I am looking for advice on obtaining an Auto Fuel STC for my 1960 Skylark /w O-470K on Aqua 2400's
I have asked a few people and the opinions are adamantly opposed.
Will Auto Gas (92 octane W/O ethanol)destroy a O-470K engine?
Are there certain techniques will allow engine longevity with Auto Fuel?
Is Marvel Mystery Oil a helpful additive?
OR should I just stay with 100LL
I am sure there are AMEs viewing and I would like your opinions.
I've got 200+ hours on an O-470K, running mo & 100 LL as a mixture or either of them straight.
The previous owner put about 600 on it too, mostly mogas.
No real problems. The engine got a top O/H at half time. That's pretty normal for an O-470.
I do notice on Mo, the shutdown isn't as clean, and often a couple of drips appear under the carb afterwards. Mo is also stinky and leaves brownish stains around the sump drains.
It's a great option to have in remote communities though and the $$ savings helps out a bit too..
First off I am not an AME but I can't really see a problem, I would recommend a 50/50 mixture of 92 MO and 100LL, just to help with shut downs. Ran a 150 hp lycoming with 87 MO gas for a little while and the damn thing would not want to shut down when the mixture was pulled. Maybe it was just the engine and the 470 Continental won't mind the MO gas but this small Lycoming seemed to like at least a 50/50 mixture for shutdowns. Maybe try experimenting first with various mixtures to see what your engine prefers and stick to that.
As for Mystery marvel oil many AME's seem to think this a is a waste of time and money but I like to add a little bit to the gas to help lubricate the valves, and top end etc. Works for me but like I said you might have AME's laugh at you for using such a product.
I am a firm believer in mystery oil. Overhaul shop(p&w1340) said to put mystery in the gas as the freshly overhauled engine had a couple sticking valves. Twenty minutes later problem was solved.
Lots of times the engine not shutting down with MOGAS is because it is leaking through the primer... ...the primer seals are shot either because of owners who use MOGAS also use SAE O-rings from NAPA instead of MS29513s.... ......or the MOGAS they used has ethanol in it and has attacked the seals.
Never had a problem with primers on 100LL machines--the 337 has TWO manual primers that never get used yet both still work fine--but with MOGAS it's almost constant.
Pretty tough to tell on an O-470 since it has a six place primer, but I've seen lots of O-300s with all of the left plugs get noticeable darker than the right plugs.
IIRC MMO is light oil with varsol. A solvent
which can be of use if you have problems
with carbon deposits.
I might point out that MMO has no paper -
it's supposed to be used for homebuilts only.
Avblend (ether) has FAA paper and is another
solvent which can legally be used in the oil of
certified aircraft. It's very effective at softening
carbon.
Avblend is not something I am familiar with and neither is MMO.
I have heard about people mixing MMO with fuel when burning super unleaded auto-gas yet it is not in the product literature.
Is this a common or acceptable use for Avblend?
I am getting smarter with each post, thanks for sharing
Lots of almost religious supporters of mmo almost to the point of elevating its powers to divine ones....
I ll just post a summary from another thread that summarized it well:
"Theory:
ADDED TO GASOLINE:
• Cleans and lubricates fuel injectors and carburetors.
• Improves gasoline mileage.
• Reduces and prevents varnish and gum build-up.
• Extends spark plug life.
ADDED TO ENGINE OIL:
• Prevents valve sticking and clatter.
• Fortifies properties of engine oil, prevents breakdown
caused by extreme temperatures.
• Promotes easier cold weather starts.
• Reduces and prevents acid and carbon formation.
Composition:
70% Light Aromatic Oil (Pale Oil)
- It is a Naphthenic Oil, so while it oxidizes faster than a Paraffinic oil, it does clean and dissolve sludge and carbon well and cleans up after itself from any oxidation. serves as base oil as well. [Naphthenic oils have more solvency and are more polar (they are attracted to metal more), but oxidize faster.
29% Mineral Spirits
- Cleans Varnish very well. General cleaner. Also acts as an antioxidant.
38 parts per million (ppm) Boron
- AW/EP agent, friction reducer, antioxidant
900 ppm Phosporous
- AW/EP agent
1/2% 1, 2 ortho-Dichlorobenzene
- EP agent as it interacts with Iron to form an Iron chloride barrier under any ZDDP or other AW additives. Also very good cleaner/solvent, and friction reducer
1/4% 1, 4 para-Dichlorobenzene
- EP agent as it interacts with Iron to form an Iron chloride barrier under any ZDDP or other AW additives. Also very good cleaner/solvent, and friction reducer
Oil of wintergreen - for the scent
- is also a cleaner/solvent
Red Dye - for the color
- well this one just colors the stuff"
Bottom line: mmo is a cleanser/solvent and that's what it was designed for in the 30's when the oils used tended to gum up systems. I ll add some to my oil close to an oil change but don't use it in the crankcase all the time. There is some literature out there supporting use of things like camguard which reduces corrosion so I will add that with my fresh oil change and some with replacement oil. Avblend is mostly mineral oil.
Great way to get a bunch of pilots into an all out bar fight, aside from tricycle versus taildragger or single versus multiengine benefits, is to get into an oil or fuel discussion!
iflyforpie wrote:Lots of times the engine not shutting down with MOGAS is because it is leaking through the primer... ...the primer seals are shot either because of owners who use MOGAS also use SAE O-rings from NAPA instead of MS29513s.... ......or the MOGAS they used has ethanol in it and has attacked the seals.
Never had a problem with primers on 100LL machines--the 337 has TWO manual primers that never get used yet both still work fine--but with MOGAS it's almost constant.
Pretty tough to tell on an O-470 since it has a six place primer, but I've seen lots of O-300s with all of the left plugs get noticeable darker than the right plugs.
Mine is only priming 4 jugs, I think.
In my case, I suspect it's to do with the poorer atomization of the fuel through the induction system, leaving drips of gas there that the engine can suck off of for a while after the mixture is cut.
The issue dissipates proportionately with less Mo & more 100 LL. With 100 LL only, the shutdowns are clean w/ no drips under the carb after.
Given the choice, I'd only run Av. The cost savings and availability / convenience make running Mo pretty much a necessity for me though.
In WW II, DC-3 were widely run on mogaz, even C-46 and all Pratt R-2800 used it IN CRUISE.
All you need is pressurize the fuel to avoid an airlock(turn the fuel pump off and the engine quit above 8,000 feet.)
Actually, for ultralight two strokes, avgas is very bad due to carbon deposits...on the other hand auto gas will gum up
if left in fuel line in storage.
Mike Busch had a GREAT webinar on lubes and such. Basically de-bunked MMO as being pretty much useless. They even had it analyzed, don't remember what the ingredients were, think one of them was pig fat or something. They tore engines down after using it and found ZERO difference using it or not but no indication that it hurt anything. Same can't be said for Cam-guard. Stuff is expensive but was HIGHLY recommended by Mike with solid data that it saves a lot of wear and tear!
These are long webinars but highly recommended watching! This is the oil one but I'm not sure if you have to be an EAA member to watch it...
Older article with lots of info on the pro's and con's of auto fuel. For those operating in remote areas, availability may be the deciding factor. It is interesting to note, that when 80/87 was phased out, not a single hour of flight testing was done by the oil companies or TC on the effects of running 100LL in low compression engines. They were approved on the basis that the engine manufacturers stated that if 87 Avgas was unavailable, fuel of a higher grade could be used. The manufactures never contemplated that this could be for several thousand hours. Hence the Lycoming "valve wobble" AD, the stuck valve indusced engine failures in the P&W radials, etc. etc.
Operating the P&W Wasp and Wasp Jnr. on Auto Fuel - One operators experience with the good, the bad, and the ugly.
Many years ago I worked for an aerial spraying company that operated its fleet of Ag Cats (P&W Radials) and Pawnees (Lycomings ) almost exclusively on auto fuel, in the process amassing tens of thousands of hours of auto fuel operational experience. We never had an engine not reach TBO nor have cylinder problems due to the use of auto fuel. In particular, we have not seen any evidence of valve seat recession. This may be due to the theory that valve seats work harden after some period of time and that we did break our engines in on Avgas. I currently fly a 450 Stearman, DeHavilland Beaver, and a Harvard on various mixtures of auto fuel and Avgas, and thought that I would pass along my observations and experiences. While directed primarily at the operation of the Wasp and Wasp Jnr. radials, I think that the lessons learned are applicable to many older low compression engine types.
History:
Octane is the measure of a fuels resistance to detonation and is actually the ratio of octane to heptane in detonation tests. 87 octane fuel has the same detonation point as a mixture of 87% octane and 13% heptane. When fuel was first refined, it was 73 octane – that is the best you can get running crude oil through a distillation column. Everything after 73 octane is due to additives and modern chemistry. When the P&W engines were designed in 1925, 73 octane was the only fuel available and that is what they were designed to run on. As both aluminum metallurgy and fuel octane improved, the rated power of the engines increased. The increased octane rating in Avgas from 73 to 80 to 87 to 91 allowed increased detonation margins and consequent increase in horsepower. I still have power charts for the operation of the engines on 73 and 80 octane fuel. The increase in octane rating was due primarily due to an increase in tetra ethyl lead. Back in the day before 100LL, the common aviation fuels were 80/87 and 100/130. The first number is the lean octane rating, the second the rich. When sales of 100/130 dwindled, the oil companies decided to phase out 80/87 and only produce one fuel, a compromise called 100LL – low lead compared to the old 100/130, but with over 400% the lead content of 80/87. If you remove the lead from 100LL it becomes 91 octane and this is sold as 91UL in Europe.
Issues with 100LL in P&W Radials:
Apart from the cost, the prolonged use of 100LL fuel in the P&W radials seems to cause two main problems – lead fouling plugs and sticking valves. The plugs can be cleaned and the “BY” or fine wire plugs seem to help some. The valve sticking problem is much more serious. When 100LL was introduced it was certified on the basis that the engine manufacturers allowed the use of a higher octane fuel if 80/87 was unavailable. It was never contemplated by P&W that their smaller radials would be run for thousands of hours on a higher octane fuel. Most frighteningly, not one single hour of engine testing was done on the effects of the operation of 100ll fuel on low compression engines. We all became the guinea pigs. It turns out that the rich running low compression engines of old do not run hot enough to scavenge the lead, and that the lead precipitates on the valves as it cools while exiting the engine. Then it forms hard, often spherical deposits on the valves that in time builds up to cause intermittent valve sticking. To give you an idea how bad this issue is, I once found a cracked cylinder head on a walkaround by noticing the lead deposits between the cylinder cooling fins, caused when the combustion gasses cooled after escaping through the crack! I have experienced several very rough running engines caused by valve sticking and know of several complete engine failures. Hence my “guinea pig” comment.
So for those of you running 100LL, what can you do? It starts with the engine overhaul. Most engine overhaul companies ream the valve guides to near maximum allowable clearances to allow for some lead buildup. Older overhauls often have tight guides and can be very problematic. The FAA approved oil supplement “AvBlend” has proven very effective at preventing lead buildup when added to the oil and we use it religiously. The downside is that it adds $50 to the cost of an oil change. Marvel Mystery Oil in the fuel is not approved but nearly everybody swears by it. One year at the Galesburg Stearman Fly In, I noted that over half the arriving airplanes had MMO in their baggage compartments! We use it, if only to provide lubrication to the fuel valves and pumps but I like to think that the decades of tribal knowledge is on our side as well. And the most effective way to solve the problem? Run unleaded auto fuel and save $$$$ in the process. If only it were that easy.
Issues with auto fuel:
Cleanliness. The delivery system for aviation fuels has crazy high quality control standards. When you buy auto fuel you don’t know if the tank truck that delivered it had some left over furnace oil in it from its last delivery. Or if the filling stations tanks are rusty, or if it was sourced cheap from some rejected load from God knows where. The solution is to buy branded fuel from suppliers that sell a lot of fuel, and then to have a high quality filtering system that the fuel has to pass through before it gets in the airplane. A “tidy tank” in the back of a pick-up works well with a fine particulate/water filter installed. A jerry can is not so good.
Storage Life: Avgas is designed to have a fairly long storage life. Auto fuel storage life has always been shorter lived but the newer “oxygenated” fuels make this issue worse. The safe storage life of auto fuel now is one to two months, no more. This is why you don’t want to buy unbranded fuel. When a big name oil company has fuel that is getting old, they usually sell it to somebody else! Also, you do not want to leave auto fuel in your airplane if you don’t regularly fly it. We frequently saw sticky needle valves in the agricultural aircraft after they had been sitting for the winter. Often, this was enough to cause the engines fuel consumption to increase significantly for the first 10 hours or so of operation. A few fillups of 100LL usually fixed this but a word to the wise – fly a tankful of 100LL and then top of the tank if you are going to store the airplane over winter.
Carb Ice: Auto fuel is somewhat more volatile and consequently cools more as it vaporizes. This is very beneficial when trying to start the Beaver in sub-zero temperatures as it is much easier to start. The downside is that when flying in moist conditions, carb icing is more likely and greater vigilance is required.
Vapor Locking: Due to the increased volatility, especially in fuels delivered in winter and used in warmer weather, vapor locking is a distinct possibility. It is unlikely below 6000’ density altitude in a gravity fed fuel system but can be problematic in pressurized fuel systems. This is why auto fuel is approved for use in a C-172 but not in a Cherokee with the same engine. We don’t use auto fuel at high altitudes, and only in a type that has been flight tested (STC’d) to demonstrate the suitability of the fuel system.
Octane: Auto fuel octane is measured slightly differently than aviation fuel octane and under certain circumstances, regular auto fuel may be slightly less resistant to detonation than the 80/87 octane aviation fuel. This could be a problem when operating at high power settings at high ambient temperatures. From my old charts at greater than about 34” MAP and 30 deg. C OAT. The solution is to use either a higher grade of auto fuel or a blend of auto fuel and 100LL. Be aware that blended fuels will separate out over time. This phenomenon is well known in the Super Cub community, and I have witnessed it occasionally in the Beaver after topping off a tank that is half full of 100LL with auto fuel. When sumping the tanks a few days later, straight auto fuel is often present in the low point fuel drain. On cool days or with un-supercharged engines, this is not an issue. One way we deal with this in aircraft with multiple fuel tanks, is to keep one tank full of 100LL for take off and landing, and the rest full of auto fuel for lower power cruising.
Legalities: A few very old aircraft (Tiger Moth for one) were certified to operate on “a good grade of motor fuel” but if you operate a type certified airplane, you will most likely require an STC to use auto fuel legally. Pederson Aviation in Nebraska supplies them along with much useful info and the peace of mind that they flight tested the airplane to check for vapor lock and carb ice issues. The price of the STC is such that you recover the cost in about 20 hours of flying.
Alcohol: This is the show stopper. Thanks to an unholy alliance between some mis-guided environmentalists and the farm lobby, many areas now require ethanol or other alcohols to be added to gasoline. Never mind that this contributes to third world hunger, or that it is not cost effective without big subsidies to both the farmers and refiners, this feel good act stupid idea came to be and we now have alcohol contaminated fuel in many areas. If you can’t find a supplier of alcohol free mogas, you can’t run mogas!
Alcohol very quickly eats most of the rubber compounds in aircraft fuel systems and over time can cause corrosion in aluminum fuel tanks. Just how fast? One day I took a load of fuel in the Beaver at a marina in the middle of nowhere in order to get home. I knew that it contained alcohol but reasoned that by the next day it would be gone and it couldn’t do any harm in a few hours. Wrong. The next day the primer seal was a gooey mess that required immediate replacement. Luckily, the engine fuel pump and fuel lines seemed to fare better but lesson learned. There are many ways to check for alcohol contamination and it is important that you check every load of new fuel.
Final Thoughts: So what do I do? I run auto fuel in our radials and small horizontally opposed engines but as you can see, using mogas is not a “fill up and forget” type of fuel. I always ensure that a tank of 100LL is available if flight at either high altitude or temperatures is contemplated. 91UL has been available in Europe for some while and the antique airplane community has been very pleased with it. I have high hopes for 91UL if and when we ever see it in North America. Until then, I run primarily mogas in our aircraft, dealing with all the consequent issues and hassles that come with it. But it still beats the fear that accompanies a backfiring, rough running engine over inhospitable terrain, due to sticking valves! And the money I save allows me to fly even more!
Kevin Maher
“You can always tell the moment a man loses his soul to flying.” Ernest Gann.
......or the MOGAS they used has ethanol in it and has attacked the seals. 
