We've been going around a bit about the location of the fuel pump and I keep
being bothered by the suggestion that pumps push and don't pull and should
therefore be placed under the tanks. As a matter of semantics, irrespective
of where it is placed, it will "pull" from one side and "push" to the other,
but there may be good reason to place a pump low rather than high. When a
fluid is moved from a low spot (the gas tank) to a high spot (a pair of
Stromburgs or Solexs) whether it is pushed up the incline from below or
pulled up to the top should not matter that much. But it does. A fluid,
when pulled from the top is subject to volatalization and becoming vapor
before it reaches the pump impellors. Most pumps do a great job of pumping
fluid but a lousy one of pumping vapor. If pushing from below, they are
merely subject to the maximum pressure they can produce.
I looked it up and found some numbers that might interest:
If we consider it to be a static system with no movement it is a matter of
how high can you suck gasoline in a straw before you get a mouthfull of
vapor instead of gas:
The operative equation here is:
Pair = Pgas + pgh
air
| |
|__| _ height (h)
| |
| |
|g |
|a | straw
|s |
|o |
|l |
|i |
|n |
|e |
____|__|_____gasoline
| |
| |
Where:
Pair is the local atmospheric pressure (101.3 kPa)
Pgas is the vapor pressure of gasoline ( 62.5 kPa)
p is the density of gasoline (718.6 kg/m^3)
g is the force of gravity ( 9.81 m/s^2)
h is the height we are looking for ( m )
101.3 kPa = 62.5 kPa + (718.6 kg/m^3)(9.81 m/s^2)(h m)/(1000 Pa/kPa)
38.8 kPa = (7.05 kPa)(h m)
h = 5.5 m or,
h = 18 feet
So this says we should be able to 'pull' gasoline up a vertical 18 feet
before running into trouble, as our Alpines are about 13 feet from tip to
tail, the distance between the tanks and the carburators should always be
less than this even when parked some of our San Francisco streets. Of
course this leaves out the discussion of head loss due to fluid flow and the
variations in local atmospheric pressure.
All this being said, I still think it is better to place it lower if
possible. But if you cannot, or do not want to, the numbers say we should
be OK.
-Patrick
SV, Richmond, CA
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