[TR] Gas line filter placement.

[Randall]

> > I'm sure opinions will vary, but I've heard that running a paper
> > element
> > filter before the pump can aggravate any tendency towards vapor lock
> > as the
> > resistance of the filter lowers the pressure slightly.
> 
> Stuff and nonsense.

Vapor lock happens when the gasoline in the line leading to the pump
vaporizes.  (The lock part comes in because the pump cannot pump vapor very
well.)  The pressure at which the gasoline vaporizes depends on the fuel
blend and fuel temperature. 

A little quick poking around shows that US law restricts gasoline to no more
than 9 psia Reid vapor pressure.  This means (approximately) that it will
start to boil at 100F when the pressure drops below 9 psia.  Since 1
atmosphere is roughly 14.7 psia, that gives us a 5.7 psi margin for pressure
drop in the inlet tube.

But, the vapor pressure goes up rapidly with temperature, and even the fuel
in the tank can easily be hotter than 100F.  I'm too lazy to work out the
equation (and I couldn't find a chart for it), but you can see some typical
curves here:
http://goo.gl/yI6ci
Remember the fuel line goes under the car, where it both gets the hot air
blown back by the radiator fan when sitting stopped in traffic, and heat
from the exhaust system.

And I have seen paper element filters that had over a 5 psi drop at a
reasonable fuel flow.  Granted, that was a sign that they needed to be
changed, but I've never been very good at changing them before they needed
to be changed.  And in probably 300,000 miles in an assortment of TR3s all
with rusty gas tanks and no filter in front of the pump, I have never had a
fuel pump quit because of dirt.

I find it interesting that you have trouble with the Facet pump on uncle
jack.  I've also got a Facet pump in my Stag (the one I drove 5000 miles
round trip to VTR 2005), mounted in the stock location, and it never had any
trouble picking up fuel.  My guess is that you have a leak or restriction on
the inlet side.  Facet pumps don't have check valves, but they should be
able to pick up fuel from at least 24 inches.  Here is a video of one
picking up more than 36":
http://www.youtube.com/watch?v=upZqF7y97h0

-- Randall