The three aromatics generally, but not always, found in gasoline are: Benzene, Toluene and Xylene-or what is slated as BTX. Benzene is extremely limited because of its toxicity and is rarely, if ever, found in California-based gasoline. Toluene is most abundant in gasoline and varies from 20 to 40 percent. Many self-proclaimed Internet chemists falsely advise dousing standard pump gas with gallons of Toluene to increase fuel octane anywhere from 3 to 8 octane numbers. Too much toluene has a negative effect on a vehicle. The car's throttle response will become lazy, and the vehicle will strain to obtain higher rpm. Aromatics for unleaded gasolines will be in the 25-percent range, sometimes higher or lower. The maximum in California gas is 35 percent.
Distillation:A gasoline contains various hydrocarbons that boil at different temperatures. As a result, the gasoline boiling range can extend from 80F to a maximum of 437F. This is in contrast to water that boils only at 212F at sea level.
Octane Numbers:Research Octane Number (RON):This number is measured under mild conditions and is more important in controlling part throttle knock.
Motor Octane Number (MON):This number is measured under more severe conditions and is most important for octane satisfaction at wide-open throttle.
Anti-Knock Index (AKI) or R+M/2This is the average of the RON and MON. This is the number posted on the retail gasoline pumps normally indicating 87-, 89-, or 91-octane. Racing gasolines have AKI's from 100 to 118.
Oxygenated Compounds:These are required in street gasoline in many areas of the U.S. to help reduce exhaust emissions. The two most common oxygenated compounds are Methyl Tertiary Butyl Ether (MTBE) and Ethanol (this is an alcohol).
Reid Vapor Pressure (RVP):Normally measured in psi, RVP is a measure of the front-end volatility of the gasoline. This is important for getting a carbureted engine started in cold weather. The national avereage for RVP in summer is 7 psi and in winter is 13.5 psi.