    Octane rating

    I was told recently that the higher the octane rating the hotter the engine burns. I've always followed bmw octane recommendation and fueled with the highest rating, but is the hotter temperature burn ok offsetting the octane benefit? What about going mid octane rating?



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  2. #2
    Lucky motorradmike's Avatar
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    There's an awful lot of misinformation around about octane.

    Octane number means only one thing: "Resistance to ignition".
    Fuel/air mixture will ignite without spark if it is compressed and heated enough. When that happens, you have no ignition timing, pistons have to compress burning mixture, and that causes undue engine stress.

    You need to use at least the octane number required in your manual. Higher is OK, lower is not.

    Does it burn hotter? I doubt it. I wish my Dad was still around because he knew all about this stuff.
    Mike Marr
    1978 Yamaha XS750 (Needs rings), 1996 BMW R1100RS, 2004 Honda CRF230F

  3. #3
    Pepperfool GSAddict's Avatar
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    Quote Originally Posted by MotorradMike View Post
    There's an awful lot of misinformation around about octane.

    Octane number means only one thing: "Resistance to ignition".
    Fuel/air mixture will ignite without spark if it is compressed and heated enough. When that happens, you have no ignition timing, pistons have to compress burning mixture, and that causes undue engine stress.

    You need to use at least the octane number required in your manual. Higher is OK, lower is not.

    Does it burn hotter? I doubt it. I wish my Dad was still around because he knew all about this stuff.
    Agreed, and higher octane will not give more power - actually the opposite.
    On my 1150GS I run 90 or 91 in winter months and 92 in summer.
    Basically you can go down in octane until pinging is heard, then go up one.
    '
    Ufda happens..........

    It's all about the details.

  4. #4
    O-Man
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    I don't know how true it is, but I heard that fuels with ethanol will burn hotter and give you less power.

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    Octane is determined by the use of a special motor with adjustable compression and load, and a very special carburetor.

    They start the motor and run on test sample at a low RPM and high load, and increase the compression until it knocks. then they switch to pure Iso-Octane and it will quit knocking. Then they blend N heptane (0 octane ) until it knocks. If it knocks at 87% Iso-Octane and 13% N- Heptane it is 87 MOTOR octane.

    Then the test is repeated at a higher RPM and lower load. The ratio that has the same knock characteristic is call Research octane. The same test sample will score higher, typically about 7 points. Lets call it 95 Research octane

    Europe mostly as I understand advertises research octane. USA uses an average of the 2 values. So for the sample, In Europe it would be 95 octane in the USA it would be (91+87)/2 or 91 octane. It has nothing to do with temperature, power, heat only resistance to knocking, hence the name AKI (AntiKnock Index)

    In general high octane will burn a bit slower, and usually has a lower BTU (heat) content so needs a bit richer mixture.

    Alcohol has an OH attached, because of carrying some heavy oxygen, it has a lower BTU content per gallon. So it will not burn as hot, there is not as much heat in it. It happens to have a fairly high octane, so can be blended with low octane gas, usually about 10% alcohol raises the USA octane rating 2 points, so 85 octane becomes 87. But it contains a few percent less BTU per gallon. You engine only runs on heat, so a lower BTU fuel (all other things being equal) will deliver less miles per gallon. The BTU rating is not directly related to the speed of flame propagation, ease of spark ignition and other characteristics.

    I know that I average about 2 to 3% less economy on 10% blends, and I have heard that E85 is about a 40% drop.

    The all things remaining equal is important. This does not happen often in the real world. The electronic engine controls do make adjustments that make the alcohol burn a bit more efficiently.

    BTW the similar factor for diesel engines is Cetane, for ease of compression ignition.

    All clear?

    Rod
    Last edited by ragtoplvr; 10-19-2009 at 07:16 PM. Reason: fix wrong word

  6. #6
    Lucky motorradmike's Avatar
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    Yes, very clear, you sound like my Dad!

    Is the 40% supposed to be 4%?

    Quote Originally Posted by ragtoplvr View Post
    I know that I average about 2 to 3% less economy on 10% blends, and I have heard that E85 is about a 40% drop.

    All clear?

    Rod
    Mike Marr
    1978 Yamaha XS750 (Needs rings), 1996 BMW R1100RS, 2004 Honda CRF230F

  7. #7
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    I do not have any direct experience, I have heard that it is 40% less.

    Gasoline is about 115000 to 120000 BTU per gallon

    Ethyl alcohol is 76000 Btu per gallon.

    Since the engine only runs on heat, much more has to be used to get the same heat.

    If an engine is designed ONLY for alcohol, you can get better efficiency, using higher compression, optimized amount of EGR, spark advance , but when it has to burn anything, then it is about a 35 to 40% drop.

    At low temperature alcohol does not vaporize well, so winter mileage suffers a bit more, starting is difficult, which is why we only have E85. E100 would not start in winter.

    Diesel is over 125,000 BTU per gallon, so part of the improvement you get is from more heat per gallon, part from higher expansion ratio or getting more work from the heat, and part from not having to expend the work to compress fuel in gas form, it is injected as liquid, since liquids are mostly incompressible, a smaller amount of work is used to get it into the combustion chamber. It all adds up.

    Rod

  8. #8
    Registered User breyfogle's Avatar
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    Quote Originally Posted by ragtoplvr View Post
    I do not have any direct experience, I have heard that it is 40% less.
    +1 on 40% less. I've heard numbers like "half" the milage of 100% ethanol versus 100% gasoline.
    '89 K75S Original Owner
    '94 (Beta) R11RS, ( RIP 12-5-2010 courtesy of blind left turning cage driver ) ....

  9. #9
    "Running Out The Clock" grafikfeat's Avatar
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    I wouldn't worry... You won't be burning E85 in your bike anytime soon anyway.
    E10 is about the max.
    It used to be classified as a winter blend to help w/ emissions on colder days when temperature inversions were most likely to occur in larger cities like LA or NYC.
    Now they use it primarily as a cutting agent to increase the profit margin.

    Side note: My cruiser is rated at 95 and I dropped it down to 87.
    Not only did I save a few bucks over a months commute, but my mileage actually increased.
    You won't notice it in one tank... It took me about three for the motronic to see and adjust.
    But I will add: YMMV.
    Oh... No knocking either.
    "Stupidity, if left untreated, is self-correcting."

  10. #10
    Benchwrenching PGlaves's Avatar
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    Quote Originally Posted by GrafikFeat View Post
    I wouldn't worry... You won't be burning E85 in your bike anytime soon anyway.
    E10 is about the max.
    It used to be classified as a winter blend to help w/ emissions on colder days when temperature inversions were most likely to occur in larger cities like LA or NYC.
    Now they use it primarily as a cutting agent to increase the profit margin.

    Side note: My cruiser is rated at 95 and I dropped it down to 87.
    Not only did I save a few bucks over a months commute, but my mileage actually increased.
    You won't notice it in one tank... It took me about three for the motronic to see and adjust.
    But I will add: YMMV.
    Oh... No knocking either.
    Careful there on two fronts. The corn lobby is pushing hard for 15% ethanol in a number of states.

    And:

    Those old dumb BMW engineers didn't know anything when they said I needed high octane fuel. I just run 87. And I can't hear any knocking.

    To which I caution, just because you can't hear it doesn't mean it isn't happening.
    Paul Glaves - "Big Bend", Texas U.S.A
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  11. #11
    "Running Out The Clock" grafikfeat's Avatar
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    Quote Originally Posted by PGlaves View Post
    Careful there on two fronts. The corn lobby is pushing hard for 15% ethanol in a number of states.

    And:

    Those old dumb BMW engineers didn't know anything when they said I needed high octane fuel. I just run 87. And I can't hear any knocking.
    To which I caution, just because you can't hear it doesn't mean it isn't happening.
    If they're so smart why are there sooo many typos in our owners manuals?

    Seriously though... It has been three years and 47K miles since w/ no ill effects.
    Again, YRMV.

    (...and don't think for a moment I don't heed your warning/advice.)
    "Stupidity, if left untreated, is self-correcting."

  12. #12
    dhgeyer
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    I bought my R850R in October 2 years ago. I ran it all that first Winter in New Hampshire on regular (87 Pump Octane). It ran fine, and I don't think I hurt anything. After that I chickened out and ran high test. Lately I've been running mid grade (89 to 91 around here). I've specifically tried to make it knock - heavy load, low RPMs, and I can't. I strongly suspect that if I was actually doing any damage I'd hear some indication of it.

    Of course one of the factors (only one among many) that comes into play in determining whether an engine will knock is the size of each cylinder. All else being equal (there's that phrase again) it's easier to control detonation in smaller cylinders. I've never heard of any bike, for example, with 250cc cylinders (like my Concours) wanting anything but regular. I don't have any idea how much less likely my 850 is to knock than an 1100 or 1150, given that all else is never equal. Dual ignition has to help in the 1150s.

    Most, if not all, of the other twins out there (mostly V-Twin cruisers) in the 800cc to 900cc range seem to specify regular gas. The 1500's and up specify high test.

    My Vulcan 1500 Classic FI ran so lean that it knocked horribly under load no matter what fuel I ran in it. It was a common problem with that bike. Basically I couldn't use half the power the engine was supposed to have. That was one of the reasons I got rid of it. I didn't want to go the Power Commander route.

    I guess what it comes down to is, you either do or don't believe, as I do, that if it's knocking badly enough to hurt anything, you'll hear it, especially if you try to make it happen and it still doesn't.

    I've also had highly qualified/experienced BMW technicians tell me to run mid grade in the Summer, regular in the Winter.

    You pays your money and takes your chances. There sure isn't any lack of free advice out there.

  13. #13
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    Quote Originally Posted by PGlaves View Post
    Those old dumb BMW engineers didn't know anything when they said I needed high octane fuel. I just run 87. And I can't hear any knocking.

    To which I caution, just because you can't hear it doesn't mean it isn't happening.
    I think part of the confusion is Europe uses only research octane. 95 research is somewhere in the 90 range USA.

    Different blends of gasoline have a different variation between research and motor octane. If the only spec you advertise is research, it stands to reason they could have bends with more than the standard 7 to 10 point difference. Depends on what is easiest to refine or perhaps what is left over after they make all the diesel they can. So USA 87 octane might actually be pretty close to European 95.

    My RS likes to knock, but in the winter I can use 87 from some stations, and 89 or 90 from anyone. In the hot hot summer, I need 91 and 93 is better still. 87 is always fresher, and stale gas loses octane.

    my $.02

    Rod

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