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 Post subject: Fuel
PostPosted: Thu Sep 28, 2017 1:55 pm 
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Joined: Tue Jan 17, 2012 3:19 am
Posts: 7699
Location: Hope, B.C Canada
About Aviation Fuels - Aviation gasoline (or "av gas") is blended specifically for use in small aircraft. It's also commonly used by many high performance two-stroke owners because of it's high stated octane rating (usually 100oct) and the relatively low price compared to racing fuel. But it’s importance to understand some details about Av-gas.

Avgas octane is rated on a different scale than gasoline's intended for ground level use. What is 100 octane "av", is not necessarily 100 octane "ground level". Besides this, there is also a big chemical difference. Normal ground level race fuels are made up of gas molecules that have a "light end" and a "heavy end". The light end of the molecule ignites easily and burns quickly with a low temperature flame (as a piece of thin newspaper would burn). The heavy end of the molecule is not so easily ignited, but it burns with a much more intense heat (as an oak log would). This heavy end of the gasoline molecule is responsible for the hotter, more powerful part of the combustion process.

Small aircraft are constructed as very weight conscious vehicles. That's because their somewhat weak engines often have difficulty taking off with any extra weight. To help reduce this weight problem, aviation gasolines are blended with no heavy molecule end. This makes a gallon of avgas weigh measurably less than a gallon of ground level fuel. Since small plane engines turn very low rpms and produce so little power, the omission of the heavy end is not a significant horsepower issue. Running 100% Avgas in a vintage two-stroke is not a good idea because there is a measurable loss of power resulting from the absent “heavy molecule ends”

All that said, for most vintage two-stroke applications, a 50/50 mix of 91-octane premium, is a mix that offers a big boost in octane, along with “enough” heavy molecule ends” to make good power. We consider it a very functional and inexpensive way to greatly reduce temperatures, and improve detonation resistance (over straight 91 octane unleaded). We would tend to recommend against it’s use in a sustained high rpm application (like road racing or pro-level 125s motocrossers). However in any vintage two-stroke that is not being run “wide open all the time” … it is a fuel that works great. It bears noting that some blends of av-gas will quickly separate from castor-based oils used in premix situations. If you are per-mixing bean oil … av gas is not a good idea.

About Octane Boosters - Octane booster additives cannot turn a gallon of average quality fuel into a gallon of racing quality fuel. These additives are essentially flame retardants. That is, they raise the octane rating of a fuel by making it resistant to burning ... not by improving the high temperature stability. The end result is that they reduce detonation risk by reducing power output. An octane booster can make 86 or 87 octane fuel into a 89-90 equivalent (from a detonation resistance standpoint). However it can only raise 91 octane by one octane point or less. Given all this, we strongly recommend against using any octane boosters in a high performance two-stroke.

About Ethanol as a Fuel – In the day of vintage two-strokes, there was no ethanol (or other oxygenates) blended into pump gas fuels. Today’s pump gas comes with a varying percentage of ethanol blended in as an oxygenate. The addition of ethanol itself is an small annoyance, but not a big problem. What is a big problem is the varying percentages of ethanol across the country, and from one retailer to the next. While the fuel pumps “state” a certain percentage, in truth, that percentage can vary a lot. Since ethanol has a much lower specific gravity than gasoline, it make jetting slightly leaner.

Ethanol (aka Alcohol) is a fuel that many racers have some experience with. 100% methanol used to be permitted in some forms of dirt track racing and is still employed by many go cart racers. It's use was abolished in many forms of racing because it's flames are virtually invisible. This very dangerous quality makes for obvious safety problems.

Inside the Engine, 100% methanol burns very clean and yields very low operating temperatures. Unfortunately it's low specific gravity means that the jetting in the carburetors had to be doubled. Jetting was very difficult because so few carbs could pass fuel at double the rate in all circuit ranges. In addition, ethanol is “very” unfriendly to non-metal fuel system parts and rubber crank seals. The final death blow for methanol use in most forms of racing was that burning twice as much fuel means that you can't cover much distance on a tank of gas. It is strictly a "short distance" fuel.

http://www.klemmvintage.com/gasolines.htm
http://www.klemmvintage.com/deto2.htm


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 Post subject: Re: Fuel
PostPosted: Thu Sep 28, 2017 3:47 pm 
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Joined: Thu Apr 29, 2010 12:53 am
Posts: 1430
Location: Norco, CA
about the Avgas comments, I have been running it for 30 years in 2 strokes, low RPM CR500 and high RPM banshee, running hard in the dunes, I run 100% with 50:1 golden spectral, I don't recall ever having detonation or oiling issues, I'm sure I'm giving up some performance over using race gas or even tuning on the edge with pump gas but I'm okay with that.
I had some Avgas tested by a freeing who used to work at the shell refinery testing their fuel, it tested at 104 octane and if memory serves me it had about 4 grams/gal of ethel lead. this was along time ago, maybe they have since found a replacement for the lead.

a comment on methanol, many years ago I worked at a high performance shop and the boss had a tape made by some professor, I still recall some points from that tape, Methanol octane I seem to recall is about 124, this makes it a great fuel for supercharged cars such as dragsters. Methanol itself does not burn so much cooler and it does not burn 2X as much as gas. you do need to approximately double the amount into the Engine but about 1/2 of the un-burnt fuel goes out of the exhaust, and that takes the heat out of the Engine. if you have ever been to see top fuellers or funny cars then you have experienced this first hand.
the information above is just from memory so don't take it as gospel.


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 Post subject: Re: Fuel
PostPosted: Fri Sep 29, 2017 12:01 am 
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Joined: Tue Jan 17, 2012 3:19 am
Posts: 7699
Location: Hope, B.C Canada
As all here know by now ---- I run Avgas ----- with no issues either.
Just thought I would post what I have found.


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