Fuel line size?
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SHOULD BE FINE IF ITS NOT BOOSTED
PROLLY STILL OK IF SMALL SHOT OF NOS TOO
PROLLY STILL OK IF SMALL SHOT OF NOS TOO
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Give us some details. Boosted, N/A, cubes, estimated HP. Where are you wanting to run the line from?
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134 Posts
Actually you can have too big a fuel line...if the line is too big and your car accelerates hard yet you don't have a massive fuel pump, you will have the additional weight of the fuel in the line pushing back and fighting the fuel pump which can cause cavitation.There is no "too big", only too small
I don't think that really starts to be an issue until you get into the -12 and larger lines though.
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what fuel pumps you running? I have a magnaflow 500 w filter and have -12 going into the pump from the cell and -10 to the regulator and -8 from regulator to carb
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Fuel cell to pump will be 12an. Pump Out I want to run the 10an to reg and 6 to carb. Motor is a 406 11 to 1 ported old brownfield heads cam I have no idea but its a hyd- flat tappet suppose to be around 560-580 lift.Carb will be a gen 3 Holley dominator 750 with a big shot plate hitting 150 hp.
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19,463 Posts
Not true. Pressure head stays the same no matter how large the lines are.
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Run it.
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134 Posts
There are a few guys at my track that have first hand experience with this issue, one being my engine builder. It has to do with the g-forces during acceleration causing the fuel to push towards the back of the car fighting against the pump.
Not the best reference but it states what I'm saying. Here is the link and at I quoted the paragraph stating it at the end.
http://www.hotrod.com/how-to/engine/ctrp-0406-fuel-systems/
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There are a few reasons why Pro Stock moved the fuel cell from the rear of the car to the front. One was G-forces.
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71,969 Posts
Should be fine as long as the pump is big enough to supply both circuits.
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Sorry, they are WRONG. Research the term "Static Head". I'll try to explain it as simply as possible.
The static head of 1 ft of water is .433psi. Converting for the density of gas, gas will have approximately .33psi. If you put the car's nose straight up in the air that will simulate "1g" acceleration. So let's say the fuel has to go 10ft up to the front of a car. That means the fuel pump will have to overcome 3.3psi of static head pressure.
No matter how large the diameter of the line is, that pressure does not change, only frictional pressure changes with line diameter. Small diameter, faster flow rate (ft/sec), more friction losses.
Another way to look at it is this: Reverse this situation and use a water tower with a water level at 100ft tall above the ground with a pipe running to the ground. If you put a pressure tap at the bottom of water tower, it will read 43.3psi (.433psi/ft of static head) - no matter what size the pipe diameter is.
Ever see a manometer used for pressure measurement? Put pressure on one leg and the other leg will rise a specific height depending on the fluid (typically mercury or water). The tubing diameter used varies between models. But as long as both legs are the same diameter, they all will all read the same.
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Sorry, there are other reasons that have nothing to do with "pressure because the diameter of the line is too large".
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I'm sorry but your theory is not sound...you are talking about only the pressure of the fuel being pushed forward and 1 times gravity so in that sense things would not change no matter the size of the line but you have to take into account the mass of the fuel, the fact that it is parallel to the ground as well as the acceleration forces are greater than 1 times gravity. I follow where you are coming from but if you follow your logic and read this information about centrifugal pumps in the link to follow, the issue at hand is that fuel pumps are only rated up to a certain head pressure but they do have a stall point. The larger fuel line is going to have more fuel in it which then will have more mass and as acceleration increases that mass will act as additional pressure/force back against the centrifugal pump and at some point it will exceed the "head of the pump" and stall out the fluid flow. Also, after reading a little more on how the centrifugal pump works, it is creating kinetic energy in the flow of the fuel which is also what is happening when the vehicle is accelerating. If the pump can not overcome that "reverse" kinetic energy which is dependent on the mass of the fuel(larger line has more fuel and overall more mass) and the movement which in this case is the acceleration of the car, then the pump looses that battle.
http://www.engineeringtoolbox.com/centrifugal-pumps-d_54.html
http://www.engineeringtoolbox.com/centrifugal-pumps-d_54.html
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What you're referring to is STATIC pressure head. While your cars nose is straight up in the air, and after you've taken your pressure reading........strap a rocket to the car and lauch it straight up.........then take another reading of the pressure and get back to us with the reading of what multiple g's do to the DYNAMIC pressure head. Or......stick your hand out the car window at 65 mph, feel the static pressure head.......then mat it up to 100 mph and let us know what happens. A well-sorted car can easily surpass 2 g's at launch, and that will assuredly affect line pressure and head pressures at the pump AND regulator based on the mass of fuel in the line. F=ma.
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I don't need to look at that link, I "passed" the test regarding pumps 3 times - once to get my Engineering degree and twice to get my Professional Engineering license. And what I have written is not based on a theory, it's based on the Engineering principals involved with fluid dynamics. And fluid dynamics is used to determine the pump characteristics.
Let's discuss fluid dynamics. The total "dynamic head" is the static head, suction head and frictional losses. Suction head is related to feeding the pump so we will assume it is the same for this example.
To determine static head during acceleration, we simply use the highest "g" and calculate the static head based on that (this is called "modeling"). In the case of a car accelerating at 2g, we simply multiply the static "1g" x 2. But the resultant pressure stays the SAME no matter what line size. So yes, acceleration affects the static pressure, but the affect it is the same for all diameter lines. So a line with 1sq-in will have the same static pressure as a 10sq-in line. (I understand the concept is "static head" pressure is counter-intuitive in some ways but the pressures are the same in large and small lines.)
Say we have to move fuel at 10lbs/min. So what happens when we do it in a line with an inside area of 1"sq-in vs. one with 10"sq-in. With the 1"sq-in line I would have to accelerate the fuel to 10x the velocity of the 10" line. (I assume everyone can figure out the math involved to calculate the velocity). So now we have fluid in the small line moving at 10x the velocity as the one in the large line. I hope everyone agrees that the frictional losses are greater for the smaller line.
So now we have a 1"sq-in line and a 10"sq-in line with the same static head but the 1"sq-in line has more friction so it will have the highest "dynamic head" - the most pressure to overcome.
Kinetic energy was mentioned. The formula for kinetic energy is 1/2mv2. Which line has the most kinetic energy? It should be obvious since we're moving the same mass in both cases and the smaller line has 10x the velocity, then it will be have 100x the kinetic energy. So it also take more energy just to accelerate the fuel mass to that flow rate.
Going back to "static pressure head". One quick example to demonstrate the concept. Take small pond and a large lake, both very deep. The small pond would be like a small diameter line, the large lake like a large diameter line. Water in each is "accelerating" at 1g.
If a diver goes down 100ft, he will experience the same pressure in both bodies of water.
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The only difference is the surface area of the line size difference, which when pressurized would compare like a hydraulic cylinder. Bigger cylinder at the same psi will have more force because there is more square inch. Just like a 600cid engine will make more power than 400cid engine at same cyl pressure, rpm, design, proportions, etc.
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Sorry, but that comparison is not really relevant to the dynamics of fluid flow which is used as the basis of fluid line sizing.
It's really quite basic - if you haven't worked with dynamic fluid flow calculations, then it may seem logical that the fluid in a larger line would have more pressure because it "pushes back" harder. But just like my original example with a 100ft tall water tower. If it has a 1ft diameter pipe, the head pressure measure at a tap on the bottom is the same as if it were a 1" diameter line.
A real world example is this - in a 20 story high rise buildings why don't they use 1/8" lines to pump water to the top floor? If would be cheaper/easier than the 3-4" lines they use. The reason is because the fluid in BOTH lines have to overcome the same gravity, but the larger line has less frictional losses.
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