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Old 08-15-2020, 07:22 PM   #1
DannL
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Default 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?
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Old 08-15-2020, 07:53 PM   #2
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Default Re: Fuel pressure at the carburetor?

Nominal
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Old 08-15-2020, 07:57 PM   #3
Jack Shaft
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Default 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
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Old 08-15-2020, 08:10 PM   #4
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Default 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.

Last edited by 1930-Pickup; 08-16-2020 at 06:36 PM.
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Old 08-15-2020, 10:47 PM   #5
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Default Re: Fuel pressure at the carburetor?

Quote:
Originally Posted by 1930-Pickup View Post
The specific gravity of car fuel at 60C 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?
Ugh........... what???????!
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Old 08-16-2020, 02:29 AM   #6
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Default Re: Fuel pressure at the carburetor?

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Quote:
Originally Posted by 1930-Pickup View Post
The specific gravity of car fuel at 60C 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?
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)
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Old 08-16-2020, 04:58 AM   #7
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Default Re: Fuel pressure at the carburetor?

Correct. About half a pound of pressure.
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Old 08-16-2020, 05:25 AM   #8
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Default Re: Fuel pressure at the carburetor?

please tell us why it matters!
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Old 08-16-2020, 05:54 AM   #9
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please tell us why it matters!
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.)

Last edited by johnbuckley; 08-16-2020 at 06:59 AM.
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Old 08-16-2020, 06:59 AM   #10
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Default 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.
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Old 08-16-2020, 07:57 AM   #11
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Default 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.
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Old 08-16-2020, 09:11 AM   #12
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Default Re: Fuel pressure at the carburetor?

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please tell us why it matters!
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.
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Old 08-16-2020, 09:28 AM   #13
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Default Re: Fuel pressure at the carburetor?

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Originally Posted by old31 View Post
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.
A wise man indeed. Your logic is what the hobby is all about. Our hobby will last forever with people like you. Hooray !
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Old 08-16-2020, 09:31 AM   #14
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Default 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.
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Old 08-16-2020, 10:07 AM   #15
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Default Re: Fuel pressure at the carburetor?

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

Methinks that you meant 60F
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Old 08-16-2020, 11:59 AM   #16
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Default Re: Fuel pressure at the carburetor?

Quote:
Originally Posted by daren007 View Post
This is called “head pressure”. For those that fly this is important especially between high wing and low wing aircraft.



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

Last edited by Patrick L.; 08-16-2020 at 12:12 PM.
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Old 08-16-2020, 12:48 PM   #17
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Default 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.
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Old 08-16-2020, 01:12 PM   #18
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Default Re: Fuel pressure at the carburetor?

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Originally Posted by eagle View Post
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.



My brain hurts, but then I could not get through/pass Physics class in College either.
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Old 08-16-2020, 05:41 PM   #19
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Default 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
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Old 08-16-2020, 05:53 PM   #20
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Default 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.
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