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Using Rec90 Fuel

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bryanhorgan

Member
After TONS of research, both anecdotal and documented, I'm having a hard time deciding whether or not Rec90 would work fine in a 0-360 with a 8.5 to 1 Compression ratio.

I know the safe answer is NO, just buy 100LL! But as a thought exercise (and the fact that I'm about to put 800+ hours on my airplane) I'd like to get past that.

While ethanol containing fuels may work, it seems there are many issues that may or may not crop up: water contamination, aluminum corrosion, rubber seals that you may or may not know about, etc. For that reason I'd like to focus on the ethanol free ones. For me, in Florida, that leaves the best available choice being Rec90.


Rec90 has an AKI rating of 90. Yes I spoke to the distributor, yes it's AKI. Lycoming's charts say an 0-360 with an 8.5 to 1 Compression ratio must have a minimum of 93 octane AKI (Lycoming Service Instruction 1070AB). Strangely Lycoming has 91 AKI on the chart, even though not 1 engine is approved for it.

BUT, according to ANY octane vs compression ratio chart I've been able to find (see below), a compression ratio of 8.5 only requires about 85 octane. Anything over that is "wasted" octane. The chart below is from energy.gov and details recommended octane ratings from auto manufacturers for specific vehicles' compression ratios over the last 90 years.

octanevscomp.png

This leads me to wonder why 93, and not 85 octane is the approved fuel. It also makes me wonder if 3 octane rating will make any significant difference in an 0-360 8.5 to 1 Compression Engine.
 
There's one way to find out! Put ethanol free Rec gas in one side tank and 100 LL in other side. TO on 100 and then switch at altitude and take some data. Ive done this on my 0320 with 8.5 to 1 ( 160 HP) without any noticeable problems. I got tired of transporting REC gas in containers and the risk of that type of refueling for the small difference on price.
 
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I ran my RV-10 with 8.5-1 pistons on 90 ethanol free with absolutely no problems. I kept one tank with 100 LL and the other with the rec fuel. I would use 100 LL for TO & Landing and switch to rec fuel in cruise. I saw no difference in engine performance or temperatures. I paid attention to fuel lines and isolation/insulation during installation to avoid vapor lock issues.
 
As long as the cylinder isn't hot the risk is small. However >400 F cylinder temp and more than 25" manifold pressure, you are moving in to a risky area.
Our engines are air cooled and the cylinders are running hotter than in a car.
Keep track of CHT.
Detonation (pinging/knocking) occurs after the fuel is ignited by the spark plug, but before the flame front has finished racing across the cylinder to burn all the fuel/air mixture. Pre-ignition occurs when the fuel is ignited before the spark occurs.

Good luck
 

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Avanza, My understanding is that it's virtually impossible to detect detonation/ pre-ignition in our engines until it's too late...
 
DennisRhodes its not a HUGE difference. Got quoted 3.46 for the Rec90 delivered. 100LL is between 4.80 and 5.99 in my area. There is quite a big convenience factor to have it waiting in the hangar for me though.
 
bryanhorgan said:
BUT, according to ANY octane vs compression ratio chart I've been able to find (see below), a compression ratio of 8.5 only requires about 85 octane. Anything over that is "wasted" octane. The chart below is from energy.gov and details recommended octane ratings from auto manufacturers for specific vehicles' compression ratios over the last 90 years.
.
Click to expand...

Compression ratio alone is mostly meaningless when comparing across different engines. It is just the ratio of cyl volume between TDC and BDC. The actual compression of the gas itself requires the inclusion of the IVC (intake valve closing) angle and some more math. The racing world calls this dynamic compression and the profile of the cam sets the IVC.
 
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All I can say, is wow. This is an example of where a little knowledge can be dangerous.
First, there is no universal definition of ‘octane’ or ‘AKI’ (anti knock index), (the two terms are interchangeable). Over the years engineers developed different tests, running at different manifold pressures, different rpm, etc., to detect the onset of knock. Unfortunately they used the same word, ‘octane’, to characterize the results, even though the tests gave different numbers. Two of the most common tests were more properly named ‘research octane’ and ‘motor octane’. Back in the 1950’s there was an ‘octane war’ between the auto gas companies, with some companies, I think Sunoco was one, selling 102 octane gas. Of course, this was ‘research octane’. But they found the public to be so uninformed that they bought the mistaken idea that ‘high octane’ meant they could get more power out of their car engines. Finally the government stepped in, and mandated that all auto gas be advertised by the average of motor octane and research octane. Today, if you read the tiny data plate at the base of the auto fuel pump, you’ll see ‘R+M/2’, or Research octane plus Motor octane divided by 2, the average. So regular car gas at 87 is actually 83 motor octane, and 91 research octane, or 87 average. BUT, aircraft engines have always used only motor octane, since the manufacturers though that that test more closely resembled their large bore, slowly turning engines. (Although you don’t see it so much anymore, aircraft engines used to also specify a third different measure of octane, using a specific full rich mixture, e.g., 80/87, where the 87 number was measured with a full rich configuration). Bottom line: if your aircraft engine specifies 92 octane, that’s motor octane. If you buy 92 octane car gas, that’s about 88 motor octane. You’re now definitely in the ‘you’re on your own, experimental’ region.
Second, the chart shown in post number 1: Virtually all newer cars have knock sensors. They automatically ‘do something’ (like retard the ignition timing) if they detect that the octane is too low for the current operating conditions. Aircraft engines generally do not have knock sensors (combination of they don’t like lead, and loud engines), so there has to be sufficient margin in the fuel to avoid knock in the worst possible conditions. e.g., the engine will run well with lower octane fuel except when the day comes when it’s hot and you need that full power takeoff. Just a minute of sustained detonation will make your engine toast.
Bottom line: Please be careful.
 
BobTurner, quite the history lesson.

Current "octane" ratings at the pump (in the USA) are in fact AKI which is defined as (RON+MON)/2. I think like you mentioned in the past this was not super clear to everyone, so it was changed to AKI.

So 93 octane at the pump is AKI 93, which is what Lycoming says the ENGINE will run on; the rest of the fuel system notwithstanding.

My issue is how much of a difference does that 3 octane rating make...
 
Change your focus

You are focusing on octane and choosing the lower in place of the higher octane 91-E10
I can only tell you that the higher the octane rating the less likely you'll end up with a detonation event.
Worrying about ethanol is unfounded after many years and hundreds of us using 91-E10.
You should worry about octane and choose the highest available, ethanol or not.
It is like a parachute jumper saying: "I need good shoes for landing but I'll settle for a substandard chute (low octane) the shoes will save me.
Do as you wish.
 
Tommy123 said:
Anecdote? 500 hours mostly over very deep water. You do know there's a STC for autogas, right?
Click to expand...

Awesome. So why even bother making such a statement instead of just referencing the STC? What was the purpose of such a statement?

Do you know that both the airframe and the PP must be STC'd?

Do you know utilizing fuels in systems not designed for lower vapor pressure fuels is a recipe for disaster?

Do you realize making blanket statements can lead others to make uniformed, dangerous decisions?

Edit = @Ross. I'm surprised by your comment here. You know that "one size doesn't fit all" here. The octane requirement is usually on the PP data plate and can be met with most premium car gas. We could at least make a reasonable assumption the OP is building a low wing aircraft because of the forum it's on. Blanket statements here or anywhere can be dangerous.
 
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OK, to sum up and keep it on track:

I have an 0-360 in an RV7A
I want to run something other than 100LL
I would like to stay away from Ethanol if possible, because of unknown factors.

If anyone can say for sure the standard build on a 2005 and up RV7 would be ethanol safe, then please explain.

Would 90 AKI (this is the rating ON the pump), when Lycoming asks for 93 AKI be a problem to run? If yes/no, why/why not?

Please remember, just saying Mogas, Car gas or Pump gas, does NOT tell us about the ethanol content OR AKI rating.
 
Freemasm said:
Edit = @Ross. I'm surprised by your comment here. You know that "one size doesn't fit all" here. The octane requirement is usually on the PP data plate and can be met with most premium car gas. We could at least make a reasonable assumption the OP is building a low wing aircraft because of the forum it's on. Blanket statements here or anywhere can be dangerous.
Click to expand...

Read what I said- for high wing 182s.

There is no mystery to running mogas in an RV, I'm doing it and so are several others, collectively a couple thousand hours on the 3 airplanes I know, more than half of that in very hot climates, some to 20,000 feet DA. All of these have EFI though but with mechanical injection, the same concepts can be applied.

We need to be able to limit ignition timing to safe levels for the octane and we need to be sure the fuel plumbing prior to the pumps follow proper practice to avoid vapor lock.

An ignition system with programmable timing curves is most useful here.

Where do you draw the line where an idea or design is acceptably safe- 1000, 10,000, 1 million flight hours?
 
No one in his right mind will tell you that it will NOT be a problem to
run unapproved gasoline in your Lyc, I certainly wouldn't!

What you'll read is what others have experienced using mogas of various octane ratings. How you interpret that information and whether or not you want to apply it to your airplane is entirely up to you.
In a nut shell, you have literally hundreds of Rv's using lower octane gasoline including the evil ethanol additive. It is hard to ignore a mountain of
"anecdotal" evidence accumulated over decades and coming from users and frequent flyers vs. the scientific
papers, where lawyers probably had more to say than scientists and petroleum engineers.

One could also assume that if you are smart enough to build an RV, you won't blindly follow some advice you read on an internet forum page.

I agree with Ross, certain system will help in mitigating your concerns of detonation and vapor lock.
The truth is that just about every RV has a few variations in how their engine compartments are set up, especially the fuel system.
Even if someone wanted to tell you that you can use mogas, you don't really know until you try it.
Experimental aviation, I love it.
 
@Ross, "Real World Experience" or product validation only applies to items with the same boundary conditions. I respect your company's products and your abilities but your statement came across as an endorsement of previous comments/examples; which, were made without any knowledge of the OP's airframe configuration, engine specifics, etc. Some of those were added later. The risks of low(er) octane MoGas (chemical compatibility, RVP, and other properties also considered) in a high wing aircraft with a PP designed low octane fuel are negligible hence the STCs (Peterson IIRC).

Extrapolating that experience to anything other configuration incorporates risks. People can say what they want. Attorneys for dead people's estates can bring action against whomever they want.

Have a safe weekend.
 
Freemasm said:
@Ross, "Real World Experience" or product validation only applies to items with the same boundary conditions. I respect your company's products and your abilities but your statement came across as an endorsement of previous comments/examples; which, were made without any knowledge of the OP's airframe configuration, engine specifics, etc. Some of those were added later. The risks of low(er) octane MoGas (chemical compatibility, RVP, and other properties also considered) in a high wing aircraft with a PP designed low octane fuel are negligible hence the STCs (Peterson IIRC).

Extrapolating that experience to anything other configuration incorporates risks. People can say what they want. Attorneys for dead people's estates can bring action against whomever they want.

Have a safe weekend.
Click to expand...

Not a blind endorsement to just fill 'er up with mogas and launch.

This whole topic has been covered before a few months back in some detail: https://vansairforce.net/community/showthread.php?t=174063&highlight=mogas+vapor+lock+sds&page=15 Read my post #144 for a synopsis of the main points.

Peterson lists many low wing aircraft in their STC list: https://www.autofuelstc.com/approved_engines_airfames.phtml

These all have magnetos and many have carbs. There are Barons, supercharged T-6s and DC-3s in here along with Cherokees and Grummans which have similar layouts and engines as RVs.

RV-12s are flown routinely on mogas as are many other low wing, Rotax powered aircraft. Look at the fuel system layouts used there and any limitations involved with its use.

I'd suggest folks evaluate their fuel system carefully, look at ignition timing carefully and start by testing with one tank full of avgas and the other with your choice of unleaded fuel to mitigate risks.

If you are not comfortable with any of this, don't do it, just use avgas.
 
My last post

in this thread.

I forgot to mention that most if not all the opposing views aginst using alternative fuels (mogas) comes from folks who have never used it, claiming scientific evidence vs heaps of anecdotal reports from actual users.

Akin to taking advice on making money:
"only take advice from those who have a lot of it"

This is a good place to start.
I'd suggest folks evaluate their fuel system carefully, look at ignition timing carefully and start by testing with one tank full of avgas and the other with your choice of unleaded fuel to mitigate risks.
Click to expand...
 
Many years ago, around 2010, I attended the Lycoming engine disassembly and reassembly forums at Air Venture that Lycoming hosted. They had their lead assembly guy there, forgot his name but he kind of reminded me of Alfred Hitchcock by his stature. During the Q&A portion of the forum, I specifically asked him about using auto fuel in my IO-540 V4A5 with 8.5 pistons as there was a lot of fretting going on at that time if 100LL would be banned. His exact response was one word, “absolutely”.
That was enough for me to begin experimenting using rec fuel in one tank when I finished my Phase 1 testing in for cruise. We are experimental and everyone has there own comfort levels, so proceed at your own risk in using auto or rec fuels.
 
Convenient availability plays a large role in use of "alternative fuels". I have no objection to using any fuel that Lycoming approves as being OK for my engine relative to both formulation and "airworthiness" but right now, 100LL is the only option here. We used to have MOGAS on the field here at the regional airport where I'm based, but they terminated that a couple of years ago for low usage....nobody was buying it. I would use it today if it was available on the truck, but not if it involves 5 gallon cans.
 
N427EF said:
in this thread.

I forgot to mention that most if not all the opposing views aginst using alternative fuels (mogas) comes from folks who have never used it, claiming scientific evidence vs heaps of anecdotal reports from actual users.

Akin to taking advice on making money:
"only take advice from those who have a lot of it"

This is a good place to start.
Click to expand...

Indeed, at some point in engineering after the theory is confirmed by controlled tests, real world validation begins. That's the actual proof that something works as envisioned.

The mogas topic is nothing new, Peterson STCs have been around a very long time and likely millions of flight hours accumulated on the mogas fueled fleet. It's well understood and proven in my view as to what it takes to make it work safely.
 
Guess you missed the point. A statement was made about an aircraft operating on Mogas for 500 hours. The PP was designed for low octane fuel (at a set timing) Both the airframe, power plant, and the combo have STCs for its use. There are probably millions of hours of validation on that specific configuration. Now, to state that experience is relevant to a then unknown power plant (still some unknowns even after the OPs addition) with an unknown fuel system configuration is unwise to say the least. Extrapolation is dangerous. Normalization of deviation has killed people. This is well documented. Stay safe.
 
Freemasm said:
Guess you missed the point. A statement was made about an aircraft operating on Mogas for 500 hours. The PP was designed for low octane fuel (at a set timing) Both the airframe, power plant, and the combo have STCs for its use. There are probably millions of hours of validation on that specific configuration. Now, to state that experience is relevant to a then unknown power plant (still some unknowns even after the OPs addition) with an unknown fuel system configuration is unwise to say the least. Extrapolation is dangerous. Normalization of deviation has killed people. This is well documented. Stay safe.
Click to expand...

Don't think I missed your point but you missed all my subsequent posts apparently about low wing aircraft. Yup high wings are different from low wings in this context. I never said they weren't.

How much extrapolation is there from a Lyc powered Grumman or Cherokee to an RV? Not much. All low wings, fuel in 2 wing tanks, fuel selector in the same area.

Follow the same recipe, do some testing with one tank of mogas and one of 100LL to the extreme edges of the envelope- over an airport if you feel better, at 5000 AGL, on a hot day before switching over to the mogas tank. Ain't rocket science and pretty safe since you can switch back to the 100LL tank is something goes sideways.

Greg with his RV-9 has been running mogas for ages, up to 20K DA in Texas. Works just fine... Copy what's he's done.

Lots of folks are already doing this every day. Copy what they've done and test it as outlined. The sky isn't falling.
 
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Very helpful, thanks

I have a 360 with AFP injection and And SDS ignition. This discussion makes Phase 1 simple.
100LL for starters and experiment with Auto Octanes and timing safely , THANKS
 
Larry DeCamp said:
I have a 360 with AFP injection and And SDS ignition. This discussion makes Phase 1 simple.
100LL for starters and experiment with Auto Octanes and timing safely , THANKS
Click to expand...

With std CR PV 360, I'd limit total timing at high MAP (above 25 inches) to 20 deg. 91 octane.

Let us know how things work.

Ross
 
rv6ejguy said:
……Indeed, at some point in engineering after the theory is confirmed by controlled tests, real world validation begins. That's the actual proof that something works as envisioned.
Click to expand...

Yes Sir. Design->development->testing->validation->field follow

Lack of attention to any of these can be vey expensive or worse


rv6ejguy said:
….How much extrapolation is there from a Lyc powered Grumman or Cherokee to an RV? Not much. All low wings, fuel in 2 wing tanks, fuel selector in the same area….

…Lots of folks are already doing this every day. Copy what they've done and test it as outlined. The sky isn't falling…
Click to expand...

Actually, there’s a lot of different stuff going on here so I’m gonna have to disagree with you here. Attached a couple of quick pix I’ve found here, below. Wish I could find the pic of one of Tom’s TSFL’s mock-ups. If was a masterful job of packaging but it does affect the system boundaries.

Never said the sky was falling. Just tried to make sure the OP and others didn’t assume the subject fuel type was good because of one person’s experience with a high wing aircraft and engine designed around low octane. Wide, overreaching, statements are dangerous. Will never back down from that position. My fuel system is designed around MoGas as are a bunch of others. I am a fan. That said, we still read stories here and other places where everything was fine until it suddenly wasn’t; summer -> winter blend oh sh!t’s seem to be a recurring theme. Something as simple as changing fuel filters to one with higher pressure loss or insufficient filter area margin (for fouling) can make the motor in-op through fuel system vapor lock. Having witnessed a lot of “benign” design changes have unexpected even catastrophic consequences throughout my lifetime has made me question everything.

Respectfully, Sir. Stay safe.

94A40D09-F738-4A33-9B8A-C631A2B759C2.jpeg
 

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In a Nutshell

I've posted this same info a couple times now. If you plan to run mogas in an RV, at least with our EFI system, we make the following recommendations:

No 90 deg drilled fittings prior to the fuel pump inlet. Madrel bent type fittings are preferred. 90 deg fittings ok after the fuel pump are fine.

Plumb with AN6/ 3/8 lines/ fittings.

Fuel pump module should be mounted on the floor.

Never use winter blend mogas in hot/high conditions. This applies to places where the formulation is seasonally adjusted (generally colder climates).

Fuel return lines must be plumbed back to the tanks, never back directly to pump inlet.

Total ignition timing will have to be retarded from data plate recommendations at high manifold pressures.

All these points are important to reduce the likelihood of vapor lock issues when using mogas in low wing aircraft.

I'll add that you should test your setup initially with 100LL in one tank and mogas in the other. Takeoff off and climb to at least 5000 AGL on 100LL, preferably within easy gliding distance of a runway. Only then switch over to the mogas tank. Carry out test at the maximum OAT you are likely to encounter in your area and maximum altitude you fly at after letting the aircraft heat soak well on the ground.

I suggest you eventually carry out high pitch and bank angle flight tests at low fuel levels (in one tank, lots of fuel in the other) to have confidence that fuel feed is maintained at these attitudes.

Check your fuel filters frequently, especially with new builds where construction debris may find its way there. A restriction here can have serious effects at the pump.

Always be aware that mogas formulations can vary widely and what works well from one supplier/ brand may not from another. There is much more variability compared to avgas.

For carbed and mechanically injected engines many of the same ideas and cautions will apply but I have no personal experience flying mogas with these setups. I'd look to see what Peterson outlines in their STC documents for similar aircraft.
 
Does ethanol create any problems with Proseal?
From experience in a Cherokee 140 and a Rotax powered Zenith 750, I can attest to vapor lock on take off in heat soaked engines using autogas. Now I try to avoid short stops and restarts when heat soaks the under cowl fuel system and I'm prepared for an engine failure on take off.
 
Cloudboy said:
Does ethanol create any problems with Proseal?
From experience in a Cherokee 140 and a Rotax powered Zenith 750, I can attest to vapor lock on take off in heat soaked engines using autogas. Now I try to avoid short stops and restarts when heat soaks the under cowl fuel system and I'm prepared for an engine failure on take off.
Click to expand...

I run ethanol quite a bit in my -8, and I haven’t noticed any problems.
 
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