Avgas, Mogas, Swift and Mix ratios

[JoeRuscito]

I know these topics have been discussed and Ive done a bunch of searching but couldn't come up with the answers Im looking for.

Im considering my fueling options. We have 100LL on the field but the price is quite high, almost $6. Im also weary that 25 hr oil changes are going to get old pretty quick. So long story short Id like to find an option that will at least give me 50 hr oil changes and not break the bank (max 30 - 50 % avgas use as I understand it). During cross country I plan on fueling with avgas as others do. Which means I should be using mostly non leaded gasoline all other times.

In our area we have 90 ethanol free car gas. Which I think is 90 AKI. My first question: Is this an option? I know the engine needs 91 AKI min, so is there a reliable way to mix avgas and 90 to get up to 91-93 to have some buffer? I could not find an AKI rating for 100ll and a good way to estimate the mix.

I did find this document https://www.rotax-owner.com/pdf/AvgasAutogasMixing.pdf from Rotax-owner which deals in MON and gives a rough way to estimate but does this method apply to other ratings? Also what is the min MON for rotax?


For those interested: I also got a quote from Swift for UL 94, 700 gallons per gallon cost delivered was $6.36. It was about $6.90 if only ordering 350 gallons. I did some rough calculations including oil changes (oil and oil filter cost me $70 with shipping from lockwood) at 25 hrs running 100 avgas and oil changes running 100% swift at 100 hrs. The cost per gallon used was almost a wash $6.50 per gallon with Swift and $6.60 per gallon with 100LL. This is independent of hrs run, it is adjusted for oil change cost per gallon. Assuming a 5 gallon per hr average.

So swift starts to sound like a decent option, likely better for the engine long term AND you save a ton of time in changing the oil. But it is pricey. Id love to be able to use local car gas and mix to 91-93 with Avgas. What would be even better is if that ratio was less than around 30% avgas, because then I believe 50 hr oil changes would be acceptable and the cost would be WAY cheaper.

So what is everyone doing? And is there a known and effective way to mix 100ll and 90 ethanol free?

Thanks!

[Radio Flyer]

With ethanol free 90 octane fuel, you should be able to add 10% ethanol to it and be above the 91 octane minimum and At or below the 10% ethanol maximum.

[JoeRuscito]

Preferably I’d like to run 0% ethanol

[Radio Flyer]

Preferably I’d like to run 0% ethanol

I figured, and I would too, but I think I would prefer to run 10% ethanol over 100LL.

[efwd]

Joe, as a point of interest, when you do your oil change, I recommend a good read through of the procedure in the maintenance manual. As they put it, "procedure must be followed meticulously". I learned the significance of this in the Rotax coarse I took back in October. In short, don't allow air to be introduced into the system. Im flying a 912iS, I use 100LL 100% of the time. Decalin Run Up added at each fill up. When you read the Line Maintenance Manual you will note as I did, Oil changes are now at 100hr unless you use 100LL 30% or more in which the frequency of oil changes are at 50hrs. I understand that those numbers are double what used to be called for. I spoke with Bryan at California Power Systems regarding the change. He states that he would continue to use the 30-35 hr frequency. This is what Stick and Rudder uses if I recall correctly and I believe Kitfox does as well. This is what I do. I just did an oil change at 35hrs and given how dark the oil was, I feel comfortable that this frequency will suit me fine.
Rotax mentions nothing about adding Decalin Run Up that I have seen. As I understand it, Run Up is a lead scavenger and facilitates burning the lead. Since Im adding product that Rotax fails to mention, I might consider (if I were hoping to increase octane level) an octane boost product that can be purchased at an automotive store. Notice, I said "might". I don't even know what the octane boost is made of.

[JoeRuscito]

Hey Eddie, I did read through it well. There is lots of conflicting info out there and the doubling of oil intervals added to it I think. I am running decalin currently as I am running 100% 100ll

[jiott]

Joe, make sure the 90 is really an AKI number. I am skeptical because all the ethanol free premium mogas available in Oregon at a couple of airports and at auto service stations is 92 AKI. If your 90 mogas is some other rating system, there are charts to convert it to AKI.

I have been using the 92 AKI mogas for over 800 hours now, with 100LL on cross-countries but never over 30%. I use 50 hour oil change intervals. Just checked my compression and it is like new, 81-82/83.

[GuppyWN]

Download the app PureGas.

[efwd]

Barring any variables pertaining to test equipment, Jim, your compression check is better than mine and mine is new.

[jiott]

Eddie, mine was also not as good when it was new. Apparently the engine just keeps slowly breaking in and improving as time goes on.

[Meyer]

I am curious about octane related to this. I am an accountant not an expert lol.

Octane for mogas will decline over time. Bryan at CPS has done a couple of accident investigations recently where "old" gas was determined to be the cause of the accident. I think at least one of these was in TX. He mentioned not running any mogas that had been sitting longer than 30 days. I know of a tank of mogas on a field near me that gets filled about every 6 months. They said "we have never had a problem".

As I understand, 100LL will maintain its octane. Will Swift?

Any thoughts on all of this? I guess not a problem if flying all the time, the mogas could never get stale.

[Dave S]

They said "we have never had a problem".

Precisely the quote of the captain of the Titantic concerning his up to then pristine record as he departed for his date with the iceberg......

My circle of friends at the 'port who use mogas (as I do) all rely on filling their transfer containers at the gas station the same day they put it in the plane - so they generally park the plane with partial tanks from their previous trip. Although nobody has been able to explain to me exactly how the miracle of "Stabil" works to preserve gasoline, all of them dispense a label recommended amount of Stabil into their transfer containers when they fill them as the fuel will not be burned up entirely when they come back to park the plane after the day's trip.

[JoeRuscito]

Swift states their shelf life is about as good as 100ll or about a year.

So I’m not sure one of my original questions was answered. Is anyone mixing Avgas and Mogas and is there a good way to estimate resulting octane. I am certain the pump gas here is 90aki.

[Radio Flyer]

Calculating octane after mixing fuels is pretty straightforward. Here's a good link that explains it: https://www.sunocoracefuels.com/tech-article/mixing-fuels-calculating-octane

[Shadowrider]

I mix avgas with ethanol 91 and find that running avgas mixed with 91mogas it keeps for long term in my snowmobiles. I run it in a boosted snowmobile and it sits all summer and never have a problem with octane the following year. Doesn’t seem to take much avgas to help stabilize the mogas.

I am currently running 91 ethanol in my plane.😬 If I where not flying often then I will put a tank of avgas in it.