Glossary of Terms

Here is a general explanation of some common fuel-related terms you might see in this web site:

Anti-Knock Index (R+M)/2:

Av Gas (Aviation Gasoline)

Av Gas is a common alternative to pump gas in performance applications. Because street pump gas is primarily designed to address emissions considerations rather than performance, Av Gas os iften preferred for performance applications. When choosing a fuel for such applications, here are some important things to consider.

Av Gas is designed for low RPM engines. A typical aircraft engine does not rev much beyond 2,200 RPM. Av Gas therefore has a very slow burn speed and is typically not well-suited to high RPM engines with their much higher exhaust and cylinder temperatures.

Since Av Gas is used in engines that typically operate at a steady RPM, acceleration and throttle response qualities are less crucial than with most racing fuels.

Av Gas octane ratings (e.g. "100/130") are derived through different methods than octane numbers for racing and street gasolines. As an example, 100/130 octane Av Gas is generally similar in performance to a racing gasoline in the mid to high 90's octane range.

Av Gas can work better than street gas in some performance applications because the quality of pump gas is often poor. However, in almost every case, the correct racing gasoline will provide more horsepower and torque, crisper acceleration, and cooler operation, as opposed to AV Gas. Racing gasolines are also usually less prone to vapor lock, more efficient at the atmospheric pressures and temperatures at ground level. For the ultimate performance, reliability, and maximum engine life, race gas is usually a much better choice.

Burning Speed:

The speed at which the fuel releases its stored energy (burns) after ignition. In a high RPM engine, the speed at which the piston travels in the cylinder allows only a microsecond for total combustion. Peak cylinder pressure should occur by no later than approximately 20° ATDC. If the fuel is still burning after that point, it will not contribute to the engine's net power output, while also exposing more of the cylinder area to extra heat.

To understand this, picture a large bed of charcoal briquettes on a grille. When you fully spray them with the lighter fluid and ignite one corner, the flames spread slowly across the entire bed. Lighter fluid has a very slow burn speed. You should never use gasoline for lighting charcoal, but if you did, the flame would flash across the briquettes at a much faster burn speed.

Detonation (Knocking):

Detonation, also known as pinging, occurs when the air/fuel mixture in the cylinder that has been ignited by the spark plug causes another flame front that interrupts smooth burning. The violent collision of these two flame fronts causes the characteristic metallic "pinging" sound. Detonation dramatically increases combustion chamber pressure, generates tremendous amounts of heat and engine wear, and can range in severity from hardly noticeable to total destruction of the engine.

Distillation:

Refers to the temperature at which various hydrocarbons in the fuel will vaporize or “boil off”. These characteristics are vital to efficient combustion. A fuel's distillation curve defines its throttle response, engine cooling ability, and tendency to resist vapor lock.

Dielectric Constant (DC):

Refers to the overall electrical charge of the molecules within a fuel. It often used as a quality control by fuel suppliers. A higher or lower DC value by itself has no bearing on the performance capability of a fuel in a given engine. DC readings can be controversial because some sanctioning bodies may associate higher readings with illegal additives. However, certain legitimate materials can raise a fuel’s DC value.

Knocking:

See "Detonation"

Lead Content (LC):

Refers to the amount of Tetraethyl Lead (TEL) in grams per gallon. TEL is added to increase a fuel's octane value and resistance to detonation. Leaded fuels are typically illegal for street use and used only in high-performance automotive and aviation applications. Leaded fuels can cause significant harm or destruction to oxygen sensors, catalytic converters and other emissions-related equipment.

Octane:

A numerical representation of a fuel's anti-knock properties. Higher octane values are more resistant to resistant to detonation. There are three commonly-used octane rating values:

Research Octane Number (RON):

This is probably the most commonly-used rating worldwide. RON is determined by running a fuel through a specific test engine with a variable compression ratio under controlled conditions, and comparing the results with those for mixtures of the chemical compounds isooctane and n-heptane. RON yields a higher octane value, but is mostly applicable to low-RPM, low-load situations.
Motor Octane Number (MON):

MON is a better measure of how a fuel behaves under load (such as in high RPM racing applications). MON testing uses a test engine similar to that used in the RON method, but with a preheated fuel mixture, a higher engine speed, and variable ignition timing to further stress the fuel's knock resistance. Depending on the fuel's composition, the MON of a modern gasoline will be about 8 to 10 points lower than the RON. Fuel specifications normally require both a minimum RON and minimum MON.
Anti-Knock Index (R+M)/2:

This is the average of a fuel's motor and research octane ratings. It is the value displayed in the yellow sticker on retail gas pumps and is therefore also known as “Road Octane” or "Pump Octane". This average considers both normal and higher range and load uses, and is becoming a commonly-used value for comparing fuel products.

Pre-Ignition:

Premature ignition of the air/fuel mixture as the piston is traveling upward during the compression cycle and prior to firing of the spark plug. It is usually caused by red hot deposits in the combustion chamber, and overheated spark plug, or incorrect tuning parameters, insufficient fuel quality or too little octane, and/or heat from compression or engine temperatures. If severe enough, pre-ignition can cause major mechanical damage. Although not the same as detonation, the resulting damages can be very similar.

Reid Vapor Pressure (RVP):

Refers to the pressure exerted by the vapors of a volatile liquid within a confined space. In simple terms, it describes a fuel's tendency to evaporate. Fuels with high RVPs tend to boil more easily and evaporate in pumps or lines. A high RVP can also increase a fuel's tendency to vapor lock or percolate. A low RVP can prevent a cold engine from starting. For racing applications, RVPs of between 5 and 9 PSI are generally considered ideal.

Specific Gravity (SG):

Refers to the ratio of a substance’s weight to the weight of an equal volume of water at a specified temperature. The chart below shows an example using water and a typical gasoline:

Liquid	lbs./ gal. @ 60°F	Specific Gravity
Water:	6.15	1.0
Gasoline:	8.33	0.7383

SG is important for fuel considerations because changes to jetting or fuel injection systems may be required to compensate for differences in fuel density. Fuels with a lower SG (less dense) have a higher BTU content and tend to be more stable at high RPM's. The lower the SG, the more crucial jetting becomes and the easier it is damage an motor due to a fuel/air mixture that is too lean. SG is often confused with "viscosity", but they are not the same.

Generally speaking, when comparing or changing fuels, it is important to consider the fuel's SG. If a fuel has a lower SG, it will typically run learner and require a larger jet. Fuels with a higher SG are commonly used with nitrous systems. Because nitrous oxide typically makes the fuel mixture leaner, a higher SG fuel can help to offset this condition.

Stoichiometric Air/Fuel Ratio:

Combustion requires the oxidation of fuel with air, or more specifically, oxygen (O2). The stoichiometric A/F value is the ratio of air mass in relation to fuel mass that is required for balanced oxidation. A racing fuel is a composition of numerous chemicals with its own unique A/F ratio. A generic value of 14.7 is often used, but this can vary significantly depending on the fuel blend. Proper fuel metering is primarily a function of volume. Specific Gravity, the A/F ratio, and to a lesser extent, viscosity, all affect the usable amount of hydrocarbon available to engine for power generation, regardless of jetting or fuel injection characteristics.

Vapor Lock:

Vapor lock occurs when a fuel changes from a liquid to gas or boils in the fuel delivery system. This can cause "cavitation" in the fuel stream and disrupt pump efficiency and performance. Fuel vaporization can be caused by engine heat, warm weather, a lower fuel boiling point, high altitudes, and or a combination of several factors.