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Thread: Mixing 100LL with gasoline

  1. #1

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    Mixing 100LL with gasoline

    I've been running straight ethanol free car gas in my VW powered plane, but on a recent trip wound up having to put some AVGAS into the tank to give me a comfortable edge in the amount of fuel I had for the trip home.

    I didn't notice any real difference in performance, to be honest - perhaps it's to subtle for the type of flying I do. Fuel burn seemed about the same as well; it might just be my imagination but it looked like I burned more 100LL than I would have gasoline. One fifty mile trip isn't a good gauge at any rate, as I rarely fly consistently on any given leg.

    Indeed, I'm not all that interesting in benefits, as in my high drag very slow aircraft there's a diminishing return on performance gains.

    The larger question is one of running 100LL causing problems. I'm thinking that I probably won't have any issues with lead gathering on the spark plugs, etc., as it's occasional and in a mix anyhow.

    A few facts:

    1) I operate at low altitudes - no more than 4,000 feet above sea level (well, with density altitude figured in we can add 2,000 feet to that in summertime. ).
    2) I may operate at freezing temperatures once or twice a year...the rest of the time it's temperate or downright hot here in Alabama.
    3) It's a bog standard Volkswagen engine with a bog standard Holley carb, with the fuel being pushed through a bog standard Facet electrical fuel pump.
    The opinions and statements of this poster are largely based on facts and portray a possible version of the actual events.

  2. #2
    rwanttaja's Avatar
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    I run a mix of 100LL and auto fuel (no ethanol) in my Fly Baby, and have had no problems with it. I generally try to make every fourth fill 100LL; it has four times the lead of the old 80 octane avgas, so I'm figuring my Continental is getting the right about of lead.

    I usually switch to running 100% 100LL in late fall, as I fly less in the winter and avgas is more stable. I've used this system for the ~23 years I've had my airplane, but this spring was the first time I developed apparent lead-fouling. I did some leaning to burn it out, and with the arrival of spring have switched back to car gas. Problem has gone away.

    Performance wise, one must remember WHAT the significance of octane is: It is to provide resistance to pre-detonation, or knock. Adding lead is the easiest way to do this, in fact, I believe it's the ONLY economic way to do this above ~93 octane. If one has an engine designed to run on 80 octane fuel, it derives no benefit from a higher-octane gas unless the engine's timing is adjusted to take advantage of it. And if one goes back to 80 octane, one must re-adjust the timing or significant detonation will occur.

    Modern car engines have electronic ignition and knock sensors, and can automagically adjust for changing octane levels of the fuel. Don't see that in aircraft engines.

    The ironic thing about this is that 100LL fuel actually had LESS energy vs. unit volume than car gas, because of the lead.

    Ron "It's a Gas" Wanttaja
    Last edited by rwanttaja; 05-05-2019 at 01:23 PM.

  3. #3
    Dana's Avatar
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    100LL occasionally won't do any harm, on a steady diet of it you might see lead fouling of plugs or valves. I ran straight 100LL in the Mosler half VW in my Fisher with no problems for the 100 hours or so I owned the plane. But there won't be any performance gain unless the engine already needs higher octane than the car gas provides.

    If, however, you usually run mogas with ethanol (I know you said ethanol free), then switching to agvas can affect jetting... avgas burns richer, in the 2-stroke Cuyuna I had in my Ultrastar I found it to be about a 1/2 jet size increment. I saw that as a safety thing, I jetted for E10 mogas (all I could get), then if I filled up with avgas on a cross country I didn't have to worry about it being too lean.

  4. #4
    FlyingRon's Avatar
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    Both the popular Autofuel STCs allow any mixture of 100LL and (allowable) AutoGas. I've never heard of it being a problem. As Dana and Ron said, the biggest issue is that 100LL will lead choke an engine that was expecting 80.

  5. #5
    cub builder's Avatar
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    I usually run a mix of 80 - 90% 93 octane alcohol free Mogas with 10 - 20% 100LL. I mix the fuels in a 110G tank/fueling rig I have in the bed of my truck. I've got roughly 1000 hours running this type of mix. I do it primarily to keep the lead content down on the O-200 in one of my planes since 100LL has roughly 4 - 5x the amount of lead contained in the 80 octane fuel the O-200 was designed to burn and causes premature valve guide wear, but also for the direct economic reasons of the fuel being less expensive. Both of my engines (O-200 & O-320) are 8.5:1 compression. I have been unable to detect any difference in performance or fuel burn between straight Mogas, 100LL, or a mix. However, since I started running a mix of fuels, I never have to dig lead out of the spark plugs during the annual, and have had no issues with sticky valves or excessive lead build up on the valve stems in the O-200.

  6. #6

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    In my vw I use a mix of 100LL and non-ethonal auto fuel. I add a product called TCP to the 100LL to keep lead at bay. When checking for water I like to see my fuel with a blue tint. The reason I use 100LL. First these engines were designed at a time when lead was being used in the fuel. Second 100LL will not vapor lock like auto gas. Now does mixing these two fuels still protect from vapor lock, I would like to think so.

  7. #7

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    While in college on the plains of Nebraska I was employed by a large FBO. The boss told me on day 1 that he expected a minimum of one quart be drained from each sump on every fuel truck and every storage tank (80, 91, 100, 115) per day, plus as much as it took to get a clean sample. What I did with it was my business but it did NOT go back into any company tanks and o by the way if I ever took a drop out of a fuel service nozzle I was fired. I bought two 5 gallon fuel cans and for four years never bought gasoline for my 1200cc, later 1500cc Beetles unless I was on a long trip out of town. The tips of the tailpipes turned purple when I burned 115. No other effects noted including average fuel economy. Your mileage may vary.

  8. #8
    FlyingRon's Avatar
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    Those old VWs were designed for leaded fuel to begin with (though nowhere near what 100/130 and 100LL have). Unless you've done something to increase the compression on these things, the extra octane isn't going to make any difference.

  9. #9

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    Softer old valve seats erode faster without a bit of lead for "lubrication". Unleaded engines have harder seats. So put a bit of lead in older engines.

  10. #10

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    So....add a bit of Marvel Mystery Oil and it'll be okay. Got it.



    Seriously, thanks for y'all's input.
    The opinions and statements of this poster are largely based on facts and portray a possible version of the actual events.

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