Vapor Pressure / Volatility

 

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.

 

 Any restrictions in fuel lines, pumps, injectors, etc., will lead to an acceleration of fuel velocity and a lowering of its pressure. That’s why engines with fuel pumps and injection systems are more susceptible to vapor lock, however the problem can occur even on naturally aspirated engines if things get too hot or the RVP is too high for the conditions around fuel systems. Simple things like a kink or pinch in a fuel line between the gascolator and a carburetor are sufficient to cause localized phase change of a fluid, bubbles of fuel vapor, and a rough-running engine.