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Thread: Mixing avgas and premium uleaded

  1. #1
    S2D's Avatar
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    Mixing avgas and premium uleaded

    Are avgas and premium unleaded miscible? or do they separate after setting a while?
    I may be wrong but that probably won't stop me from arguing about it.

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    Steve Pierce's Avatar
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    I know a lot of people who mix the two with no issues so I assume they are mix able.
    Steve Pierce

    Everybody is ignorant, only on different subjects.
    Will Rogers

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    180Marty's Avatar
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    As we all know, gasoline is a mixture of many different molecules. I've wondered when a tanker load of fuel is dropped at the station, what keeps the heptane, toluene,benzene, and the many others from separating with the heaviest going to the bottom. It's not like it's being agitated when stored.
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    mvivion's Avatar
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    I mixed avgas and auto gas about 50-50 for years for use in an O-320. Never saw any signs of problems. Engine ran fine and clean.

    MTV
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    They stay mixed forever as far as I can tell. Round engines love super unleaded with 1/3 100ll
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    Our round engine has 6 1/4 compression ratio. I wonder if even regular auto is octane overkill?
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  7. #7
    180Marty's Avatar
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    It'd probably run fine on the 84 sub-octane being produced today.

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    cubdriver2's Avatar
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    Quote Originally Posted by bob turner View Post
    Our round engine has 6 1/4 compression ratio. I wonder if even regular auto is octane overkill?
    Stearman only required 73oct

    Glenn
    "Optimism is going after Moby Dick in a rowboat and taking the tartar sauce with you!"

  9. #9
    Jerry Burr's Avatar
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    I'm sorry but the two don't stay mixed. Do your own test. Clear plastic 5 gal container and pour in 1/2 & 1/2 shake it up and let it sit two days. You will have yellow on the bottom and blue on the top. Very clear line. I have submitted my test results on here before and after awhile they seem to go away. For those who have run it (mixed for years) you are simply burning the car gas first and the 100 second and the engine doesn't care. You could burn all car gas for all it cares. You may be getting some valve protection from the lead however. The only way to truly do what you are trying to do is keep 100 in the left ( TO and LD) tank and car gas in the right for cruise. That way you TO/LD/Park on 100. When running the car gas I kept getting small crumbles that looked like baby granola in the fuel strainer. Went back to 100 and the granola eventually went away. This isn't my opinion, it's simply the results of my testing. Try it for yourself and see what happens. Take Care. Jerry B.
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    Granola might have been tank sloshing compound. It kinda looks like black granola.

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    Quote Originally Posted by Jerry Burr View Post
    I'm sorry but the two don't stay mixed. Do your own test. Clear plastic 5 gal container and pour in 1/2 & 1/2 shake it up and let it sit two days. You will have yellow on the bottom and blue on the top. Very clear line. I have submitted my test results on here before and after awhile they seem to go away. For those who have run it (mixed for years) you are simply burning the car gas first and the 100 second and the engine doesn't care. You could burn all car gas for all it cares. You may be getting some valve protection from the lead however. The only way to truly do what you are trying to do is keep 100 in the left ( TO and LD) tank and car gas in the right for cruise. That way you TO/LD/Park on 100. When running the car gas I kept getting small crumbles that looked like baby granola in the fuel strainer. Went back to 100 and the granola eventually went away. This isn't my opinion, it's simply the results of my testing. Try it for yourself and see what happensTake Care. Jerry B.
    Which is why I mixed them in a tank in the back of a pickup. They got agitated every day.

    MTV
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  12. #12
    S2D's Avatar
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    Quote Originally Posted by Jerry Burr View Post
    I'm sorry but the two don't stay mixed. Do your own test. Clear plastic 5 gal container and pour in 1/2 & 1/2 shake it up and let it sit two days. You will have yellow on the bottom and blue on the top. Very clear line. I have submitted my test results on here before and after awhile they seem to go away. For those who have run it (mixed for years) you are simply burning the car gas first and the 100 second and the engine doesn't care. You could burn all car gas for all it cares. You may be getting some valve protection from the lead however. The only way to truly do what you are trying to do is keep 100 in the left ( TO and LD) tank and car gas in the right for cruise. That way you TO/LD/Park on 100. When running the car gas I kept getting small crumbles that looked like baby granola in the fuel strainer. Went back to 100 and the granola eventually went away. This isn't my opinion, it's simply the results of my testing. Try it for yourself and see what happens. Take Care. Jerry B.
    This is why I brought it up. I remember your post form the early days.
    I may be wrong but that probably won't stop me from arguing about it.

  13. #13
    180Marty's Avatar
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    I found this forum on race engines that might explain the separation a little---specific gravity.

    "Also as a certified Aircraft and Power-plant technician and private pilot, I also know that Av-Gas is not formulated like auto gas, i.e., they are quite different. Av-Gas is formulated for longer shelf life as some air planes will sit for extended periods between fill ups. Av-Gas is also formulated to withstand the extreme temperatures of higher altitude and for many other factors that is not taken into account with auto gas.
    This different formulation alters Av-Gas so much that its specific Gravity is much less than that of Auto Gas. That means for given volume, there is actually less mass, (less dense, i.e., in a running engine, Av-gas burns leaner for a given jet size/injector pulse width compared to auto gas). With that means for us Auto guys, is that if you just switch to Av-Gas from auto gas, you will have to run slightly larger jets in the carb or slightly longer pulse widths for the injectors if the engine is fuel injected. You may have heard old timers talk about running Av-Gas in their cars, motorcycles, lawn mower, etc and mention that the Av-Gas is so much hotter that they burned a valve or melted a piston! Uh, yeah they burned the valves and melted pistons, but not from Av-gas being hotter. Av-Gas is NOT hotter, i.e. no more BTU in Av-gas vs Autogas, it is less dense, those bone heads didn’t compensate for that and as such, their engines were running too lean and THAT is what burned their valves and melted their pistons. 
    Another attribute that Av-Gas has a slower burn rate, ie. flame speed. I have only read this in one other place, but the source was credible so I believe it to hold water. A slower flame speed could be a hindrance at elevated engine speeds as the rate at which pressure rises due to combustion is slower and therefore depending on the combustion chamber size, the engine could become less efficient at making power the higher it revs, i.e. burn event is not happening fast enough in the time available at high RPM."
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    Great information input!! I have heard that the gas will separate before and will trust Jerry's input. Most all of my fuel comes from the tank in the back of my truck so I would think it is well mixed. However, I will take more care to fill after having driven the truck. Now the problem of fuel separation in the tank. I am not one to do long warmups and usually do a lower power first takeoff when at home base. I would think that fuel would mix well during flight. I propose a test of mixed fuel that has separated in a jar placed on the floor of a cub and see if it will remix during flight? It will take me a while to get to it but that should tell us if it will remix in flight. A glass jar full of fuel on the floor of the plane, what could go wrong! Even better would be someone that has access to fuel testing. Blend 50/50 100 LL and 87 octane, let sit for 2 days, sump, fly, sump, compare the two. I will spring 50 bucks for the cause.
    DENNY

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    180Marty's Avatar
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    I like to try different things so yesterday I took some E10 87 octane and mixed with 100LL---about an inch of both. After 24 hrs there is no separation. I got some E0 91 and mixed with 100LL this morning----no separation after 3 hrs. It is mid 80's temp in my hangar. This pic isn't as good as me looking from different angles. Are things just that different in the Midwest? 91 and 100 on the left.
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    S2D's Avatar
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    I might have to go buy 2 bottles of vodka tonight so I have some glass containers to test too!! then maybe tomorrow I'll put 1/3 of each in the bottle and try it. Or maybe I'll have to wait til sunday to try it if I can get out of bed by then.
    I may be wrong but that probably won't stop me from arguing about it.

  17. #17
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    In my Acroduster 2 I simply go up at least once a week and "MIX" it. Two snap rolls followed by an outside snap should do it
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  18. #18
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    Quote Originally Posted by 180Marty View Post
    I like to try different things so yesterday I took some E10 87 octane and mixed with 100LL---about an inch of both. After 24 hrs there is no separation. I got some E0 91 and mixed with 100LL this morning----no separation after 3 hrs. ......
    I was just thinking of doing the same thing.
    I imagine that you gave the bottles a pretty good shake after putting in the two fuels.
    Unless you own something "snappy" like n40ff's Acroduster,
    I imagine about all the mixing action most of us will get is from the pump nozzle swooshing the 100LL into the mogas.
    Makes me wonder just how well it would be mixed, even before sitting for a period of time?
    Cessna Skywagon-- accept no substitute!

  19. #19
    180Marty's Avatar
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    Both samples were the auto fuel first and 100LL drizzled in. Looked well mixed and still looks the same a day later. Cooled off last night so got into the 60's in my hangar. Might put them in the refrig to see if temp affects them. Also, wonder what makes the dye stay mixed with the 100LL----is it electrical charges?????
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  20. #20

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    Used to blend into my 10000 gal tank all the time. Might sit for a whole winter only burning a little cub fuel and I have never noticed any separation. been doing it since 2008.
    Dave
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  21. #21
    cruiser's Avatar
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    Left to right, ethanol free, 100LL and 50/50 mix just over 72 hours old.
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

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  22. #22
    spinner2's Avatar
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    So I wonder why lead doesn’t settle out of 100LL? It has a higher specific gravity. TEL is 1.63 and gasoline is .70.
    Last edited by spinner2; 06-03-2018 at 08:37 PM.
    "Fast is fine, but accuracy is everything." Wyatt Earp
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  23. #23
    cubdriver2's Avatar
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    Parked this last October with E0 in the tank, started 2nd flip today.



    Glenn
    "Optimism is going after Moby Dick in a rowboat and taking the tartar sauce with you!"
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  24. #24

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    I had some 87 auto gas for my lawn mower and so mixed that equally with 100 LL and after two days no color separation appears in a clear glass jar.

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    hotrod180's Avatar
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    Does anyone actually "mix" mogas & avgas before putting it into the airplane?
    Or do y'all (like me) just pour it in separately?
    The lead gets to the valves either way,
    but if you're blending in 100LL to add octane to 87 or 92 mogas for a high compression engine,
    that mixing first wouldn't be a bad idea.
    Cessna Skywagon-- accept no substitute!

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    Does the Lead actually matter?

    Quote Originally Posted by hotrod180 View Post
    Does anyone actually "mix" mogas & avgas before putting it into the airplane?
    Or do y'all (like me) just pour it in separately?
    The lead gets to the valves either way,
    but if you're blending in 100LL to add octane to 87 or 92 mogas for a high compression engine,
    that mixing first wouldn't be a bad idea.

    I mix it by putting a 5 gallon can of mogas in each tank when I am filling up with 100, not very scientific.
    1) Does the lead really protect your valves (320)?
    2) If so, what is the optimal ratio to do that?

  27. #27
    mvivion's Avatar
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    Quote Originally Posted by MadDog18A View Post
    I mix it by putting a 5 gallon can of mogas in each tank when I am filling up with 100, not very scientific.
    1) Does the lead really protect your valves (320)?
    2) If so, what is the optimal ratio to do that?
    Peterson in their STC for the small Continentals requires that you use 10 % 100 LL with auto gas, to maintain valve train health.

    MTV
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    MadDog
    The lead helps protect the valve seats if you do not have hardened valve seats. As far as the valve stem I think lead does more harm than good. http://www.dtic.mil/dtic/tr/fulltext/u2/a218743.pdf
    https://oldschool.co.nz/index.php?/t...ion-explained/

    Do a search on aircraft valve seat recession, lots of info out on the web.
    DENNY
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    180Marty's Avatar
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    First time I have seen a stuck exhaust valve thanks to lead was yesterday. Cessna 150 from Idaho stopped for gas. Guy was a mechanic so had it figured out which one was the problem by feeling for colder cylinder. Mobile mechanic from 20 miles away came down and was able to unstick it with air pressure and not the rope trick. Didn't pull the cylinder. On his way after a few hours.

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    Quote Originally Posted by hotrod180 View Post
    Does anyone actually "mix" mogas & avgas before putting it into the airplane?
    Or do y'all (like me) just pour it in separately?
    The lead gets to the valves either way,
    but if you're blending in 100LL to add octane to 87 or 92 mogas for a high compression engine,
    that mixing first wouldn't be a bad idea.
    Click image for larger version. 

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    I mix it as a matter of convenience. I have a 110G tank in the back of my truck. I buy 105 gal of 91 AKI mogas at the gas station. Pump out 10 or 15 gallons to mix gas for the boat, chainsaw, leaf blower, etc, then add some 100LL to the tank to mix 10 - 20% 100LL. I'm feeding an 8.5:1 O-200 and an 8.5:1 160 hp O-320 from the tank. I want just a little bit of lead for valve health, but don't want all the lead found in straight 100LL to choke the guides on the O-200.

    -Cub Builder
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    I mix it in 55 gallon drums. 40 gallons of 87 E0 / 15 gallons 100 LL. Mixes in the truck bed on the way home and sometimes it sits for months before use. No problems yet.....


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    Gentlemen: Great information, There is a ton of info in the articles Denny posted. Thank you.

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    The one thing to watch for if using auto fuel is, get Ethanol FREE (E0) fuel. Ethanol will eat your engine from the inside out.
    Sorry for the dumb questions, I'm an FNG!

  35. #35

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    We get ethanol-free mogas (89 octane) here in the Canadian arctic. I get 100 litres of mogas in a 205 litre (55 US gallon) drum, then fill it up with 100 LL. The drum sits in the bed of my truck and I feed the mix to my O-320-160. Seems to work well and the plugs are cleaner.

  36. #36
    Cub Special Ed's Avatar
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    Quote Originally Posted by hotrod180 View Post
    Does anyone actually "mix" mogas & avgas before putting it into the airplane?
    Or do y'all (like me) just pour it in separately?
    The lead gets to the valves either way,
    but if you're blending in 100LL to add octane to 87 or 92 mogas for a high compression engine,
    that mixing first wouldn't be a bad idea.
    I mix in pickup tank with fine micron filter.
    Click image for larger version. 

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    "There are 3 kinds of men. The one that learns by reading. The few who learn by observation. The rest of them have to pee on the electric fence for themselves." Will Rogers

  37. #37
    aktango58's Avatar
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    I use it as race gas in our sleds.

    Things get shaken up pretty quickly when running wot!
    I don't know where you've been me lad, but I see you won first Prize!

  38. #38
    RatCub Redux FrankO's Avatar
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    Quote Originally Posted by hotrod180 View Post
    Does anyone actually "mix" mogas & avgas before putting it into the airplane?
    Or do y'all (like me) just pour it in separately?
    The lead gets to the valves either way,
    but if you're blending in 100LL to add octane to 87 or 92 mogas for a high compression engine,
    that mixing first wouldn't be a bad idea.
    I mix it 60 (mogas) to 40 (avgas) in one of the 30 gallon gas caddys you see advertised on ebay & amazon, burn it in my a658 (Luscombe) and C90 (PA18-95). Works just fine, no issues with clinkers, stuck valves, etc...


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