The Reid Vapor Pressure (RVP) is a number expressed in pounds per square inch (PSI). It represents the fuel’s volatility at its initial boiling point. It tells you how quickly a gasoline will vaporize. This is important when starting a cold engine on a chilly evening. If you are using a leaded racing gas, this number is not very important. It becomes critical if you are using unleaded pump gasoline.

The oil companies change the formulation of unleaded gas at least seven times a year, changing with ambient temperature for the area of the country in which it is used. Each of the different RVP. The colder the climate becomes, the greater the variation from summer to winter gasoline. In Minnesota and Michigan where it gets really cold in the winter, they end up with a much higher vapor pressure gasoline for the street.

While the higher vapor pressure helps the cars to start, it can lead to vapor lock. Gasolines with RVP of 12 to 14 are highly volatile. If the fuel line is routed close to a heat source, like the headers, and if the ambient air conditions are right, the fuel could physically boil in the fuel line. When this happens, vapor, not liquid, makes its way to the fuel pump and the engine dies.

Most racing fuels have an RVP between 5 and 7 pounds. The lower the number, the more difficult it is to start a cold engine because it doesn’t vaporize as well. In a race car, the ignition is generally hot enough to vaporize the fuel in the combustion chamber, so starting the engine is not a problem.