Reid Vapor Pressure (RVP)
RVP Low = Poor Cold Weather Starting but good "vapor Lock"
resistance
RVP High = Good cold weather starting characteristics but
poor "vapor Lock" resistance
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.
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 inthe
combustion chamber, so starting the engine is not a
problem.
:
The other prime requirement is
RVP, which controls both cold start volatility (RVP too low is bad) and vapor
lock resistance (RVP too high is bad). Those limits are narrower for aircraft
(5.5 to 7.0 psi) than they are for automotive use.
I must admit, I had never paid any
attention to the RVP of gasoline (mainly because it didn't seem like truly
useful information), but it might be time to get an RVP tester
{:>)
Ed
Edward L. Anderson
Anderson Electronic Enterprises
LLC
305 Reefton Road
Weddington, NC 28104
http://www.andersonee.com
http://www.eicommander.com