What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

100LL Future and Engine Purchase Concerns

Mudfly

Well Known Member
I'm probably a year or so away from looking for some type of propulsion system for my -14 project:). As decision time gets closer I become more concerned about what I?m hearing about the uncertain future of 100LL. When I discuss these concerns with others, the typical statement I hear is ?they?ll come up with something?. Well, this issue has been around for many years now, and so far a suitable replacement is yet to be produced. Apparently, it?s a little more challenging than expected. I wonder will the whole 100LL phase out plan just go away, or will some type of ?band-aid? fuel be introduced until engines can catch up with the times. I realize this can be a controversial subject, and I may be overreacting. I imagine the folks that already own the 100LL burning engines are likely hoping for some type of 100LL replacement, or that the FAA/EPA will give up on the AVGAS phase out plan all together. On the other hand, the group that have yet to buy engines are hoping for some type of miracle, and a new engine will be developed that doesn't require 100LL,..and will be similar in weight, shape and fuel burn of the Lycomings. Big wish list I know.
Again, I may be overreacting to this, but this 100LL thing has kind of taken the wind out of my sail for my build motivation. I enjoy my project, and have no intention of throwing in the towel. However, I just can't bring myself to buying an engine that requires a fuel with an uncertain future.
I would love to put a small turbine, or diesel up front. However, I just don't believe the current options are a good fit for the RV-14. I'm keeping my fingers crossed though!
It appears the most likely scenario may be sticking with the lycoming and modding the engine to burn something other than 100LL. I've heard the engine for the 14 (Lycoming IO-390) with a compression of 8:7:1 is just over the limit for running a lower octane unleaded fuel. The 390 would need to be derated, and have hardened valve seats. I have not researched what the derated HP would be, or if it would it be sufficient for the RV-14. My knowledge of piston airplane engines is pretty basic, but I'm learning.
So...anybody else out there have issues with buying an Avgas burning engine given the 100LL issue, or is it just me??

Here's some links that help "fuel" my concerns:

FAA PAFI program updatehttps://www.faa.gov/about/initiatives/avgas/

AVGAS facts and Future
https://www.shell.com/business-customers/aviation/aeroshell/knowledge-centre/technical-talk/techart12-30071515.html

100LL Replacement or not?
https://www.avweb.com/blogs/insider/Are-We-Gonna-Get-a-100LL-Replacement-for-Not-230953-1.html


The need for leaded Avgas (This site has interesting info of all kinds)
http://www.epi-eng.com/aircraft_engine_products/demise_of_avgas.htm


and on the R&D front:

What happened to this NASA project?
https://www.grc.nasa.gov/WWW/PAO/PAIS/fs01grc.htm

Give Continental credit..at least they're trying!
http://www.continentalmotors.aero/diesel/diesel-engines.aspx


Turbine Aeronautics (wishing these guys luck!)
https://www.turb.aero/

PBS (there was one of these in a 10) very pricey and fuel burn issues. would love one though!
http://www.pbsaerospace.com/our-products/tp-100-turboprop-engine
 
GAP Piston Engine

This is all my my ancient memory banks. The specs were quite aggressive and any qualifying product would be drool worthy. A very long time ago, a Continental Engineer working on the GAP piston engine told me (right or wrong) that the engine had met all of the design criteria. If just one of the OEMs would pick it up, it would go into production. Assuming that was true, they missed the market; not their fault. The spec called out a 200HP engine. At that time, the exp and commercial market were going to bigger powerplants. Continental was subsequently sold to the Chinese so it's doubtful we'll ever see anything for our market from these tax dollars.

Maybe someone else has better info/memory. Hopefully they will reply.
 
Not at all.

I don't think you will get many answers on this one as no one knows what the final look of the fuel change over will be at this time. I can say that many of us have planed for this many years ago using our dealings with engines, engine parts and systems. You have a good starting point by looking at the compression ratio as a guide. The new fuels are targeting the standard of 8.5:1 as that is the set that most of the old aircraft engines have used down throw the years. We know that we can burn 92-93 Oct. UL fuels in these engines now. And if you look the engine companies are starting to go back and approve the use of Mow. gas of this grade in these new and older engines with that set-up or configuration. The newer FAA approved fuels will have to target at least that market if not higher Octane ratings. So I have no personal problem with an engine that burns fuel of that grade or standard. I personally think that the fuel or fuels that will be given the green light will be closer to the 100 Octane level in the end. WE built our fuel system to be able to be used with any "E" fuels for just these reason and compromised by putting 9:1 compression pistons in, instead of 8.5:1.
The big catch in all of this is "WARRENTY", or who will pay if the new fuels do damage to my nice costly engine. If you don't have big sponsors and have to swallow that pill for yourself, I can more than see the stress this would generate. I think they have good replacements already, it is I think the liability and the jockeying for a place at the profit table that is the thing that is dragging out the time table. In the engine world, fuels are often hand blended to meet the need of that use. Some of us still do it a little.

This may not help you but it is another two pennies to throw in the pot.
Yours, R.E.A. III # 80888
 
Last edited:
My $.02 if you are early enough, use high pressure auto fuel injection hoses in any thing fuel related and make provision for a possible fuel return.This type of
thinking allows for any fuel/system in future. I know it?s not politically correct,
but an acquaintance used high test auto fuel in his Bonanza for 30 years !
 
I'm rolling up on 275 hours now on my IO360 running Walmart-grade 91 premium autogas with ethanol on 8.7:1 compression with no issues. I did build the entire fuel system on the airplane with ethanol exposure in mind and eliminated all but one natural rubber O-ring which I found a couple weeks ago, it finally started leaking at about 250 hours.

The engine is quite happy burning 91E10, in-flight performance is within a percent or two of the 100LL standard, I can't tell the difference without digging into the Dynon performance data on a long cross-country.

Handling the fuel here is going to be the trick for you and most others though. I am based on a private strip just over 30 miles from the nearest "real" airport with fuel - so I have my own tank on my strip. I took a used milk container, 125 gallons stainless, and steam cleaned it, set it on a 275-gallon chemical tote frame so I can pick it up with forks on a tractor, and put a pump and battery on the frame. I can fork it onto the back of my pickup and take it to town for a fill when needed, easy-peasy.
 
Be careful here

My $.02 if you are early enough, use high pressure auto fuel injection hoses in any thing fuel related and make provision for a possible fuel return.This type of
thinking allows for any fuel/system in future. I know it’s not politically correct,
but an acquaintance used high test auto fuel in his Bonanza for 30 years !

I don't like disputing people and hate the back and forth that ensues but please be careful here. Your last sentence is a bit cavalier in my opinion. Most aviation engines are designed to operate on relatively low octane levels. MoGas is fine for that application. Octane rating does not change Specific Fuel Consumption (though ethanol f's that up). Airframes have to be designed (certified where applicable) for the fuel as well. Most MoGas fails the Reid Vapor Pressure requirement so use in a low wing aircraft without proper design provisions can have serious consequences. "Drawing" fuel through a line can easily cause vapor lock. I also have known of people that put MoGas into airframes that were not designed/certified for such (the bell curve exists in every population.) It works fine until it suddenly doesn't. It's always the alligator you don't see that gets you.

Fly safe.
 
Last edited:
As decision time gets closer I become more concerned about what I’m hearing about the uncertain future of 100LL.

I don't think the FAA or anyone else is going to ground the entire GA fleet until everybody upgrades to a new fuel system. I'm sure there will be a transition period where new airplanes will be required to run the new fuel to get their pink slip. Then, over time, as supply and demand works its magic from gradual transition it will force the prices of 100LL to rise and will push folks to upgrade their fuel systems/engines. I think it's safe to say build on!
 
Last edited:
I?m hearing about the uncertain future of 100LL.
Just a few years ago their was major concern (worry) we were about to run out of ALL petroleum (Peak Oil). Don't worry about 100LL.... worry about choosing a reliable engine with reliable systems... get the best available today that you can afford.

Again, I may be overreacting to this, but this 100LL thing has kind of taken the wind out of my sail for my build motivation. I enjoy my project, and have no intention of throwing in the towel. However, I just can't bring myself to buying an engine that requires a fuel with an uncertain future.

No fuel has a certain future. But, before the EPA tries to ban 100LL they'll try (again) to beat coal, woodstoves, and diesel. There are many thousands of 100LL engines in service and nobody has a replacement fuel... its not a priority. Just fly the heck out your 100LL machine and wear it out before a replacement comes along. My guess is we've all got at least 20 years.
 
Not a definitive answer, but FWIW

The EPA and FAA have been battling and saber rattling this since lead was removed from cars. I worried about that too, but proceeded ahead with my M1B. The serious issues to be resolved for a replacement fuel are for turbocharged engines. Many are out there and cost a lot more than our normally aspirated units. Owned by people with lawyers.

My invested opinion was to build on, fly and enjoy. If it becomes an issue there is a lot of pressure to apply to the situation, politically, (AOPA, EAA, etc) and technically.

On an optimistic note, I am sure that Ross will have a fully developed EFI system ready with knock sensors, O2 sensors and full timing control with diagnostics (OBD-X) ready for the task. :D
 
What Lycoming has to say

I like to go back to the manufacturer of the engine to see what they have to say.

Lycoming on Unleaded fuel part 1.

Lycoming on Unleaded fuel part 2.

Lycoming on Unleaded fuel part 3.

Lycoming Service Instruction 1070Z.

If that is too much to read, most of the parallel Lycoming engines (150, 160, 180 HP) used in our RVs will operate just find on alcohol free premium auto fuel.

As a side note, the Superior Vantage 180 HP engine was certificated on 100LL and auto fuel. Check the type certificate data sheet. That engine is same bore and stroke as the Lycoming and the parts used to fabricate it are for the most part FAA / PMA approved for Lycoming engines.
 
Last edited:
On an optimistic note, I am sure that Ross will have a fully developed EFI system ready with knock sensors, O2 sensors and full timing control with diagnostics (OBD-X) ready for the task. :D

We already have lots of folks running mogas with our EFI for many years without issue and few running Swift unleaded avgas. When the universal unleaded avgas becomes available, we'll already have technology in place to take full advantage of the benefits it would afford.

With the present SDS EFI/EI systems, it's easy to retune for fuels with different specific gravity, octane rating and stoichiometric ratio.
 
This is all my my ancient memory banks. The specs were quite aggressive and any qualifying product would be drool worthy. A very long time ago, a Continental Engineer working on the GAP piston engine told me (right or wrong) that the engine had met all of the design criteria. If just one of the OEMs would pick it up, it would go into production. Assuming that was true, they missed the market; not their fault. The spec called out a 200HP engine. At that time, the exp and commercial market were going to bigger powerplants. Continental was subsequently sold to the Chinese so it's doubtful we'll ever see anything for our market from these tax dollars.

Maybe someone else has better info/memory. Hopefully they will reply.

Thanks for the info Freemasm. Too bad the GAP engine didn't get picked up by an engine manufacturer back in 2000. Might be close to getting certified by now:).
Here's a couple more articles I found with some additional info on the GAP engine:
https://www.nasa.gov/centers/dryden/news/NewsReleases/2000/00-58_pf.html
https://www.avweb.com/news/reviews/182838-1.html
 
Last edited:
15 years ago I wondered the same thing and went with standard 8.5 CR, new engine is going to be high compression, I'll worry about UL fuel when it gets here if I live that long, until then I'm going to enjoy a few more ponies :D
 
I like to go back to the manufacturer of the engine to see what they have to say.

Lycoming on Unleaded fuel part 1.

Lycoming on Unleaded fuel part 2.

Lycoming on Unleaded fuel part 3.

Lycoming Service Instruction 1070Z.

If that is too much to read, most of the parallel Lycoming engines (150, 160, 180 HP) used in our RVs will operate just find on alcohol free premium auto fuel.

As a side note, the Superior Vantage 180 HP engine was certificated on 100LL and auto fuel. Check the type certificate data sheet. That engine is same bore and stroke as the Lycoming and the parts used to fabricate it are for the most part FAA / PMA approved for Lycoming engines.

Thanks Gary. Very interesting information. It's clear Lycoming has no problem with the use of "MOGAS" in certain engines. However, they are also clear that MOGAS is not pump gas. When/If 100LL goes away, what would be required to produce//transport/store and deliver MOGAS to the pump at the local airport..and how much will that cost?
 
Most MoGas fails the Reid Vapor Pressure requirement so use in a low wing aircraft without proper design provisions can have serious consequences. "Drawing" fuel through a line can easily cause vapor lock.
Fly safe.

Would love to hear your explanation on why 100LL can be "drawn" through a line without vapor lock, but Mogas can't.

Larry
 
Last edited:
I have no direct experience with the 390, but significant evidence suggests that 8.5 CR is not a hard limit for various grades of existing auto fuel even with fixed timing magnetos. In that case, the 8.7 in the 390 and its efficient combustion chamber, coupled with a tailorable ignition curve such as the SDS products feature means that auto fuel is a very likely possibility. Build the fuel system to be compatable with auto fuel additives and vapor pressure issues and you should be all set for any future demise of 100LL.
 
To the Op

Your concerns are not overblown in my opinion as an engine for your 14 will be a big investment. I have been eyeballing the RV-14. When I visited the Swift fuel tent at Oshkosh 2017, I asked them about their 100ll replacement for the IO390. They showed me a chart on compatible engines for their replacement fuel. The IO390 wasn?t on their laminated sheet. Bummer.

I built and own a Rans S-20 Raven that burns auto gas with up to 10% ethanol. I use only top tier super unleaded. Inexpensive flying. $15 an hour for fuel. I have several Alaskan Airframes bush bags that I use at Costco gas stations. The have recently told me that my bags don?t have DOT stamps on them so I cannot fill them at Costco anymore. I have other gas cans that are acceptable to them that I now use. The don?t allow cans to be filled that are over 12 gallons. Racing cans with big filler necks are not allowed as well. I spilled some fuel on the ground approximately the size of quarter. They raced over to my pump and sprayed some fluid on it like the fuel I spilled contained hydrazine?

The demographics are a changing and there is pressure on the epa. I won?t build another plane until I see what the FAA?s decision is concerning replacement fuel slated for late 2019 or later.

The 94UL fuel I have seen for sale at several different airports and it was expensive. Swift told me that they have no control over the price airports sell it for.

If you lower the compression on an IO390 it will you cost you some horsepower. But you could burn Mogas. Mogas isn?t readily available at airports at least out west. There is currently 2 airports in the state Oregon that sell it.

Lots of pieces to this puzzle but few answers.

Best,
Jim
 
Would love to hear your explanation on why 100LL can be "drawn" through a line without vapor lock, but Mogas can't.

Larry

Too low of a vapor pressure (related to the fluid property proper) will cause fuel to vaporize (vapor lock if in fuel line) and subsequent fuel starvation. This is realized when the local static pressure of the fluid is too low. This can happen when “drawing” fuel through a line e.g. engine fuel pump above low wing tanks. Mogas RVP isn’t held to as tight a tolerance as Avgas. In fact, MoGas tends to vary with seasonal blends/region where supplied. That is why an airframe must also be approved/STC’d for MoGas, not just the engine. High wing planes tend to be no issue as there is inherent head pressure. Low wing airframes tend to need full time duty boost pumps or a header tank to meet the requirements for the lower RVP. There’s plenty of smarter people than me in these forums that will tell me if i’m off. Maybe you’re telling me. Let me know. I’m always ready to learn more.
 
Last edited:
Would love to hear your explanation on why 100LL can be "drawn" through a line without vapor lock, but Mogas can't.

Larry

Hey Larry, Not sure if this relates to your statement on vapor lock or not, but the following quote is from "Lycoming on Unleaded fuel part 1" posted by RV6_flyer. Sorry if this is completely off. I have no experience at all with vapor lock.

"Lycoming approved "mogas" by controlling automotive gasoline properties differently than what is done for ground transport vehicle "pump gas." The specifics:

93 AKI for detonation margin (hot day OAT and 500F cylinder heads).
Vapor pressure Class A-4 to prevent vapor lock.
No ethanol and maximum 1% oxygenates.
ASTM D4814 Revision 09b and EN228 Revision 2008:E."
 
Pump Gas

If "Pump Gas" can be used in autos, our aircraft systems can be designed to use it too.

I've been running 93 octane E10 in an IO-360 with 8.7:1 compression for the last 1000+ hours. My fuel system utilizes automotive fuel system design parameters to facilitate problem free operation. The fuel is pressure driven to the engine. The additional Aux pump will automatically be turned on (with an independent fuel pressure sensor and circuit) if the fuel pressure goes below a specific set point. All fuel hoses are either teflon or alum. All fuel system "O" rings have been changed to buttal rubber.
Testing with winter blended fuels on hot days and high altitudes has never shown any signs of vapor lock issues.
Use of "Pump Gas" in aircraft can be done, but the fuel system designs require the same solutions that the automotive industry implemented decades ago.


I'm rolling up on 275 hours now on my IO360 running Walmart-grade 91 premium autogas with ethanol on 8.7:1 compression with no issues. I did build the entire fuel system on the airplane with ethanol exposure in mind and eliminated all but one natural rubber O-ring which I found a couple weeks ago, it finally started leaking at about 250 hours.

The engine is quite happy burning 91E10, in-flight performance is within a percent or two of the 100LL standard, I can't tell the difference without digging into the Dynon performance data on a long cross-country.

Handling the fuel here is going to be the trick for you and most others though. I am based on a private strip just over 30 miles from the nearest "real" airport with fuel - so I have my own tank on my strip. I took a used milk container, 125 gallons stainless, and steam cleaned it, set it on a 275-gallon chemical tote frame so I can pick it up with forks on a tractor, and put a pump and battery on the frame. I can fork it onto the back of my pickup and take it to town for a fill when needed, easy-peasy.
 
Do not stress.

The PAFI process was fatally flawed right from the beginning. Recently proven.

There is a certification project almost complete and has taken time due funding and apathy of the market to get behind it, but is literally "months" or "weeks" from a major milestone.

G100UL is the likely fuel you will have when the time comes. So relax. Build on.
 
Too low of a vapor pressure (related to the fluid property proper) will cause fuel to vaporize (vapor lock if in fuel line) and subsequent fuel starvation. This is realized when the local static pressure of the fluid is too low. This can happen when ?drawing? fuel through a line e.g. engine fuel pump above low wing tanks. Mogas RVP isn?t held to as tight a tolerance as Avgas. In fact, MoGas tends to vary with seasonal blends/region where supplied. That is why an airframe must also be approved/STC?d for MoGas, not just the engine. High wing planes tend to be no issue as there is inherent head pressure. Low wing airframes tend to need full time duty boost pumps or a header tank to meet the requirements for the lower RVP. There?s plenty of smarter people than me in these forums that will tell me if i?m off. Maybe you?re telling me. Let me know. I?m always ready to learn more.

If "Pump Gas" can be used in autos, our aircraft systems can be designed to use it too.
Can be, yes, but at what cost? Autos typically have the fuel pump inside the fuel tank, so there's no real "draw;" at worst, a couple of inches of rise, and a similar horizontal run. In an airplane, it can be several inches of rise (maybe as much as a foot, if you weren't careful with your design), and several feet of horizontal run, plus several bends.


Yes, you could use the automotive design of putting the pump in the tank, but now you're looking at multiple pumps, and you're also dealing with a tank that's shallower than the typical auto tank. Can it be done? Yes. Is it practical and cost-effective? Maybe, maybe not, and I lean toward "not."
 
Can be, yes, but at what cost? Autos typically have the fuel pump inside the fuel tank, so there's no real "draw;" at worst, a couple of inches of rise, and a similar horizontal run. In an airplane, it can be several inches of rise (maybe as much as a foot, if you weren't careful with your design), and several feet of horizontal run, plus several bends.


Yes, you could use the automotive design of putting the pump in the tank, but now you're looking at multiple pumps, and you're also dealing with a tank that's shallower than the typical auto tank. Can it be done? Yes. Is it practical and cost-effective? Maybe, maybe not, and I lean toward "not."

People have been flying low wing planes like RVs on mogas for years successfully. Follow a few simple rules and it works just fine. I've been doing it for 9 years now. No issues other than reducing the MAP limit down with my high compression turbo engine.

?A person who says it cannot be done should not interrupt the man doing it.?
 
Do not stress.

The PAFI process was fatally flawed right from the beginning. Recently proven.

There is a certification project almost complete and has taken time due funding and apathy of the market to get behind it, but is literally "months" or "weeks" from a major milestone.

G100UL is the likely fuel you will have when the time comes. So relax. Build on.

Thanks David. Somehow I have missed this G100UL fuel development project during my frantic internet research of the demise of 100LL. This sounds very interesting. I have taken a step back from the ledge:). Lets hope they can keep the FAA satisfied and get the fuel refineries and suppliers to keep the cost reasonable.
This video is several years old, but has some good info for those not familiar with this G100UL fuel. For some reason I trust this guy and believe what he is saying.
https://www.youtube.com/watch?time_continue=138&v=vEcHW2B0vXE
 
I had absolutely no problems with vapor lock when I ran ethanol free 90 octane "recreational fuel" in my IO-540 RV-10, even on the hottest days here in Michigan. Rec Fuel is nothing more than ethanol free mogas some gas stations carry for our boats and other recreation vehicles. My engine was a Lycoming IO-540V4A5 with 8.5 pistons. I took some basic precautions during the build knowing I was going to run mogas by insulating the fuel lines FWF and shielding them from heat and a heat shroud with blast tube on the engine fuel pump. When I ran my plans to burn mogas by the engine guy from Lycoming during an Airventure Lycoming rebuild seminar that Lycoming put on he said "absolutely".
 
Interesting comments

Forgive the small rant. The Spirit of Experimental/Homebuilt Aircraft is unique. It should challenge convention and often does. What is disheartening and dangerous are open ended statements asserting something is OK or correct without supporting facts. Because something was done once or even several times without consequence, doesn't make it technically sound. There are numerous statements in this thread that offer technical reasoning in support of one side of an argument while the other is essentially "been fine, so far." The Normalization of Deviation was central in the death of seven Astronauts and countless aviators. I've been closer to these situations than I ever could have imagined.

I probably need to take a break from reading these forums for a while. I can hear the "good riddance" going through many minds. If you want to call me the big hole, probably need to email or PM me. Build safe. Fly safe. Be careful what you post here. I've learned a lot from you guys.

Scott F
 
Can be, yes, but at what cost? Autos typically have the fuel pump inside the fuel tank, so there's no real "draw;" at worst, a couple of inches of rise, and a similar horizontal run. In an airplane, it can be several inches of rise (maybe as much as a foot, if you weren't careful with your design), and several feet of horizontal run, plus several bends.


Yes, you could use the automotive design of putting the pump in the tank, but now you're looking at multiple pumps, and you're also dealing with a tank that's shallower than the typical auto tank. Can it be done? Yes. Is it practical and cost-effective? Maybe, maybe not, and I lean toward "not."

The trick here, as you noted, is to reduce the pressure drop in the draw to the suction of the pump as much as possible. I eliminated the engine-driven pump and installed two electric pumps on the floor in the cockpit in the standard boost pump location, so the pumps are already as low as the bottom of the fuel tank. I used stainless-braided Teflon core hoses from Tom Swearingen for my fuel lines in from the tank to my Andair duplex fuel valve to eliminate tight bends and 90's to reduce the pressure drop, and I put 85-micron filtration in each line just prior to the pump, with one single 35-micron filter downstream of the pump. Once the fuel gets TO the pump, then it's pressurized and everything becomes automotive-standard from there.

By virtue of removing the engine-driven fuel pump, I'm automatically at a single-point-of-failure on the pump system if I don't have multiple pumps - I just installed an SDS dual pump module that has a pair of them in parallel, so a backup pump is plumbed in just waiting for the switch to flip. Total cost for this? I dunno, didn't track it very closely honestly, but certainly less than $1500 including the lines and insulation. I'm saving easily $1 per gallon with a conservative average fuel flow of 8 gph and I've been doing it for about 275 hours now - so it's already well paid for itself.
 
Last edited:
I probably need to take a break from reading these forums for a while. I can hear the "good riddance" going through many minds. If you want to call me the big hole, probably need to email or PM me. Build safe. Fly safe. Be careful what you post here. I've learned a lot from you guys.

Scott F

Don't leave on our account - there is always value in discussion and even occasional dissention, as long as it's kept in the realm of civil discourse. Your first post on this thread came across as "You can't do that" and that automatically will start a (sometimes heated) discussion amongst the experimental community - especially when people are already quite obviously doing it and have been for some time.

As you know, there are blanket rules designed to keep the tourists from falling into the volcano and there's nothing wrong with that - but occasionally you get people with enough knowledge and experience that they can duck under the rope and walk right up to the edge of volcano safely. It happens everyday - just not in large quantities and not always in public view - but it does happen.
 
There is a difference between blindly dumping a load of auto gas into an airplane and "seeing what happens" and designing the system to use it. As others have done, my system has zero 90 degree fittings (to include changing the standard Van's in tank pickup), the fuel pump is mounted at BL 0 against the spar (as low as possible without punching out the bottom of the OML), and is chemically resilient for modern Mogas. While this does not guarantee success, it's stacking the deck as much in my favor as possible. A detailed flight test series will show if it's enough.

This forum has long fought the battle between presenting technical information to consenting adults vs. "protecting the children"... It's the latter that prompted the warning statement on my signature line.
 
Forgive the small rant. The Spirit of Experimental/Homebuilt Aircraft is unique. It should challenge convention and often does. What is disheartening and dangerous are open ended statements asserting something is OK or correct without supporting facts. Because something was done once or even several times without consequence, doesn't make it technically sound. There are numerous statements in this thread that offer technical reasoning in support of one side of an argument while the other is essentially "been fine, so far." The Normalization of Deviation was central in the death of seven Astronauts and countless aviators. I've been closer to these situations than I ever could have imagined.

I probably need to take a break from reading these forums for a while. I can hear the "good riddance" going through many minds. If you want to call me the big hole, probably need to email or PM me. Build safe. Fly safe. Be careful what you post here. I've learned a lot from you guys.

Scott F

While theory is important, in the end, real world experience validates or invalidates theory. The fact that there are many hundreds or even thousands of low wing aircraft using mogas with hundreds of thousands of successful flight hours on them seems to prove that it can be done safely.

Any aircraft which did have problems, probably broke some of the important rules in their fuel system designs which have been discussed at length on various threads here on VAF.
 
Thanks

Ok guys. I did include the following statement in my initial post of this thread.
"I realize this can be a controversial subject, and I may be overreacting."
That does seem to be the case, and that's...Ok I think. I have learned a a lot from all the responses.
The debate over the use of pump gas seems to a point of contention. What I learned from the replies is that it does seem completely feasible to run SOME engines on "pump gas". However, in order for this to take place safely, several steps need to be taken. Everything from building the airplane fuel system to accept this fuel, to transporting, and eventually the proper operation of the engine needs to be carefully planned and executed. I think the problem some of the posters have with stating that "it's ok to do..I've done it for years", is that you get guys like me on here that are not that familiar with piston engines, and I run down to the corner gas station and fill up my 5 gal lawn-mower gas can and head to the airport. That's just an inherent problem of the internet and msg boards specifically. It sounds like the posters that are using some fuel other than 100LL have in fact taken all the necessary precautions and are operating safely. I think that's great and one of the huge benefits of experimental aviation. However, the majority of operators may not be able to safely operate within all the "benefits" of experimental aviation. Regarding this 100LL phaseout issue, most pilots just need a fuel that will be easily accessible at their local airport, and at a fair price. Hopefully RV10inOz speaks the truth and the issue will "soon" be a thing of the past. Fingers crossed!
Thanks again for all the information. It's time to head to the shop and work on flaps:)
 
Why do we run into knock problems using MOGAS when we have CRs above 8.5?
Modern cars have CR well above 10 and racing bikes even above 12.

So what's the problem with our aircraft engines? Too low air density/pressure? Would it be easier with turbo charged engines?

Malte
 
Why do we run into knock problems using MOGAS when we have CRs above 8.5?
Modern cars have CR well above 10 and racing bikes even above 12.

So what's the problem with our aircraft engines? Too low air density/pressure? Would it be easier with turbo charged engines?

Malte

Purely speculation but maybe it has something to do with the larger displacements? I've wondered the same thing though, if someone with more knowledge could explain I'd really appreciate it.
 
Water cooled heads remove heat more efficiently and less prone to hot spots, cylinder head combustion chamber design, smaller piston diameter, ECT.
 
Purely speculation but maybe it has something to do with the larger displacements? I've wondered the same thing though, if someone with more knowledge could explain I'd really appreciate it.

I'll try.

The larger the cylinder volume (all other things equal), the longer it takes the flame front to burn all the way to the corners of the combustion chamber. If the combustion event takes too long the heat and pressure inside the cylinder reach the point where the unburnt mixture at the far (from the plugs) corners of the chamber will spontaneously combust (AKA detonate). Add in a low RPM engine like ours, and we play close to the detonation margins.

On an engine with smaller combustion chambers, the flame front burns all the way to the extents of the chamber faster, reducing the risk of detonation, since the fuel burns before the conditions inside the combustion chamber cause detonation.

This is why we see 10+:1 compression ratios on today's smaller, higher revving engines, but not on yesterday's engines with big combustion chambers and which turn relatively slowly.

Notice when you get pinging in your car - generally under load and at relatively low RPM, like starting off going up a hill. Kinda/sorta like our engines see all of the time.
 
Last edited:
This is from the Shell link I listed on post 1. (Avgas Fact and Future).
Is this relative to mjanduda's question? Last paragraph mainly.

"Octane rating is a measure of how resistant a fuel is to detonation or "pinking"; the higher the Octane rating, the more the fuel / air mixture can be compressed without detonation happening. To make this clear, octane rating is not a measure of the amount energy in the fuel, but is a measure of its resistance to detonation.

The advantage or higher octane fuels is that a higher compression ratio or supercharging ratio can be used, which then leads to a higher engine cycle efficiency, which in turn means more power output for a given fuel burn. However, to confuse things further, there are four principal ways to measure Octane rating, RON, MON, Lean Mixture and Rich Mixture ratings.

Road fuels tend to be measured on a RON scale, for which unleaded fuels tend to be 95 - 98 RON but are only 85 - 87 MON. Avgas is measured on Lean Mixture (similar to MON) but also has a Rich Mixture Octane rating.

The Lean Mixture rating is 100 octane (15 octane higher than the comparable 85 MON for unleaded Mogas) but Avgas also has a Rich Mixture rating of 130 which allows higher supercharger boost pressures to be used without detonation occurring. This is particularly a problem when using high power settings at low altitude, for example during take off.

As you can see TEL in Avgas makes a significant difference to the octane rating and without it Octane ratings would be back down to 80 - 85 Lean Mixture - the level for road fuels - instead of 100 / 130. This is not a problem for most typical modern normally aspirated engines as their compression ratios are quite modest and detonation would not be a problem with 80 - 85 Lean Mixture Octane fuel."
 
Modern chambers are designed with a lot of computer simulation of the mixing and combustion events and then validated in real life on a instrumented dyno.

Lycomings had none of this tech or science available when they were designed.

Cars are running up to 14 to 1 CRs on 87 octane these days. Even turbo engines are running 10.5 CRs on 87 octane.

Computer controlled fuel and spark has lots to do with it too. At least we can add those parts to help out the old Lyc these days. We can retard the spark at high MAP to avoid detonation and re-advance when MAP is lower to gain back the power and efficiency. Can't do that with mags.
 
Computer controlled fuel and spark has lots to do with it too. At least we can add those parts to help out the old Lyc these days. We can retard the spark at high MAP to avoid detonation and re-advance when MAP is lower to gain back the power and efficiency. Can't do that with mags.

Back to the first question in this thread:
You think by using modern ignition systems you can use MOGAS on an IO-390 without engine modifications and without losing lots of power or lifetime?
 
Back to the first question in this thread:
You think by using modern ignition systems you can use MOGAS on an IO-390 without engine modifications and without losing lots of power or lifetime?

Yes. 390s are base timed at 20 degrees already which is conservative. The 8.7 CR is not too high. Once the MAP drops below about 27 inches, you should be able to run the full 20 degrees on 91 octane. You might give up 5-7hp running 18 degrees at SL. No negative impact on engine life.
 
Yes. 390s are base timed at 20 degrees already which is conservative. The 8.7 CR is not too high. Once the MAP drops below about 27 inches, you should be able to run the full 20 degrees on 91 octane. You might give up 5-7hp running 18 degrees at SL. No negative impact on engine life.

How about 87E10 in a IO-540 with 7.2:1? :O
 
Last edited:
How about 87E10 in a IO-540 with 7.2:1? :O

Same thing applies, reduce total timing down to 20ish at sea level MAPs, by 25 inches, I'd think you could be back up to 24-25 degrees total. Would cost you a few ponies at SL vs. 91 octane. You'd have to weigh the cost vs. power de-rating of the two fuels.
 
Have a look at this

How about 87E10 in a IO-540 with 7.2:1? :O

K46IJV0Lg24


Here is an STC'd $12,000 water injection system for IO 520 Continentals
If you watch the first 2 minutes you will see the MP and CHT values the system starts flowing at. Mostly at take off power settings with MOGAS

A slightly different tool to address pre ignition / detonation that lets you maintain high MP and normal advance for rated HP output.
 
Last edited:
This is from the Shell link I listed on post 1. (Avgas Fact and Future).
Is this relative to mjanduda's question? Last paragraph mainly.

"Octane rating is a measure of how resistant a fuel is to detonation or "pinking"; the higher the Octane rating, the more the fuel / air mixture can be compressed without detonation happening. To make this clear, octane rating is not a measure of the amount energy in the fuel, but is a measure of its resistance to detonation.

The advantage or higher octane fuels is that a higher compression ratio or supercharging ratio can be used, which then leads to a higher engine cycle efficiency, which in turn means more power output for a given fuel burn. However, to confuse things further, there are four principal ways to measure Octane rating, RON, MON, Lean Mixture and Rich Mixture ratings.

Road fuels tend to be measured on a RON scale, for which unleaded fuels tend to be 95 - 98 RON but are only 85 - 87 MON. Avgas is measured on Lean Mixture (similar to MON) but also has a Rich Mixture Octane rating.

The Lean Mixture rating is 100 octane (15 octane higher than the comparable 85 MON for unleaded Mogas) but Avgas also has a Rich Mixture rating of 130 which allows higher supercharger boost pressures to be used without detonation occurring. This is particularly a problem when using high power settings at low altitude, for example during take off.

As you can see TEL in Avgas makes a significant difference to the octane rating and without it Octane ratings would be back down to 80 - 85 Lean Mixture - the level for road fuels - instead of 100 / 130. This is not a problem for most typical modern normally aspirated engines as their compression ratios are quite modest and detonation would not be a problem with 80 - 85 Lean Mixture Octane fuel."

1 - "pinking" should be "pinging"
2 - road fuels in the US are rated in AKI not RON. AKI is (RON+MON)/2. You'll see that formula shown on pumps
 
1 - "pinking" should be "pinging"
2 - road fuels in the US are rated in AKI not RON. AKI is (RON+MON)/2. You'll see that formula shown on pumps

Likely from Shell UK where the term used is "pinking" and also where they rate their road fuels differently than North America.

In any case, under 9 to 1 Lycomings can safely use 91 AKI with proper timing adjustments and care with AFRs. Under 7.5 CRs ones can run on 87 AKI with the same care.

The Swift UL avgas is higher octane and certainly suitable as well.

The "universal" UL avgas, when it's introduced, is likely to be similar in octane rating to the current 100LL so as not to have to derate the current turbocharged engines too far, if at all.
 
Likely from Shell UK where the term used is "pinking" and also where they rate their road fuels differently than North America.

In any case, under 9 to 1 Lycomings can safely use 91 AKI with proper timing adjustments and care with AFRs. Under 7.5 CRs ones can run on 87 AKI with the same care.

The Swift UL avgas is higher octane and certainly suitable as well.

The "universal" UL avgas, when it's introduced, is likely to be similar in octane rating to the current 100LL so as not to have to derate the current turbocharged engines too far, if at all.

"pinking" is a new one to me! I thought it was a typo. Shell UK would also explain the use of RON. The link was to shell.com though that could be their international site.

Has anyone tried to use in-tank pumps with EFI? You'd nearly eliminate the vapor lock issues (pretty hard to vapor lock at 2-3 bar), extend pump life, and have automatic redundancy.
 
K46IJV0Lg24


Here is an STC'd $12,000 water injection system for IO 520 Continentals
If you watch the first 2 minutes you will see the MP and CHT values the system starts flowing at. Mostly at take off power settings with MOGAS

A slightly different tool to address pre ignition / detonation that lets you maintain high MP and normal advance for rated HP output.

Huh, that's interesting thanks for the link!

Same thing applies, reduce total timing down to 20ish at sea level MAPs, by 25 inches, I'd think you could be back up to 24-25 degrees total. Would cost you a few ponies at SL vs. 91 octane. You'd have to weigh the cost vs. power de-rating of the two fuels.

Awesome. With most options away from home being 100LL would one need two configurations or could you run those settings for higher octanes as well?
 
"pinking" is a new one to me! I thought it was a typo. Shell UK would also explain the use of RON. The link was to shell.com though that could be their international site.

Has anyone tried to use in-tank pumps with EFI? You'd nearly eliminate the vapor lock issues (pretty hard to vapor lock at 2-3 bar), extend pump life, and have automatic redundancy.

If an in tank pump fails, you can't access the fuel in that tank. The current pump life is excellent and vapor lock hasn't been an issue if proper location and plumbing practices are followed.

We strongly recommend that people use what's been proven over a half million flight hours with regards to the EFI fuel system if you don't want surprises or to be on a test program to validate a new design.
 
Back
Top