Fuel pressure.

[mycar]

I have a question about fuel pressure.

I have a fuel tank and out of the bottom of the fuel tank comes a fuel line which feeds a pressure gauge.

The fuel tank is above the pressure gauge, we'll say it's a meter above the pressure gauge.

If the fuel tank is filled one quarter full, the pressure gauge will show a value, what I think is called a static head.

If the tank is then filled to the top, will there be an increased reading on the pressure gauge ?

[Bruce M]

Yes.

[Gary71]

As Bruce said.

Static head.

Not as good as dynamic head obviously, but still head.

[mycar]

Thanks.

Is it the weight of the extra fuel or is it the increased height of the extra fuel that increases the pressure ?

[MT]

Height. 2.31 ft of water or approx. 3.3ft of petrol = 1psi.
..... so not a lot.

Mick

[Bruce M]

I think of it more as "depth" cause it make sense in my head

If you swim down roughly 10m in water, the pressure increases by 1 bar or 12.5psi. This can be in a lake, swimming pool, very deep bath or even a tube (if you are skinny enough).

[fetuhoe]

Both the height of the head and the density of the fluid have an impact as it is the mass of the 'column' that produces the pressure - hence old imperial units were often quoted in inches of Water.

1 atmosphere is about 34 feet using water and about 2.5 ft for Mercury in a traditional barometer.

[MT]

To tie it all together ....
1atm = 14.7psi x 2.31ft of water = 33.57ft of water.
Petrol with SG of ~ 0.7 translates 2.31ft into 3.3ft per psi.

Simples

[mycar]

Thanks, I'm trying to understand the types of pressures involved in a 356 fuel system and how much that pressure is increased by filling the tank to the top.

It has been put to me that filling the tank with 40 litres of fuel will increase the pressure significantly because 40 litres of fuel weighs 30 kg.

So initially I was thinking that a full tank of 40 litres of fuel would create 4 times as much pressure as a tank containing just 10 litres of fuel but that isn't the case is it ? The difference in pressure between 1/4 full and full full isn't the weight of the extra 30 litres of fuel, it's actually the difference in height of the fuel. In a 356 tank that's about 6 inches, which, if I've got my thinking right, is a minuscule increase in fuel pressure ?

I'm just trying to get my head around this generally accepted view that if you fill a 356 fuel tank to the top, you're just asking for trouble because the pressures involved are so great they are just bound to overcome the needle valve in the carburetor.

I know it's just easier to turn the petcock off but I want to be able to forget to turn my petcock off without having the cylinders fill up with petrol.

The next consideration is levels. There's a very good diagram somewhere showing a cut through section of a 356 showing how much higher the petrol tank is than the carburetors... but I can't find it at the moment.

[hot66]

I'm just trying to get my head around this generally accepted view that if you fill a 356 fuel tank to the top, you're just asking for trouble because the pressures involved are so great they are just bound to overcome the needle valve in the carburetor.

Just my thoughts on a monday morning :

If this had been a real issue when the cars were new, then I can't imagine Porsche and their engineering focus wouldn't have designed it out on subsequent updates. Are pre A or B & C owners reporting the same problems ?

re the needle valves , what pressure does the fuel pump supply at ? This shoud give an indication of the pressure range the needle valves need to work up to ? Work back from that and calculating the head of a full tank of fuel and you should be able to see if in theory the needle valves will hold when the tank is full and the car parked flat

[fetuhoe]

The pump will change what is going on as the design of the pump will affect the delivery pressure on the outlet because the pump basically sucks.

See here:

http://www.pumpschool.com/applications/npsh.pdf

[hot66]

yes, but do you see what I'm getting at ... say the pump supplies fuel at 3psi to the carbs, then the inlet valves on the carbs must be able to open at say 2.5psi for the bowls to fill. If the needle valves have a spec of 2.5psi, any pressure below that and they should stay shut. So , if the head caused by a full tank is 2.2psi , result = no leaks into carb bowl... if head is 2.7 psi, needle valves open and bowls keep filling until head drops below 2.5 psi ?

So in this case , is the needle valves are rated at 2.5, and the head is only 2.2 .. but the bowls are still filling then that points to incorrect spec or failed needle valves

All these values are theoretical / made up by me , but thats how i imagine the principle ? Or am I missing something ?

[fetuhoe]

The head doesn't get past the pump so its really not relevant.

You simply won't have any fuel pressure until the pump delivers fuel.

If I understand a 356 has a mechanical diaphragm pump and the float level in the carbs is above the position of the fuel pump..

The internal suction valve in the fuel pump should prevent any fuel from flowing, unless the pump is being driven, due the 'inlet' head.

Diaphragm pumps tend to self regulate delivery pressure depending on the internal spring and the level pivot lengths so thre is no real method of increasing the deliver pressure with this type of pump.

[Gary71]

The principal is sound, but I'd really hope the margins on the actual numbers are a bit wider than that!

Also does the pump offer any restriction when it's off?

[hot66]

The head doesn't get past the pump so its really not relevant.

You simply won't have any fuel pressure until the pump delivers fuel.

If I understand a 356 has a mechanical diaphragm pump and the float level in the carbs is above the position of the fuel pump..

The internal suction valve in the fuel pump should prevent any fuel from flowing, unless the pump is being driven, due the 'inlet' head.

Diaphragm pumps tend to self regulate delivery pressure depending on the internal spring and the level pivot lengths so thre is no real method of increasing the deliver pressure with this type of pump.

so this is just like my old VW's then ...

which brings it back to how come the carbs are filling and overflowing if the fuel can't get past the diaphram pump ? ... unless there lies a fault / leak past with the pump too ?