Fuel pressure

[Dtolson9]

Does anyone know how many psi the gravity puts out on a model a?

[WHN]

At sea level?

14.7 PSI. Atmosphere.

There is a formula for this. Water at 10 feet is around 14.5 PSI.

Is tank full or quarter full?

Hard to answer.

[Dick Steinkamp]

Originally Posted by WHN

At sea level?

14.7 PSI.

That's atmospheric pressure. I think the OP would like to know the fuel pressure of a stock Model A gravity feed.

I've never measured it, but I'd guess .5 to maybe 3 pounds depending upon how full the tank is.

[Synchro909]

By my calculations, about 1 psi as a max.

[hardtimes]

Originally Posted by Dtolson9

Does anyone know how many psi the gravity puts out on a model a?

Hey David,
Not very much !
I do not know how to measure ...not very much, but would guess that that is maybe less than 1psi.

BTW...I do not like electric fuel pump/fuel regulator/fuel pressure guage, hoses pipes clamps, etc, etc on my A.
There is an alternative to electric and/or gravity.
An OLD guy I know is going to 'teach' me how to use air pump and what's involved to put 2/3 psi on my gas, like he did when he raced. Sounds pretty simple and no unsightly equipment that may or may not work

[Tom Wesenberg]

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It only gives about 1/2 PSI at best from gravity.

[Jim/GA]

The pressure at the carburetor inlet depends on the fuel level in your gas tank.

You can measure the vertical height from inlet to top of gas tank to get maximum pressure.

Pressure(psi) = Height (inches) x 0.026 psi/inch

You can also see what the pressure goes down to with 1/4 tank of gas or less by measuring carb inlet to bottom of tank.

I'm too lazy to walk out to my shop and measure actual heights. You all can figure it out.

[mike657894]

if im understanding the equation 10in of fuel would give .26 psi. my math homework often got the red pen though.

[40 Deluxe]

Doesn't any one have an accurate low pressure gauge to Tee in at the fuel inlet? Seems like all we have are vague guesses based on presumed/assumed knowledge.

[Tom Wesenberg]

Originally Posted by 40 Deluxe

Doesn't any one have an accurate low pressure gauge to Tee in at the fuel inlet? Seems like all we have are vague guesses based on presumed/assumed knowledge.

I have a very low pressure gauge, but it measures the pressure in inches.

Air pressure at 14.7 PSI will support a column of water at 33.8 feet. Gasoline has a specific gravity of .74, so someone can do the math and see what the pressure of gasoline at 1 foot would be.

.32 PSI per foot of gasoline, so a full tank would be just under 1/2 PSI.

[Dtolson9]

Thanks for all the input. I'm really trying to figure out if I need to run a fuel pump and regulator for a single 94 or 97 on my downdraft. Stock engine

[Richard in Anaheim CA]

Depends on distance between carburetor input and tank outlet "when installed" in car. Intake manifolds can vary tremendously. Most 94s & 97s need a fuel pump.

I run a Weber 32/36 without a fuel pump. Most run a pump. The Weber is shorter than your choices and my manifold is home made with very little riser.

At the bottom of Pikes Peak, gas guage showed 1/4 tank. I found a gas station and filled up. Never missed a beat.

Richard
Anaheim CA

[Dtolson9]

When I put the 94 everything will be stock. Fuel line, tank locations etc. the stock fuel line will hook to a 94. Cragar manifold

[Jim/GA]

Originally Posted by Dtolson9

When I put the 94 everything will be stock. Fuel line, tank locations etc. the stock fuel line will hook to a 94. Cragar manifold

You better run with a full tank of gas all the time then!

Originally Posted by mike657894

if im understanding the equation 10in of fuel would give .26 psi. my math homework often got the red pen though.

You are correct. That's what it is. And 10 inches does not get you up very far from the inlet of a stock Zenith carb. A 3/4 to full tank of gas will probably be up about 20" or so, in which case you can double it to a whopping 0.52 psi.

Move the carb to the top of the manifold for a downdraft and you'll be lucky to get 10" of head between carb inlet and fuel level in tank, even with a full tank.

This answer is based on an assumed average or typical density of gasoline (although it does change with temperature) that is equivalent to 0.026 psi per inch of head (or 0.312 psi per foot of head; compare to water which is 0.433 psi/foot). The only thing I don't know is how full your gas tank is and where you have mounted your carburetor (where the inlet is). But if you know how to work a yardstick, you can go measure from inlet to fuel level and quickly figure it out. It does not matter what your elevation is (sea level or mountain peak).

Reporting an actual pressure reading with my low pressure gauge on my stock Model A with 3/4 full gas tank would not have helped the original poster answer his question, because he does not plan to mount a stock carb on a stock manifold. My answer lets him go measure from various fuel levels in tank to his carb inlet, as he plans to install it, and calculate the pressures he will get at his carb.

.

[Jim/GA]

Originally Posted by Richard in Anaheim CA

Depends on distance between carburetor input and tank outlet "when installed" in car. Intake manifolds can vary tremendously. Most 94s & 97s need a fuel pump.

<snip>

Richard
Anaheim CA

No, Richard, not distance between carb inlet and tank outlet... distance from installed carb inlet and fuel level in the tank at the time (full tank, 3/4 tank, 1/2 tank, etc.).

You have the right idea, though.

.

[BILL WILLIAMSON]

Even back in the '50s, we decided a fuel pump is a MUST, with a downdraft!
Bill W.

[hardtimes]

Originally Posted by Dtolson9

Thanks for all the input. I'm really trying to figure out if I need to run a fuel pump and regulator for a single 94 or 97 on my downdraft. Stock engine

Hey David,
I've run two 81s for many years and even now. Last 5 years without any fuel pump.
I experimented with pumps/regs/etc. Also , have had pumps/regulators fail ! Besides failing equipment, this stuff does not have the clean look that I like in the engine bay.
So, I experimented without fuel pump and the other associated equipment.
I found that on flatland (San Fern Vly), that I did not run out of gas till tank was nearly empty.

It must be pointed up here that I'm running a '30 A, which has a higher tank than '28/'29. So, figure that in also.

I figured if that is the case on flatland, I'd just keep half tank or more in my gas tank and should be alright for hilly areas also. It has worked out that way exactly. Have gone over mountains around valley with lots of gas in tank with no problems. Just went 40 miles up in La Canada hills. You probably know that they are steep. However, if a pump makes you feel more secure over the long run, then install one. Now problem becomes, which pump to buy that fits your needs and application that is RELIABLE..good luck

[amodel25]

Not sure about gasoline but sea water is .4463 psi per foot.

[CarlG]

I have run both Webers & Stromberg 97 with & without fuel pumps. They all worked great at idle and around town without the pump. However at sustained speed and in any kind of incline, even with a full tank of fuel would run out of gas frequently.

[Big hammer]

Some antique aircraft with gravity flow fuel system use ram air to pressurize the tanks !
The gas caps have a small diameter tube coming out the top in a 90* bend. How much air pressure would be created at 30-40 mph ? Might be worth a try !