For those concerned about the formation of
vapor in a pressurized fuel rail, I've attached a plot of
data collected from local samples of 100 LL avgas, 87
octane auto fuel with no ethanol, 91 octane auto fuel with
10% ethanol, and tap water. The water was measured just
as a check on the method.
The data would be considered more of a true
vapor pressure rather than a Reid vapor pressure due to
the method used.
The data indicates to me that if the fuel
pressure in the fuel rail is 35 psi as measured with a
regular gauge referenced to the atmosphere at sea level,
the temperature of 100 LL or 91 octane 10% ethanol in the
rail would have to be in the neighborhood of 240 deg F for
it to form bubbles of vapor (boil). The sample of 87
octane would require a temperature of about 215 deg F to
form a vapor phase.
My take on this is it may be more productive
to be concerned about the fuel supply to the high pressure
pumps rather than worrying about "vapor lock" downstream
of those pumps. This seems to be the conclusion reached
by the recent thread on this subject, possibly now
supported by actual data. Of course the data only applies
to the samples I obtained.
The higher temperature tolerance of the auto
fuel with ethanol compared to the auto fuel without
ethanol was surprising to me.
But I only collect data --- it is up to an
engineer to make sense of it ;>)
Steve Boese
RV6A, 1986 13B NA,
RD1A, EC2