"Frank" wrote in message
...
Tony Cox wrote:

My 'vapour lock' occurred at 7500' on a cold November day
with low humidity (ceilings were well above us). If anything,
switching to one tank rather than both should actually *lower*
the fuel pressure slightly - faster flow = greater friction loss.


Right, lower fuel pressure means less oomph to get it through the
pipes....Seems consistent with the 'theory'. I don't know enough about it
to say whether it's right or wrong.


To get maximum oomph, it would seem that 'both' should be
selected. Which is just what they tell you not to do!


I know the explanation wasn't very satisfying, I'd love to hear it
explained
better too. But, as you have apparently also experienced, the phenomenon
is
real. So there is something going on and pressure differences due to
venting/plumbing seems to explain it better than temperature related ones.


I just draw a blank on this. Unless there is a heat source near
the selector. Then I can rationalize an explanation. If there's some
venting/plumbing explanation, I'd expect some admonition to
"select the fullest tank" or "don't fly above 5000' without more
than 10 gallons in a tank". Makes no sense as is.

tc