Mixing 100ll With Mogas

Stingray Don

Stingray Don

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Stingray Don
I have an auto fuel STC for my 172N. The minimum octane requirement is 91. Yesterday I went to my local supplier of ethanol-free liquid altitude and they changed from 91 to 90 octane. I know mixing 110ll with mogas is not an issue, but can I blend 100ll with the mogas to achieve the required 91 octane? Theoretically, adding 10% 100ll with 90 octane mogas should yield 91 octane. However, will the fuels blend together or will they stratify in the tank? I am assuming they would just blend together. Are there legal or other reasons to avoid using 100ll to meet the minimum octane requirements?
 
I can't answer the legality issue, but I can attest that it is 100% fine to mix the two in infinite combinations. Though I've been warned to stay away from 100% mogas during hot summer months for vapor lock issues.
 
Does the STC say you MUST use Mogas? If not, you're good to go. And yeah, they mix just fine.
 
Having a chemistry background...just trace addition of tetraethyllead will boost that mogas 5 points or more...I bet a 1:50 blend would do it...legal, moral, or ethical issues no comment...
 
Mtns2Skies said:
I can't answer the legality issue, but I can attest that it is 100% fine to mix the two in infinite combinations. Though I've been warned to stay away from 100% mogas during hot summer months for vapor lock issues.
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Yes, the STC does state it’s fine to mix the two but doesn’t state whether or not the 100ll can be used to boost the octane to meet the requirement.
 
The specific gravity of 100ll is .68-.74 and of gasoline is .71-.74. That means that they won't necessarily be homogeneous in the tank. However, I believe the mogas will "wash" the TEL out of the 100ll and boost the octane of the 90 octane gas (if that is really what it was when it came out of the pump) and you won't see any knocking in your engine. You may have to run a little richer to keep the engine cool.
 
Seems like it would be easier to just find anther gas station than deal with the hassle of mixing 100ll. On my plane which uses mogas, running 30% or greater 100ll causes oil change intervals to cut in half, meaning oil change every 25 hours instead of 50. And other problems caused by lead build up.
 
Ben2k9 said:
Seems like it would be easier to just find anther gas station than deal with the hassle of mixing 100ll. On my plane which uses mogas, running 30% or greater 100ll causes oil change intervals to cut in half, meaning oil change every 25 hours instead of 50. And other problems caused by lead build up.
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You aren't leaning enough if you're getting lead build up. Mixing 100LL isn't a hassle. It consists of pulling up to the pump and putting 100LL in the tank just like you would with MoGas. Actually less of a hassle if you're bringing 5 gallon jugs of Mogas to the airport. I also doubt the oil change being necessitated at 25 hours rather than 50.
 
Ben2k9 said:
Seems like it would be easier to just find anther gas station than deal with the hassle of mixing 100ll. On my plane which uses mogas, running 30% or greater 100ll causes oil change intervals to cut in half, meaning oil change every 25 hours instead of 50. And other problems caused by lead build up.
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There isn’t any 91 octane nearby, unfortunately.
 
Hard to actually measure an octane change of 1.
The effect of TEL on octane is non-linear, the first little bit changes the octane a lot, but then you get into diminishing returns. Your plan would work from a practical standpoint.
 
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EdFred said:
You aren't leaning enough if you're getting lead build up. Mixing 100LL isn't a hassle. It consists of pulling up to the pump and putting 100LL in the tank just like you would with MoGas. Actually less of a hassle if you're bringing 5 gallon jugs of Mogas to the airport. I also doubt the oil change being necessitated at 25 hours rather than 50.
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My engine is a Rotax. You are going to learn something new today.

https://aviation.stackexchange.com/...ure-control-how-do-they-handle-less-dense-air

As for oil changes, I’d rather follow the recommendations of the top mechanics for this type aircraft. In their worlds, “oil changes are a lot cheaper than new engines.”
 
Ben2k9 said:
My engine is a Rotax. You are going to learn something new today.

https://aviation.stackexchange.com/...ure-control-how-do-they-handle-less-dense-air

As for oil changes, I’d rather follow the recommendations of the top mechanics for this type aircraft. In their worlds, “oil changes are a lot cheaper than new engines.”
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The "top mechanics" at Lycoming and Continental also say we "must" overhaul our engines every 12 years or we will fall out of the sky. I still stand by the statement of not lean enough. Whether you can control it or not is irrelevant. Lead build up means not lean enough. Never had lead build up in any of my engines, and I've only ever used 100LL.
 
EdFred said:
The "top mechanics" at Lycoming and Continental also say we "must" overhaul our engines every 12 years or we will fall out of the sky. I still stand by the statement of not lean enough. Whether you can control it or not is irrelevant. Lead build up means not lean enough. Never had lead build up in any of my engines, and I've only ever used 100LL.
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Since mixture can’t be controlled, I’m guessing that’s why lead buildup occurs and oil changes are more frequently needed in this engine, or better yet 100ll avoided. So I will also stand by my statement. Comparing preventative maintence like an oil change to a TBO/calendar overhaul is sort of absurd. In fact, doing the former actually prolongs the latter.
 
Ben2k9 said:
On my plane which uses mogas, running 30% or greater 100ll causes oil change intervals to cut in half, meaning oil change every 25 hours instead of 50. And other problems caused by lead build up.
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MOGAS with less than 30% AVGAS use = 100 hour oil change interval
AVGAS 30% or more of the time = 50 hour interval
AVGAS 50% or more of the time = 25 hour interval

I got this directly from the Rotax line maintenance manual and their own SI, confirmed by a conversation with a guy in the Rotax tent at Airventure year before last. I didn't get his name, but I'm pretty sure it was Phil Lockwood.
 
I'm pretty sure the OP's C-172 does not have a Rotax...
 
cgrab said:
The specific gravity of 100ll is .68-.74 and of gasoline is .71-.74. That means that they won't necessarily be homogeneous in the tank. However, I believe the mogas will "wash" the TEL out of the 100ll and boost the octane of the 90 octane gas (if that is really what it was when it came out of the pump) and you won't see any knocking in your engine. You may have to run a little richer to keep the engine cool.
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They are overlapping mixtures of various miscible hydrocarbon molecules. They are going to mix when you stick the fuel nozzle in the fill. No way to prevent it.
 
Stingray Don said:
Theoretically, adding 10% 100ll with 90 octane mogas should yield 91 octane.
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I haven't seen it mentioned yet, and it's worth mentioning. The aviation octane scale is different than the auto gas octane scale. 87 octane auto gas is roughly equivalent to 80 octane on the aviation scale. Conversely, 100LL is roughly 108 to 110 octane on the MoGas scale.
 
Ben2k9 said:
Since mixture can’t be controlled, I’m guessing that’s why lead buildup occurs and oil changes are more frequently needed in this engine, or better yet 100ll avoided. So I will also stand by my statement. Comparing preventative maintence like an oil change to a TBO/calendar overhaul is sort of absurd. In fact, doing the former actually prolongs the latter.
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You are correct. The Rotax was never meant to be operated on leaded avgas. If they are, better pay close attention to Rotax's oil change interval reduction or you will be buying a new engine prematurely. We have several folks with first hand experience with that on five separate Rotax engines at our airport. They ran them only on 100LL, did not pay heed to the reduced interval in the Rotax maintenance instructions. They thought they could run them like their previous Lycomings/Continentals. Expensive mistake.
 
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Capt. Geoffrey Thorpe said:
They are overlapping mixtures of various miscible hydrocarbon molecules. They are going to mix when you stick the fuel nozzle in the fill. No way to prevent it.
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Yes, but they may separate while standing in the tanks. I, for one, would not be concerned with what the OP is doing but I wanted to answer the question.
 
with the 0-200/0-300 we have had great luck with a 3 to 2 mix ( 3 auto/2 100LL)
 
cgrab said:
Yes, but they may separate while standing in the tanks.
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Seems a good theory, but I believe it will require about 100 years to achieve total separation.
 
Capt. Geoffrey Thorpe said:
They are overlapping mixtures of various miscible hydrocarbon molecules. They are going to mix when you stick the fuel nozzle in the fill. No way to prevent it.
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I'm not a chemist by any stretch of the imagination, but the poster's assertion that the two fuels will stratify because the specific gravities differ doesn't make sense.
 
DaleB said:
MOGAS with less than 30% AVGAS use = 100 hour oil change interval
AVGAS 30% or more of the time = 50 hour interval
AVGAS 50% or more of the time = 25 hour interval

I got this directly from the Rotax line maintenance manual and their own SI, confirmed by a conversation with a guy in the Rotax tent at Airventure year before last. I didn't get his name, but I'm pretty sure it was Phil Lockwood.
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https://rotax-owner.com/en/rotax-forum/4-general-discussion/5758-oil-change-interval
 
To the mixing question:
They will mix fine. All are approximately C5-C9 hydrocarbons. If they were not soluble in one another you could just take crude oil and let it sit and settle and pull out all your hydrocarbon fractions. Doesn’t work that way. (Been in the chem/refining industry 25 yrs, if it matters).
 
EdFred said:
I don't think there was a conclusion in that thread. The way I read it first it was 50 then 25 then 50 then 25 then 50 then whatever you want to do. LOL
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Sounds just like a PoA forum. ;)

Basically they are a glorified snowmobile engine. They were never designed to run on high octane leaded fuel.

And now we return to regular programming...
 
EdFred said:
I don't think there was a conclusion in that thread. The way I read it first it was 50 then 25 then 50 then 25 then 50 then whatever you want to do. LOL
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Advice from well meaning perople on line is great. Factory documentation backed up by the US distributor is better. The SI says 25 hours if you're burning 100LL... over 50% of the time. The maintenance manual says to change it every 50 IF you're burning leaded gas over 30% of the time, otherwise the normal schedule with unleaded gas is 100 hours.

But hey... if people want to burn unleaded gas and change their oil every 25 hours, I'm not gonna stop them.
 
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