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  1. #1
    I dream skis
    Join Date
    Sep 2007
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    Portland, Oregon
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    692

    Compression versus Octane chart?

    Anyone got such a beast?

    I'm thinking I'm going to shave a bit off the head of my 650SN but I don't want to get into crazy expensive gas territory.
    ************
    '78 & 87 Suzuki WetBike's. Yup - 2 of them now.
    '94 Blaster - Piped, footholds, trim (wifes boat )
    EnrollerConnect - Stay busy with your insurance license this year!

  2. #2
    ger87410
    Guest

    Re: Compression versus Octane chart?

    http://www.howstuffworks.com/question90.htm

    What does octane mean?
    by Marshall Brain

    If you've read How Car Engines Work, you know that almost all cars use four-stroke gasoline engines. One of the strokes is the compression stroke, where the engine compresses a cylinder-full of air and gas into a much smaller volume before igniting it with a spark plug. The amount of compression is called the compression ratio of the engine. A typical engine might have a compression ratio of 8-to-1. (See How Car Engines Work for details.)
    The octane rating of gasoline tells you how much the fuel can be compressed before it spontaneously ignites. When gas ignites by compression rather than because of the spark from the spark plug, it causes knocking in the engine. Knocking can damage an engine, so it is not something you want to have happening. Lower-octane gas (like "regular" 87-octane gasoline) can handle the least amount of compression before igniting.
    The compression ratio of your engine determines the octane rating of the gas you must use in the car. One way to increase the horsepower of an engine of a given displacement is to increase its compression ratio. So a "high-performance engine" has a higher compression ratio and requires higher-octane fuel. The advantage of a high compression ratio is that it gives your engine a higher horsepower rating for a given engine weight -- that is what makes the engine "high performance." The disadvantage is that the gasoline for your engine costs more.
    The name "octane" comes from the following fact: When you take crude oil and "crack" it in a refinery, you end up getting hydrocarbon chains of different lengths. These different chain lengths can then be separated from each other and blended to form different fuels. For example, you may have heard of methane, propane and butane. All three of them are hydrocarbons. Methane has just a single carbon atom. Propane has three carbon atoms chained together. Butane has four carbon atoms chained together. Pentane has five, hexane has six, heptane has seven and octane has eight carbons chained together.
    It turns out that heptane handles compression very poorly. Compress it just a little and it ignites spontaneously. Octane handles compression very well -- you can compress it a lot and nothing happens. Eighty-seven-octane gasoline is gasoline that contains 87-percent octane and 13-percent heptane (or some other combination of fuels that has the same performance of the 87/13 combination of octane/heptane). It spontaneously ignites at a given compression level, and can only be used in engines that do not exceed that compression ratio.
    During WWI, it was discovered that you can add a chemical called tetraethyl lead (TEL) to gasoline and significantly improve its octane rating above the octane/heptane combination. Cheaper grades of gasoline could be made usable by adding TEL. This led to the widespread use of "ethyl" or "leaded" gasoline. Unfortunately, the side effects of adding lead to gasoline are:
    • Lead clogs a catalytic converter and renders it inoperable within minutes.
    • The Earth became covered in a thin layer of lead, and lead is toxic to many living things (including humans).
    When lead was banned, gasoline got more expensive because refineries could not boost the octane ratings of cheaper grades any more. Airplanes are still allowed to use leaded gasoline (known as AvGas), and octane ratings of 100 or more are commonly used in super-high-performance piston airplane engines. In the case of AvGas, 100 is the gasoline's performance rating, not the percentage of actual octane in the gas. The addition of TEL boosts the compression level of the gasoline -- it doesn't add more octane. Currently engineers are trying to develop airplane engines that can use unleaded gasoline. Jet engines burn kerosene, by the way.

  3. #3
    PWCToday.com Is My Home Away From Home MikePeavler's Avatar
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    Re: Compression versus Octane chart?

    170's w/ pump 93 fresh ..up to a "cooler" running 180 if you keep it cool w/dual cooling and the squish angle is right.

    M

  4. #4
    PWCToday.com Is My Home Away From Home rasper99's Avatar
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    Re: Compression versus Octane chart?

    I usually point people to this info on the Blowsion head/dome web page:
    Blowsion
    You gotta figure they don't want their customers buying something too hot and getting tee-ed off having to buy race gas.
    Check out the videos of people riding PWCs in the wild Pacific Northwest surf:
    http://www.barry-blanchard.com/


  5. #5
    ger87410
    Guest

    Re: Compression versus Octane chart?

    http://www.faqs.org/faqs/autos/gasol...section-1.html

    7.2 What is the effect of Compression ratio?

    Most people know that an increase in Compression Ratio will require an
    increase in fuel octane for the same engine design. Increasing the
    compression ratio increases the theoretical thermodynamic efficiency of an
    engine according to the standard equation

    Efficiency = 1 - (1/compression ratio)^gamma-1

    where gamma = ratio of specific heats at constant pressure and constant
    volume of the working fluid ( for most purposes air is the working fluid,
    and is treated as an ideal gas ). There are indications that thermal
    efficiency reaches a maximum at a compression ratio of about 17:1 for
    gasoline fuels in an SI engine [23].

    The efficiency gains are best when the engine is at incipient knock, that's
    why knock sensors ( actually vibration sensors ) are used. Low compression
    ratio engines are less efficient because they can not deliver as much of the
    ideal combustion power to the flywheel. For a typical carburetted engine,
    without engine management [27,38]:-

    Compression Octane Number Brake Thermal Efficiency
    Ratio Requirement ( Full Throttle )
    5:1 72 -
    6:1 81 25 %
    7:1 87 28 %
    8:1 92 30 %
    9:1 96 32 %
    10:1 100 33 %
    11:1 104 34 %
    12:1 108 35 %

    Modern engines have improved significantly on this, and the changing fuel
    specifications and engine design should see more improvements, but
    significant gains may have to await improved engine materials and fuels.

  6. #6
    PWCToday.com Is My Home Away From Home rasper99's Avatar
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    Re: Compression versus Octane chart?

    HTML Code:
    That chart might look a little better with a fixed font and an some HTML tags.  
    
    Compression       Octane Number    Brake Thermal Efficiency       
         Ratio            Requirement         ( Full Throttle )
          5:1                 72                      -
          6:1                 81                     25 %
          7:1                 87                     28 %
          8:1                 92                     30 %
          9:1                 96                     32 %
         10:1                100                     33 %
         11:1                104                     34 %
         12:1                108                     35 %
    Check out the videos of people riding PWCs in the wild Pacific Northwest surf:
    http://www.barry-blanchard.com/


  7. #7
    I dream skis
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    Sep 2007
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    Portland, Oregon
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    Re: Compression versus Octane chart?

    Well that was a helluva lot more technical that what I was looking for.

    I was thinking something more along the lines of ..

    up to 170PSI - 87 Octance
    170 - 185 - 90Octane

    etc etc etc.

    I'd like to get my compression bumped to around 165PSI, which I think is still safe with 87 octane fuel.
    ************
    '78 & 87 Suzuki WetBike's. Yup - 2 of them now.
    '94 Blaster - Piped, footholds, trim (wifes boat )
    EnrollerConnect - Stay busy with your insurance license this year!

  8. #8
    PWCToday Guru subhard's Avatar
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    Re: Compression versus Octane chart?

    Quote Originally Posted by bnovak View Post
    Well that was a helluva lot more technical that what I was looking for.

    I was thinking something more along the lines of ..

    up to 170PSI - 87 Octance
    170 - 185 - 90Octane

    etc etc etc.

    I'd like to get my compression bumped to around 165PSI, which I think is still safe with 87 octane fuel.
    A simple list like that would be awesome. Anyone?

  9. #9
    PWCToday Newbie flat2damat's Avatar
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    Re: Compression versus Octane chart?

    Closest i've seen to a simple cross reference is at...

    http://www.adaracing.com/personal-wa...01/ya701lhdkit

    ...not based on 650 (so ignore the dome cc's) but you still get psi V octane for 701 which should be very similar.

    BTW...the 'Octane' value ADA refer to is Ron+Mon/2

  10. #10
    PWCToday.com Is My Home Away From Home teamzulu's Avatar
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    Re: Compression versus Octane chart?

    Quote Originally Posted by bnovak View Post
    Well that was a helluva lot more technical that what I was looking for.

    I was thinking something more along the lines of ..

    up to 170PSI - 87 Octance
    170 - 185 - 90Octane

    etc etc etc.

    I'd like to get my compression bumped to around 165PSI, which I think is still safe with 87 octane fuel.

    So you use what 50 gallons of gas a year in your 650. And premium will cost you like another $25 a year over 87? Some people are so tight i want to punch them in the face. Just use premium regardless. Like shells premium has better additives and detergents in it that will make your ski last longer anyways.
    94 WB w/extras -Wetness Specialist
    94 FX-1 w/extras -
    Get It Up, Stick It in
    95 WB Stock
    96 WB w/extras

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