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Thread: 100LL vs compression ratio

  1. #1
    bwilson4web's Avatar
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    100LL vs compression ratio

    Hi,

    I need to order an 1835 cc engine and have options for compression ratios:

    9.4:1 - 100 octane
    8.1:1 - 92 octane
    6.7:1 - 89 octane

    I would prefer the higher compression ratio but I notice the EPA wants to 'get the lead out'. Understand, my old Cherokee 140 was designed to run on 80 octane but all I could buy was 100LL . . . all of the drawbacks of higher lead with no benefits from the higher octane. But all of this muddle about 100LL potentially going away doesn't help.

    It looks like there are cylinder head inserts that can decrease the compression ratio. So my thinking is order the higher compression ratio engine and if 100LL goes the way of 80 octane, do a top-end overhaul and add the 'de-compression' rings.

    Reasonable solution?

    Bob Wilson

  2. #2
    CarlOrton's Avatar
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    Hi, Bob;

    What type of engine? If a VW, then yes, there are rings between the cylinder and case that you can vary to adjust the deck height of the piston in the cylinder, determining compression. I have an AeroVee, and when building, we have the option of selecting either 7 or 8:1 ratios.

    I built mine assuming that 100LL or the next gen fuel will be available; if not, I can always make some relatively (key word there) simple adjustments. Bear in mind; you'll also have to probably adjust the valve pushrods as well (as in, cut a new set).

    Carl Orton
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    http://mykitlog.com/corton

  3. #3
    bwilson4web's Avatar
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    Thanks!

    Yes, it is a VW engine. A second set of pushrods and shims seems a small price to pay. But you mentioned "either 7 or 8:1 ratios."

    In my ignorance, I'm wondering what is the practical upper limit of compression ratios are for VW engines . . . no doubt driven by what the engine can mechanically take. After all, it is not a candidate for converting to a diesel.

    Bob Wilson

  4. #4

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    Just to thow another thought out there...94 octane alternatives appear to be relatively easily produced...even if 100LL "goes away" you would likely be safe with the middle option.
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  5. #5
    CarlOrton's Avatar
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    Quote Originally Posted by bwilson4web View Post
    In my ignorance, I'm wondering what is the practical upper limit of compression ratios are for VW engines . . . no doubt driven by what the engine can mechanically take.
    Well, there's the upper limit, and the upper limit for aviation use!

    I am saying more than I know here, but I would assume that for things like drag racing, they're somewhere above 10:1. But, they probably change the heads on a pretty regular basis.

    The aero applications of a VW (or really, any high HP utilization) begin to stretch the limitations of Dr. Porsche's design. Head cracks around the spark plugs are not that uncommon; fortunately they are still somewhat readily available. Heat (CHT) is the biggest problem. Some folks have added additional cooling fin area. Going much higher than 8:1 isn't going to get you that big a boost compared to the stresses on the engine. I believe (again, ignorantly) that dropping the ratio to 7:1 will only drop about 4-5 hp. Going the other direction, for 5 hp that you'll typically use for just a few minutes, do you really want to tax your engine that much? I'll let the designers with empirical data chime in now.

    Carl Orton
    Sonex #1170
    http://mykitlog.com/corton

  6. #6
    Looking abroad at the way things are heading, the only two aviation fuels we're likely to have in the future are 91+ AKI autogas (no ethanol of course) and Jet-A. Any future plane I build or buy must operate on one or the other. Autogas will eventually be available at more airports, and without ethanol, I am convinced.

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    Octane Ratings

    Please make sure that the octane ratings that you are discussing are measured the same way. The last post mentioned AKI (anti-knock index). Remember that automobile fuel is based on (R+M)/2 and aviation fuel is different. On average aviation fuel is labeled ~5 points above automobile fuel. In other words, 100LL is about equivalent to 94/95 octane (AKI) automotive fuel. Are the numbers in your first post for aviation or automotive fuel octane? Just trying to be careful.

    In addition, remember that autofuel may or may not have alcohol (some states require that to be labeled on the automotive pumps). Also, there are summer and winter blends of autofuel. The RVP (Reid Vapor Pressure) is changed (made more volatile) to make cars easier to start in the winter. This can be an issue with vapor lock if used in hotter or heat soaked conditions on airplanes (and more prevalent on low wing airplanes).

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    bwilson4web's Avatar
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    Quote Originally Posted by Ron Blum View Post
    Please make sure that the octane ratings that you are discussing are measured the same way. The last post mentioned AKI (anti-knock index). Remember that automobile fuel is based on (R+M)/2 and aviation fuel is different. On average aviation fuel is labeled ~5 points above automobile fuel. In other words, 100LL is about equivalent to 94/95 octane (AKI) automotive fuel. Are the numbers in your first post for aviation or automotive fuel octane? Just trying to be careful. . . .
    Thanks!

    The compression-octane table came from Great Plains engine assembly manual. But upon further reflection and everyone's good notes, it makes sense to go conservative. Their stock build has an 8.0 to 1 compression ratio and they list it as good for 92 octane. I'll double check the octane rating system but it sounds like a good solution.

    Since the aircraft is already registered, N19WT, my rework will try to be as true to the original as possible with well documented changes that improve safety. Once flying again, there will be plenty of time for any tweaking later to the baseline.

    Bob Wilson

  9. #9
    Quote Originally Posted by Ron Blum View Post
    Please make sure that the octane ratings that you are discussing are measured the same way. The last post mentioned AKI (anti-knock index). Remember that automobile fuel is based on (R+M)/2 and aviation fuel is different. On average aviation fuel is labeled ~5 points above automobile fuel. In other words, 100LL is about equivalent to 94/95 octane (AKI) automotive fuel. Are the numbers in your first post for aviation or automotive fuel octane? Just trying to be careful.

    In addition, remember that autofuel may or may not have alcohol (some states require that to be labeled on the automotive pumps). Also, there are summer and winter blends of autofuel. The RVP (Reid Vapor Pressure) is changed (made more volatile) to make cars easier to start in the winter. This can be an issue with vapor lock if used in hotter or heat soaked conditions on airplanes (and more prevalent on low wing airplanes).
    Note that I stated "91+ AKI autogas", not avgas. AKI = (RON+MON)/2. Avgas is rated according to the MON (Motor Octane Number) method of determining anti-detonation. RON = Research Octane Number. In the US, we use an average of the two numbers for auto fuel, but not 100LL, go figure....

    Tons of related information on this topic on our GAfuels blog at http://www.generalaviationnews.com/c...inion/gafuels/

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