I agree with you totally. The distance doesn't matter in our little cars.
I hadn't considered the volatility issue. A very important factor on a
really hot day. But you are correct. The difference in our vehicles is
almost nothing.
Thanks for the calcs.
Terry Smith
'67 Series V
-----Original Message-----
From: K. Patrick Wheeler <pwheeler(at)ix.netcom.com>
To: alpines(at)autox.team.net <alpines(at)autox.team.net>
Date: Friday, February 12, 1999 1:29 AM
Subject: RE: Fuel pump location and fluid statics
>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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