"Donald H. Locker" wrote:
>
> Fellow Spitsters,
>
> Not to be too contrary, but even though the initial loading (priming)
> of the fuel pump will be with suction, after the pipe is full from the
> bottom of the tank to the pump, there will no longer be any gravity
> for the pump to overcome -- the rising fuel is exactly balanced by the
> falling fuel, and as long as the pump is lower than the in-tank
> opening of the pick-up, this [siphon] action will reduce the lift
> requirement of the pump. Now the other side of this story is that the
> level of the outlet of the pump also affects the pressure differential
> that the pump operates against, and since the pickup point and the
> delivery are at a constant differential height, (depending on road
> grade, though) the pump operates against a constant head. So it
> doesn't really matter to the pump, (who probably doesn't even see this
> simple situation -- remember the carb float valve is closed most of
> the time, so the output is deadheaded. And as long as the pump is in
> the trunk, the fuel should be cool enough to stay away from vapour
> lock, so the fact that there may be suction on the supply side of the
> pump is nearly irrelevent. But wait! there's more!
All very true, but on the siphon type tank, there is the distinct
possibility that when the pump shuts off both the up and down portions
of the suction line will drain, making the pump work harder on startup
in order to fill the lines.
> (Sorry to be so long-winded.) For safety, if you power the fuel pump
> through a relay controlled by lube pressure, you should be safer in
> case of serious shunt. And this would not significantly alter
> starting characteristics -- either there is sufficient fuel in the
> carb bowls to start, (in which case oil pressure rises quickly, pump
> begins delivering fuel, and all is ducky) or the cranking raises the
> oil pressure (albeit with a bit of delay) and the pump begins
> delivering, the bowls fill, engine starts, and THEY'RE OFF!
I don't know if it's a safe bet to expect the bowls to remain full.
Many I have seen leak down if they set for any length of time!
Joe
--
"If you can't excel with talent, triumph with effort."
-- Dave Weinbaum in National Enquirer
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