Re: Fuel pressure

David J. Gall

Sorry, Pat, I should have said 0.026 >PSI< per inch of height.

Long-winded explanation follows:

0.026 psi per inch of height. That's pounds per square inch (pressure) per
inch of height of the column. If you put two of my described columns
side-by-side you'll have 12 lbs of fuel pushing down on 2 sq inches of
surface area, again for a total pressure of 0.026 psi per inch of column
height (231 inches height). Another way to say that is that the weight of a
single CUBIC inch of fuel is 0.026 lbs.

If you have a 3/8" I.D. fuel line, then your total weight of fuel in the
line would be 3.14 x (3/8in)^2 x 0.026 psi per inch x inches of height. Note
that psi per inch equals lbs per cubic inch: lbs/(in)^2/in = lbs/(in)^3.
Note also that the result of that calculation will give you pounds, not psi,
since (3/8in)^2 x inches of column height gives in^3 (volume) and wieght per
volume times volume yields weight. That would be the pounds of weight of
that particular column of fuel but not its pressure at the bottom of the
column. The weight of the column divided by the square inches of the base of
the column, in this case, the cross-sectional area of the fuel line, 3.14 x
(3/8in)^2, gives the pressure.

But, notice that we've multiplied and then divided by that cross sectional
area during the course of all this figuring. Those steps cancel each other
out; Just leave out those extraneous steps and the simple calculation is
left: 0.026 psi per inch of column height times column height gives
pressure.... Ta da!

Thanks for checking my work.

David J. Gall

-----Original Message-----
From: Q-LIST@... [mailto:Q-LIST@...]
On Behalf Of Patrick Panzera
Sent: Wednesday, August 09, 2006 5:41 AM
To: Q-LIST@...
Subject: Re: [Q-LIST] Re: Fuel Vent Line

Fuel weighs 6 lbs. per gallon. One gallon holds 231 cubic
inches. If we
stack all those cubic inches atop one another, we get all 231 cubic
inches pressing down on one square inch of surface area at
the bottom
of the container. Therefore, there is 6 lbs per square inch of fuel
pressure at the bottom of a column of fuel 231 inches tall.
Dividing 6 lbs by 231 inches yields a usable value of head
pressure of
0.026 lbs per inch of height. So if your header tank bottom
(figuring a
nearly empty tank) is 20 inches above the carb needle valve,
then the
fuel pressure at the carb is .52 psi or about ONE inch of
mercury. Not
a lot, and easily overcome by misguided ram air pressure.
Wouldn't that only be true if your fuel line was equal to one
square inch? 3/8" line is quite a bit smaller that one square inch.


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