Fuel pressure at the carburetor?
 

For you math nuts. What is the fuel pressure at the carburetor fuel inlet when the tank has 10 gallons of fuel, and fuel shut-off in the cab is wide open?

Re: Fuel pressure at the carburetor?
 

Nominal

Re: Fuel pressure at the carburetor?
 

30/31 has more than a 28/29,..over 2psi on a full tank,it will run a Stromberg 97 no problem

Re: Fuel pressure at the carburetor?
 

The specific gravity of car fuel at 60F is about 0.73 (Google).
The vertical distance from the carb inlet to the top of a late 1930 full tank is about 22 inches.

If the liquid was water, then the pressure would be 22 in/H2O.
But this is fuel, so
22 x .73 = 16.06 in/H2O

16.06 = 0.58 PSI.

What do you guys think? Sound about right?


Revised 8-16-2020. Was 60C, is 60F.

Re: Fuel pressure at the carburetor?
 

Re: Fuel pressure at the carburetor?
 

Yes that's how I reckoned too. Now what is the difference in the carb fuel bowl with a full tank compared with an almost empty tank . Spoiler alert (I reckoned c 1mm, but I may be wrong)

Re: Fuel pressure at the carburetor?
 

Correct. About half a pound of pressure.

Re: Fuel pressure at the carburetor?
 

Re: Fuel pressure at the carburetor?
 

Well, it could matter if one decided to put a fuel tank at the back of the car and utilise a pump. (It is also a neat math exercise.;))

Re: Fuel pressure at the carburetor?
 

Oh you guys are killing me. My head hurts. I'm confused. I'm old. What math, arithmetic pleeeeeze save me.

All I know and care about is when I turn the valve the gas flows down hill. That has something to do with gravity, and that is good enough for me thank you.:confused:

Re: Fuel pressure at the carburetor?
 

This is called “head pressure”. For those that fly this is important especially between high wing and low wing aircraft.

Re: Fuel pressure at the carburetor?
 

Thats funny. Two carburators and multiple shut-off valve changes and still if I don't take special steps fuel will drip from the intake. Having had to drain the tank multiple times over the past year, I was surprised how little pressure there was at the fuel line on a full tank. I figure more than 60 lbs of fuel. Thought it would be an interesting question.

Re: Fuel pressure at the carburetor?
 

Re: Fuel pressure at the carburetor?
 

The equation for the static pressure at the bottom of the tank Pressure = density of gasoline x acceleration due to gravity x height of tank. This is high school science and it came before rocket science! As a matter of fact, Daniel Bernoulli (1700 to 1782), a Swiss mathematician and physicist, developed this science in his theories of fluid mechanics.

Re: Fuel pressure at the carburetor?
 

The specific gravity of car fuel at 60C is about 0.73 (Google).

Methinks that you meant 60°F

Re: Fuel pressure at the carburetor?
 

???????
I've been a CFII/MEII for 40 years. Your gonna have to explain that one to me.

Re: Fuel pressure at the carburetor?
 

It's not about the weight of the volume of the tank, it's about the weight of the column of fuel. A water tower with water level at 100 feet will have the same pressure if it hold 100 gallons or 1 million gallons.

Re: Fuel pressure at the carburetor?
 

My brain hurts, but then I could not get through/pass Physics class in College either.

Re: Fuel pressure at the carburetor?
 

The pressure a liquid exerts depends on the height/depth of the liquid, its weight density and force of gravity.
Two equations: P = mgh or P = Dw(h) where m = liquid mass, g = gravity, h = liquid height/depth and Dw = liquid weight density.

Either way you do it the answer here, ( 22" height ) = .58 lbs/sq inch.

What no one figured tho' is the # of sq inches of area in the cross section of the tube that feeds the gas to the carburetor. It certainly is not 1 sq inch. The ID of the tube might be 1/8" or .125 ".

Some of you are asleep now with these #'s. If not you can continue on. It is an interesting exercise. The only answer needed here is that the system in a Model A works so the Ford engineers had it figured out OK. Bill

Re: Fuel pressure at the carburetor?
 

If I wanted to know the pressure [ I don't ] I would just hook up a gauge. Maybe the needle would give a little wiggle.