Racing Gasoline Verses Aviation Gasoline We have received many phone calls and all wanted an explanation from us regarding the use of AvGas. As a Moderator we are impartial and want only to help our constituents by offering some facts they can decide on. So we thought it best to chime in a bit with some information. As you know us by now we tend to offer enough information for all levels of expertise.
Avgas 80/87: this product is used in low compression ratio aircraft engines, contains little or no lead, is red in color, and should not be used in any automotive engine due to a low motor octane number of about 80.
Avgas 100/130: this product that can be used in some automotive engines. It has both research and motor octane numbers slightly over 100. Avgas 100/130 is green in color, contains four grams of lead per gallon, and is becoming harder to find.
Avgas 100 LL: the LL stands for "low-lead" which means two grams per gallon, low compared to the avgas 100/130 that it was designed to replace. It has research and motor octane numbers very similar to the 100/130 product previously discussed. The color is blue. This product sometimes has a high level of aromatics, which can contribute to lazy throttle response, and dissatisfaction of the consumer
Avgas 115/145: this product was developed for high performance piston aircraft engines used in world war II and in the Korean war. It is very hard to find anymore due to lack of demand although it is of very high-octane quality. The color is purple.
Avgas is less dense and has a lower specific gravity than most racing gasolines. Instead of weighing about 6.1 to 6.3 pounds per gallon like racing gasoline, it weighs 5.8 to 5.9 pounds per gallon. The racer must compensate for this by changing to richer (larger) jets in the carburetor when changing from racing gasoline to avgas.
The other major difference is octane quality. Avgas is short on octane, has a lower volatility and doesn't evaporate as quickly, which is important for high-altitude use. Gasoline/Race fuels are more volatile fuel than Avgas, diesel or kerosene. The reason for this is not only the base constituents, but also the additives that are put into it. The final control of volatility is often via blending of butane. The desired volatility depends on the ambient temperature: the hotter the weather, the lower the volatility
AvGas is a good gasoline for low speed aircraft engines that run at 2700 to 2800 RPM at 10,000 feet or higher. This does not mean it is a good gasoline for racing engines operating at 8,000 to 10,000 RPM.
AvGas can be illegal to use in anything except aircraft engines because of on aromatics in general, benzene in particular, and olefins (alkene) content. Violations can carry a potential penalty of $25,000 per day of violation.
AvGas octane numbers are determined in a different test than motor gasoline octane numbers. Do not be confused by the big numbers from the AvGas test method. They are not comparable to motor gasoline test numbers. A 111 octane Racing Fuel tests at 160 on the AvGas Scale.
Engines running AvGas with "quick" spark advance curves or with no centrifugal advance have more spark advance at low rpm that racing gasolines can handle.
The result is detonation; Inadequate octane quality is one of the quickest ways to destroy an engine.
A Quote released from SUNOCO
Aviation gasolines are leaded and not taxed as road fuels, either of which make them illegal for use on public highways. Aircraft engines are also essentially low engine speed, constant throttle applications that must work well at very low atmospheric pressure and temperature. Aviation gasolines are blended to work well only under these very specific circumstances, and do not perform as well under high rpm and high load applications.
Only our opinion: For maximum engine performance, stability and power use race fuels with octane levels designed to mactch your engine specifications.
Quick Reference
Octane rating
The most important characteristic of gasoline is its Research Octane Number (RON) or octane rating, which is a measure of how resistant gasoline is to premature detonation (knocking). It is measured relative to a mixture of isooctane (2,2,4-trimethylpentane) and n-heptane. So an 87-octane gasoline has the same knock resistance as a mixture of 87% isooctane and 13% n-heptane.
There is another type of Octane, called "Motor Octane Number" (MON), which is a better measure of how the fuel behaves when under load. Its definition is also based on the mixture of isooctane and n-heptane that has the same performance. Depending on the composition of the fuel, the MON of a modern gasoline will be about 10 points lower than the RON. Normally fuel specifications require both a minimum RON and a minimum MON.
Volatility
Gasoline is a more volatile fuel than Avgas, diesel or kerosene. The reason for this is not only the base constituents, but also the additives that are put into it. The final control of volatility is often via blending of butane. The desired volatility depends on the ambient temperature: the hotter the weather, the lower the volatility. The maximum volatility of gasoline in many countries has been reduced in recent years to reduce the fugitive emissions during refueling.
Knock resistance
An air and fuel mixtures' ability to resist rapid auto ignition - when compressed in a spark ignited engine running at a specific temperature and compression ratio - long enough to avoid detonation or Engine knocking. Detonation occurs when the fuel mix not yet consumed by the flames from the spark plug auto ignites all at once (or sometimes perhaps when the flame front goes supersonic because of early ignition timing). A fuel with an high auto ignition temperature that burns reasonably fast and thus doesn't need early ignition timing, will therefore most often have high practical value knock resistance. Ethyl alcohol is such a fuel.
Interesting Fact during World War II and octane
One interesting historical issue involving octane rating took place during WWII. Germany received the vast majority of her oil from Romania, and set up huge distilling plants in Germany to produce gasoline from it.
In the US the oil was not 'as good' and the oil industry instead had to invest heavily in various expensive boosting systems. This turned out to be a huge blessing in disguise. US industry was soon delivering fuels of ever-increasing octane ratings by adding more of the boosting agents, with cost no longer a factor during wartime.
By war's end American aviation fuel was commonly 130 to 150 octanes, which could easily be put to use in existing engines to deliver much more power by increasing the compression delivered by the superchargers. The Germans, relying entirely on 'good' gasoline, had no such industry, and instead had to rely on ever-larger engines to deliver more power. The result is that British and US engines consistently outperformed German ones during the war, playing no small part in the defeat of the Luftwaffe.
