a testament to av gas

[Duke W.]

Apparently, the boiling point of 100LL is not as high as a racing fuel such as VP Vintage Leaded or VP110.

Gasoline does not have a specific boiling point. This is because it is made up of scores, if not hundreds, of hydrocarbon species, each of which has a specific boiling point. So the vaporization characteristics of gasoline is expressed as a "distillation curve". Without actually looking at the curves, one can express the percent at or above their boiling point. For example, a typical summer mogas blend may have 10 percent at or above the boiling point at 120F, but 100F for a winter blend, and both may have 90 percent at or above their boiling point at 300F.

Modern cars may actually account for more hydrocarbon emissions due to evaporation from the fuel system than from the tailpipe. Thus modern mogas blends are primary concerned with Reid Vapor Pressure, which is measured at 100F. Winter blends have higher RVP than summer blends and modern mogas blends generally have lower RVP than pre-emission era blends/cars with carburetors. Lower RVP usually aids starting in cold weather, but modern fuel injected cars are less effected due to good fuel atomization at the injectors, and pressuized fuel systems in modern cars have eliminated vapor lock/percolation issues from the past.

Even though modern mogas blends generally have lower RVP the percent evaporated in the 140-200F is greater than traditional straight gasoline blends from the pre-emission era due to the addition of ethanol, which boils at 170F. Percolation was an issue in decades past, but it is more of an issue today with the vintage cars due to the greater percent of current E10 gasoline at or above the boiling point in the 150-200 degree range.

Off the top of my head I don't know if avgas or racegas is least prone to vapor lock/percolation in vintage cars, but both are certainly more resistant than modern E10 blends. ASTM specs control the distillation curve of avgas and blenders of race gas should be able to provide the distillation curves for their products.

You can also look at this thread from about ten years ago, or maybe you shouldn't.

https://www.forums.ncrs.org/forum/te...llation-curves

Duke

[Duke W.]

Mistake... second paragraph, third sentence should read "Higher vapor pressure usually aids starting..."

I wish the edit function would stay active longer.

Duke

[Joe L.]

It's both against federal law and state law for any FBO to pump avgas into a licensed road vehicle (Don't worry, I'm not going to tattle on anyone.) due to the fact that 100LL has TEL, which is a federal offense to use in road vehicles and federal and state tax issues. Some will let you fill jugs, but not necessarily all, especially in busy urban area general aviation airports.

Anecdotal evidence indicates that avgas has a longer shelf life than mogas. I've never had a problem storing cars with E10 mogas for up to eighteen months, although they have "closed" fuel systems (EVAP systems with vacuum and pressure relief valves), and I always store with full fuel to minimize the vapor dome at the top of the tank, to minimize any "breathing" because if humid air at or below the dew point is drawn in and water condenses in the tank, the only way to to remove it is to drain or consume the fuel.

Back in the winter of '64 my SWC would not start during a Seattle cold snap with below freezing temperatures. I figure a slug of water at the bottom of the tank froze. After a few days and warmer temperatures it started. I poured in a can of HEET, which is basically ethanol, which acts as a cosolvent and allows the water to homogeneously mix with the gasoline and be consumed with the fuel.

Modern E10 allows much more water to be homogeneously absorbed and corrosion isn't an issue until there so much water that a water alcohol blend drops out to the bottom of the tank, but that takes 10 times more water than with straight gasoline.

Cars with vented fuel systems are much more susceptible to water intrusion especially if they are exposed to condensing humidity (like being outside), but filling the tank fully to minimize the volume of air that might be drawn in is best as is inside storage where temperatures don't often drop below the dew point.

Also the colder the storage temperature the longer the shelf life because chemical reactions like oxidation increase/decrease exponentially with temperature.

I have a friend who formerly managed air operations for his employer. (He's now an airline pilot.) The fleet included piston engine aircraft that used 100LL avgas. Certain maintenance operations require fuel to be drained, and it's against FAA regulations to put drained fuel back in any aircraft, so it has to be "disposed", and he disposed of it by pouring it into his C2 rather than paying for it to be "recycled".

Too bad we all can't get such a deal!

BTW, the "Aviation Method" for rating octane yields about the same number as the Motor Method for mogas, and since the "sensitivity" - the difference between MON and RON is 8-10 for most gasoline blends - the RON of 100LL avgas is about 108-110 and the PON is 104-105; and since avgas is blended to prevent vapor lock at high altitude, it has a higher distillation curve, than any mogas which will delay the onset of percolation in vintage cars with non pressurized fuel systems (and this includes the old Rochester FI system since the fuel bowl in not pressurized), but they may be a little harder to start, especially vintage carbureted or Rockester FI engines.

Duke

Duke-----

Yes, using AVGAS for any NON-AVIATION use is illegal under federal law. ALL gasolines, including AVGAS, are supposed to be free of added lead as of many years ago. However, aviation received a special exemption FOR SAFETY purposes until an acceptable UNLEADED aviation fuel could be developed. The exemption has been extended several times. The use of AVGAS for non aviation motor vehicle applications is just the kind of thing that could threaten the exemption if some news organization should expose it. The loss of the exemption could threaten the entire general aviation industry.

However, it will all be a moot point in the not-so-distant future inasmuch as a few UNLEADED aviation gasolines have been approved for use and more will be soon. I expect the complete phase-out of leaded aviation fuel is very much on the horizon now. That's actually a good thing for aviation, too, as I understand there is only one plant manufacturing tetraethyl lead in the world and they want to get out of the business for product liability concerns.

[Larry E.]

Joe: Do not know if you know this. Always there is two sides to a coin. I'm always dubious
when I hear "fuel news" from California.
New FAA Reauthorization Bill Would Prohibit Removing 100LL From Airports.

The just-passed House FAA Reauthorization Bill would prevent airports from removing 100LL or offering unleaded fuels in lieu of leaded avgas under penalty of losing airport improvement grants. If the language remains in the Senate version of the bill, it would raise a nearly insurmountable barrier to fielding competitive unleaded fuels because many airports can’t afford or don’t want dual tankage for both leaded and unleaded fuels. The bill requires any fuel being sold on Oct. 5, 2018—the date of the previous reauthorization bill—to remain available to aircraft operators. (See correction at end of story.)

Bottom Line>Your "moot point" is decades away. JHMO>Larry

[Mark F.]

Duke----- Yes, using AVGAS for any NON-AVIATION use is illegal under federal law. ALL gasolines, including AVGAS, are supposed to be free of added lead as of many years ago. However, aviation received a special exemption FOR SAFETY purposes until an acceptable UNLEADED aviation fuel could be developed. The exemption has been extended several times. The use of AVGAS for non aviation motor vehicle applications is just the kind of thing that could threaten the exemption if some news organization should expose it. The loss of the exemption could threaten the entire general aviation industry.

However, it will all be a moot point in the not-so-distant future inasmuch as a few UNLEADED aviation gasolines have been approved for use and more will be soon. I expect the complete phase-out of leaded aviation fuel is very much on the horizon now. That's actually a good thing for aviation, too, as I understand there is only one plant manufacturing tetraethyl lead in the world and they want to get out of the business for product liability concerns.

Joe,
Per the web, Innospec is the name of the company...
and they manufacture tetraethyl lead (TEL) at their Ellesmere Port facility in Cheshire, England...Innospec is the only known manufacturer and distributor of TEL for leaded gasoline.

https://innospec.com/fuel-additives/...EL)%20products.

[Michael G.]

I used 100LL av gas when I lived down near Detroit, but Up North here, with no airport nearby, I mix 18 gal of 90 octane Rec gas and 2 gal of 110 octane no ethanol racing gas. Never have a problem in the Spring. The 65 L84 cranked two or three times and fired right up.

[Duke W.]

Don't forget the 90 is PON, and the 110 is RON, (RON + MON)/2, and sensitivity, RON-PON = 8-10, so the PON of the racegas is about 105.

The PON of the blend can be reasonably estimated by linear proportioning, so:

(0.9(90) + 0.1(105))/2 = 91.5 PON or about 96 RON.

Reid vapor pressure is probably fairly high, but more importantly the 10-50 percent boiling points will be higher than E10 (see distillation curves in post #16) and probably even higher than the traditional (non-ethanol) summer blend, so much less prone to percolation and/or vapor lock than current E10, any commercial PON.

Duke

[Larry E.]

Some added info: George Braly of General Aviation Modifications Inc., the only U.S. company with an FAA-approved 100-octane fuel, said the bill will “almost certainly stop dead” early efforts to develop an unleaded market in California and other parts of the West. “Our position on the bill is that the law, will be impossible for anybody, GAMI or Swift or anybody, to deliver a high-octane unleaded fuel,” says Braly. “There’s not enough money to put in additional tanks and it would be years to get them installed.” Also many of OEM of small airplanes have said:


One OEM has already advised its customers not to use the unleaded fuel developed by General Aviation Modifications Inc. (GAMI)

In June 2024, Cirrus Aircraft released a Service Advisory that warned owners that using the STC-approved G100UL would void their warranties.

This gas is unreliable and not safe. Man is not smart enough to make a substitute for 100LL at this time.
Of course. JMHO - Per Continental and Lycoming, only approved fuels may be used for an engine to be covered by warranty. As the GAMI G100UL fuel is a non-approved fuel per Continental and Lycoming, engines known to have run this fuel may not be covered by the current OEM engine warranty.” Case Closed-Larry

[Duke W.]

Don't forget the 90 is PON, and the 110 is RON, (RON + MON)/2, and sensitivity, RON-PON = 8-10, so the PON of the racegas is about 105.

Sorry, sensitivity, the difference between RON an MON is... RON-MON = 8-10. I need to do a better job proofreading.

Duke