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Old 29-07-2016, 08:18   #1
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aviation gas for a 20hp yamaha

I love my new dinghy, a WM 350 with a yamaha 20hp 4 stroke. With my son and I we can do 27mph and with 4 adults we're still able to get up on plane.

But frankly I noticed quickly that Marylands ethanol fuel is a big pain and was causing it to stall. Carburetors are much more sensitive to ethanol than fuel injected engines. I switched to special, expensive as sh*t, ethanol free racing fuel and it runs now very smoothly (expect for for it always needing about 5 pulls to start it cold, but after that only one).

I'm thinking of getting something that doesn't cost $70 for 5 gallons and was looking for some solid advice on Aviation fuel. From what I understand, it's 100LL, low lead. This is my understanding of avgas:

1) even though my outboard isn't tuned to use 100 octane fuel, it won't hurt the engine. The rumors that it doesn't burn completely are untrue.

2) The low lead is very low, and also won't hurt the engine because it doesn't have a catalytic converter.

3) Avgas has special additives that give it shelf stability of 2 to 3 years.

Last, ethanol for a fact WILL cause the carburator to gum over and gradually make it start to stall out. And to me an unreliable engine is something that's useless.

This is what I think is the case regarding Avgas from doing research on lots of different boards and talking to people, but I'd like to hear from an expert on whether this is true or false. If someone has been running avgas in their outboard for years, I'd be interested in hearing their long term experience as well.

And yes, for those who are unfamiliar, there is NO non ethanol gas in Maryland except for aviation fuel and racing fuel. None, not in marinas, not anywhere. I know the websites to check for pure gas and the closest is about 150 miles away.

Thank you!
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Old 29-07-2016, 08:36   #2
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Re: aviation gas for a 20hp yamaha

I've run Avgas in things for years, I have a little motorized bicycle with a four stroke Honda weed eater motor, carburetor the size of your little finger and run only Avgas, but it's was carried in the airplane, so I just drained a cup or two whenever I wanted it. An engine jetted for car gas will burn a little lean on Avgas, not enough to matter usually but if your engine stumbles on acceleration, it due to an improperly jetted carb, not the fuel. Most engines run fine though.

Avgas does have lead, is technically illegal to manufacture, just the Feds have ignored it forever as it's only a tiny portion of gasoline anyway but the "Friends of the Earth" and I'm sure others have lawsuits trying to stop it's manufacture.
It is illegal to use it for anything other than aircraft, usually nobody cares though, if you can get it five gallons at a time go for it.
Shelf life is almost forever, as in lord knows how many years, I've seen it run fine in aircraft that hadn't moved in years.
I'm fixing to essentially abandon my airplane for a few years in the hangar and I just will top off the tanks and I'm sure in four or five years it will be OK.

Lead does build up in the engine oil, anybody that has changed the oil in an aircraft can tell you about the fine grey sludge, that is the lead. It does build up on spark plugs too, but isn't hard to clean off, so I would change oil a little more often and expect to have to clean the plugs, where with UL fuel you don't have to.
Really one of the best things to happen to engines was unleaded fuel, that is why you can go 100,000 miles on spark plugs now and 5,000 miles between oil changes.
Also the lead causes problems somehow with pure synthetic oil, for that reason there is NO pure synthetic aircraft oil, so run a good grade of regular oil, not synthetic.
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Old 29-07-2016, 08:37   #3
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Re: aviation gas for a 20hp yamaha

The term "low lead" is a misnomer. It should be called "less lead". As in less lead than the fuel that it replaced. There is a LOT of lead in avgas. I would be concerned about lead deposits on piston rings, valve stems, etc.

The shelf life is much greater than 2 or 3 years. I recently ran some 8 year old avgas. No problems.

Steve
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Old 29-07-2016, 08:38   #4
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Re: aviation gas for a 20hp yamaha

Are you not able to find regular 87 or 89 octane ethanol free gas in your area? Uausually it is only 15-20 cents more per gallon.

Pure-Gas.org : Map of Ethanol-Free Gas Stations in the U.S. and Canada
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Old 29-07-2016, 08:41   #5
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Re: aviation gas for a 20hp yamaha

I tried avgas in my Yamaha 15hp 4 stroke a few years ago. It ran but it didn't run well. It didn't seem to have the power it had with regular gas and it wasn't as smooth at idle. It did clean out the fuel system and the engine ran better for a year or so after I used it. When my last tank was about half empty I topped it off with regular gas and that seemed to solve the power problem and the idle problem. Avgas is not stabilized by additives so much as it is a tighter fraction of the distilling process so all the molecules are of a similar size and evaporation rate. Regular gas is a mixture of light and heavy molecules. The light molecules evaporate faster and change the characteristics of the mixture. The carburetors are designed for regular gas so the jets are not ideal for avgas, which is probably why I experienced the power and idling issues. Also one should note that one of the primary ingredients in avgas is toluene, which if you read the label on most carburetor cleaner cans is also their primary ingredient. That probably explains why after running 5 gallons of avgas through it it ran so well for a year.
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Old 29-07-2016, 08:44   #6
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Re: aviation gas for a 20hp yamaha

Avgas doesn't have additives that make it last, it is a completely different different fuel from automotive gasoline, it's like it was Naptha based or something, let Avgas evaporate on your hand say and they will be completely dry, leaving maybe a little white color which is I suppose the TEL, where car gas will leave an oily film.
I've used Avgas for years to clean parts, it is great for that, but being flammable be careful of course.

The vapor pressure, and other properties are completely different than car gas, base Avgas without any lead is called 93UL and has been for years proposed and will run fine in almost all airplane engines, but has not happened, only a very few percent of engines need the extra Octane, but they burn the most fuel.
Octane in Avgas does not correspond to car gas Octane, completely different methods used to determine Octane
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Old 29-07-2016, 08:51   #7
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Re: aviation gas for a 20hp yamaha

More and more pilots are obtaining STCs for their aircraft to use autogas. Why? Because autogas is less expensive and the FAA has found no statistically significant safety-related issues with using autogas as long as the gasoline meets the octane requirements for your engine. You have probably heard the term, "caveat emptor." It means, "buyer beware." The wrong fuel octane used in your engine can mean the early demise of your engine and you, if you are not careful.

Octane 101

The octane rating of gasoline refers to the fuel's anti-knock quality, Autogas octane is rated differently than that of avgas. Two CFR (Cooperative Fuel Research) knock-test engines, adopted by ASTM, are used to test automotive gasoline according to ASTM D 2699 and D2700 standards. A modified version of the CFR engine is used to test avgas. The ASTM D 2699 test refers to the Research Octane Method (RON) and serves as the essential index of acceleration knock. ASTM D 2700 refers to the Motor Octane Number (MON) and provides an index of knock at high engine speeds. The (MON) method engine test differs from the (RON) method by using preheated fuel mixtures, higher engine speeds and variable ignition timing, placing more stringent thermal demands on the fuel under test. The (MON) number is typically 8 to 10 octane numbers lower than the (RON) number.

The autogas (MON) octane number is similar to the aviation rating of octane up to 100 octane, according to ASTM D 2700. When you purchase autogas, the octane rating is the average of the RON and MON, (R+M)/2 and the formula is posted on the gasoline dispenser. However, you have no way of knowing if the (MON) octane number meets the requirements of your aircraft engine unless you have documentation or a means to test gasoline octane on-site.

Another potential problem is the mixing of turbine fuel with avgas, When turbine fuel is added to avgas, the octane level drops significantly. A 10-percent mixture of turbine fuel and 90-percent 82-octane avgas can lower the octane of the gasoline over two-octane numbers.

Many people think high-octane gasoline is more powerful than low octane gasoline. This is not true. The energy produced from a gallon of high and low octane gasoline is almost the same. Any minor variation depends on what additives are used by refiners and blenders. The key features of high-octane gasoline are a higher ignition temperature and a slower burning rate.

The higher ignition temperature of high octane gasoline reduces the chance of detonation from "hot spots" within the engine's cylinders and minimizes pre-ignition. A slower burn rate allows for more efficient use of the ignited fuel's pressure buildup to be converted to mechanical energy instead of heat. That is why a high performance engine will run smoother and will feel more powerful when high-octane gasoline is used.

Using a low-octane gasoline whose ignition temperature is too low causes pre-ignition. Low-octane automotive gasoline (87-octane) has a typical ignition temperature of 300 degrees Celsius; high-octane (93-octane) automotive gasoline has a typical ignition temperature of 400 degrees Celsius. Aviation gasoline is blended to ignite at 500 degrees Celsius. High compression and high cylinder temperature will cause the fuel to ignite before the sparkplug fires.
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Old 29-07-2016, 08:52   #8
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Re: aviation gas for a 20hp yamaha

Quote:
Originally Posted by Hoosiersailor View Post
Are you not able to find regular 87 or 89 octane ethanol free gas in your area? Uausually it is only 15-20 cents more per gallon.

Pure-Gas.org : Map of Ethanol-Free Gas Stations in the U.S. and Canada

Nope, I know the website well, and the ONLY sources of ethanol free gas in Maryland is avgas or racing fuel for $14 per gallon. Virginia, yes, Pennsylvania, yes, but Maryland and Delaware, we are way too environmentally concerned to allow the sin of ethanol free gasoline.
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Old 29-07-2016, 08:54   #9
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Re: aviation gas for a 20hp yamaha

Warning: Thread drift.

Hey, a64pilot

I have not been following the 100LL replacement saga. Any developments or decisions made recently?

I run a 520 powered Bonanza and I probably should be paying more attention.

Steve
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Old 29-07-2016, 09:02   #10
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Re: aviation gas for a 20hp yamaha

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Originally Posted by Panope View Post
Warning: Thread drift.

Hey, a64pilot

I have not been following the 100LL replacement saga. Any developments or decisions made recently?

I run a 520 powered Bonanza and I probably should be paying more attention.

Steve
Panope, If you are not a member of BEECHTALK.COM you should stop reading this now and head there. The very best information and group there is. No handles, real names only. All the info you need.
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Old 29-07-2016, 09:03   #11
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Re: aviation gas for a 20hp yamaha

Av gas is color coded Or was, now there is only one grade 100LL, it's light blue. There used to be red blue and purple. Purple was 145 oct. If you put it your car it burned so slow it turned the whole tail pipe red.
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Old 29-07-2016, 09:04   #12
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Re: aviation gas for a 20hp yamaha

I gave up being concerned about Avgas years ago, I don't follow it either. It's called a "boutique" fuel and I think as a percentage it's WAY less than one percent of gasoline manufactured, it is statistically insignificant, but some people have to have a cause I guess.
Only airplane I have now is a 1946 C-140 with a Continental C-85 engine, min octane required for the C-85 is 73.
Be real tough to find a fuel that the C-85 isn't happy with, when I was flying it much I burned car gas as it's cheaper and easier on the engine, the difference in price alone will pay for an engine overhaul at 2,000 hours, burn car gas and your overhauls are free

I don't see the demise of Avgas, until it simply becomes no longer profitable, I think there is only one manufacturer and it has to be transported specially in trucks only and these trucks can only be used for Avgas, it can't be pumped by pipeline even if there were enough to justify that, and there isn't.
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Old 29-07-2016, 09:06   #13
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Re: aviation gas for a 20hp yamaha

Artisanmach, it sounds like burning low octane fuel in a high octane engine would cause pre-ignition and engine problems. But it sounds like running higher octane in an engine meant for lower octane will not necessarily cause problems?
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Old 29-07-2016, 09:14   #14
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Re: aviation gas for a 20hp yamaha

100LL has so much lead in it that I used to take lead BBs out of my spark plugs, stuck down beside the ceramic electrode cover, when I cleaned them. My 0-320 Lycoming ran better off 87 octane car gas, for which it was certified. It appeared that long idles contributed.
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Old 29-07-2016, 09:16   #15
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Re: aviation gas for a 20hp yamaha

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Originally Posted by schoonerdog View Post
Artisanmach, it sounds like burning low octane fuel in a high octane engine would cause pre-ignition and engine problems. But it sounds like running higher octane in an engine meant for lower octane will not necessarily cause problems?
Necessarily is the key word. Higher octane fuels burn slower and have higher ignition temperatures. Engines designed for low octane may not be able to handle that well. My Cessna 172 was designed for 80 octane no lead avgas. Most of the time I could not by it and had to use 100ll. There was a place in Elizabeth City NC where I could buy it and i did when I was in the area. My plane would run smoother and climb 75 feet/min faster on the 80NL than on the 100LL.
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