Technical Fuel line size effect on pressure?

Yep... I am trying to restrict myself and keep my input simple. I do have a tendency to wonder off the subject.


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Perhaps you are a analytical/detail oriented person too......my family and friends would describe me as “ask him what time it is and he’ll tell you how to make a watch”........I’ve learned to look for the telltale “eyes glazing over” and try to move on at first indication.....

 

I did this experiment a few years back using a drip/seep type of irrigation hose . I hooked one 20' hose to the end of a regular garden hose, which was hooked to my out side spigot. 1/2" copper line to the spigot , 5/8" hoses, with house pressure varying between 40-55 lbs. the first irrigation hose had a cap at the the end, that, when removed water would come out, when I hooked a second hose up, water barely trickled out the end, third hose......nothing out the end. I guess it would have taken more volume to run the third hose effectively? the pressure never went below the 40lbs in the house, and the first 2/3 of the plants [carburetors] had plenty of water, [fuel].

 

A motor (yeah, I said motor) needs .5 lbs of fuel per horsepower hour. One could convert gallons to lbs (8lbs/gal I think), then the desired HP at max multiplied by .5 , where:

500 HP=250lbs per HP hr

at 8lbs/gal thats just short of 32GPH.

But, But, BUT, every fitting, foot of fuel line, current delivered to the electric pump, gravity, all cause a drop in the required delivery or volume by the time it's sprayed into the engine. This why race pumps have such high outputs like 150, 180, 250, even 400 GPH ratings. And yes, the pressures that volume gets delivered is also important. Fuel injection is even more complicated and always requires the return system alluded to above just to maintain the high pressures needed at the injectors. In the glory days of mechanical race injection that was "the pill" tuners referred to when changing mixtures to go lean (bigger pill) or rich (smaller).

"WTF does all this have to do with line size Highlander!?!"
Same/same, the basics, just lower volumes and pressures. A big line needs a bigger pump to pull or push the fuel to the engine. Using a pusher pump out back that likes a 5/16" line will work harder and likely run warmer than normal, and a mechanical that pulls might eventually strain the diaphragm to the point of failure if line size was reduced. If the input feed line was increased it might not have enough ass to pull it from the tank.

This would be a better topic if the desired goal was known. If more pressure is needed, why? Did output get bumped up (more HP)? More carbs? Delivery issue? I guess I'm in the same church pew 31Vick is in.

 

Get a long silly straw and suck up some soda.
Get the same length of garden hose and try, see how much harder you’ll need to work.

 

Ha, same for my friends and family, and my coworkers, too.

 

You guys are like a never ending game of trivial pursuit , lot of info to get to the answer of a yes or no question ....yawn !

 

Politicians have reduced that to an art form.

 

But that is just the point. Most questions asked here are not adequately answered by a simple 'yes' or 'no'................unfortunately....

 

Exactly.

 

You would hate to learn something along the way....


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According to whom ???

 

Why.....by those who know, of course!

 

Exactly!

 

Yes.

 

My high school buddies' dad would say "I'm gonna find a woman who can suck a golf ball through a garden hose".
Never understood whether this would be considered a pressure debate or a volume debate.

 

Neither.

 

At least you're maintaining a sense of humor , now we gotta work on humility
BTW , most on here ask the wrong questions ..LOL

 

When I was running three Rochester H carburetors ( Corvair units ) on my 250 inline six, it always ran richer then I liked. If I leaned out the jets, it I would loose power at higher rpms. One of the carb inlets was leaking, so I pulled all the fuel lines and made new ones since I had three 1/4" inlets, the old ones were 5/16". Once I replaced the fuel line with smaller 1/4" line, I was able to increase the jet size back up and have decent power at all rpms without any overfueling or fuel rich smells. It took me awhile a figure out why it ran better. The larger fuel line had enough pressure to overpower the floats a little bit, the smaller line didn't.

 

Her name was Mama Judy and she hung out at Ruth's Bar in Rosemead, CA, in 1968. Pressure or volume made no difference to Judy.

 

My first thought was if you increase the pressure you must raise the temperature it takes for the fuel to vapor lock right? Great discussion!

 

When I was asking about my well pump pipe diameter something about friction relating to pipe size entered the discussion, but I can't pretend to understand any of this.

 

For long runs line friction becomes a factor and you will use bigger pipe for irrigation systems.


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That would be a debate about consistancy or performance...

 

Did you change all or just the carb lines? If just the carb lines the volume was adequately fed by the 5/16 main line, but also reduced as needed at the carbs. Volume and pressure was easier to manage at the inlets. Makes sense. I ran a 3/8 to a block of lines, 1/4", on a GTO tri-power that the race shop was fighting with. Was able to provide a constant 5PSI at the inlets based on the gauge at the end of the block. Always had full pressure even under high loads (WFO).

 

Forget all the theory. An electric fuel pump basically pumps fuel in a circle...from the tank thru a tube and back to the tank. The reason it does this is because the electric pump needs fuel to flow thru it and keep it cool. Some newer pumps may have exceptions but in general that is what happens. Along the way it passes near the engine and a portion of the fuel is diverted for use by the engine. At this point you have fuel flowing (volume) but basically there is no pressure (humor me). In order to develop pressure to accompany the volume a "pressure regulator" is inserted into the circle at a point after the fuel has passed the engine. (Yes I know this isn't exactly correct, but humor me for explanation purposes) So now the fuel is pumped in a circle and we have a "pressure regulator" inserted just after the fuel has passed the engine but before it returns to the tank.
The regulator is simply a spring loaded diaphragm that resists the attempts of the pump to push fuel thru the line and back to the tank. Depending on the ability designed into the pump this resistance has to be overcome and it takes a certain amount of pump pressure to push the diaphragm off its seat and allow SOME fuel to return to the tank.
All of the line prior to the regulator will then be pressurized the same pressure. The fuel that bypasses the diaphragm will return to the tank with little or virtually no pressure.

Now, here is what is important to the OP. In the high pressure area ahead of the pressure regulator, fuel is supplied to either fuel injectors or to a carb. The fuel injectors need a somewhat higher pressure to function as they spray fuel exactly like a garden hose sprays water.
A carburetor on the other hand does not need (at this point) to spray or atomize the fuel. At this point it only needs to be able to move a float needle off its seat and allow fuel to rise into a float bowl. This is the same thing that happens in your commode when you flush.So the pressure needed is only a few lbs. All it has to do is push the needle against a very small spring with minimal spring pressure.
Now the electric pump will have plenty of capacity and since its pushing fuel thru the line it should be able to provide sufficient fuel easily. The fuel will stay cool since its continuously flowing. You simply need to purchase a pressure regulator that is adjustable so you can set it for your pressure needs. Maybe 4 lbs or so for a carb, and up to 60 or more lbs for injection.
If you are running an engine that produces less than about 450 hp, a 3/8 fuel line and fittings will probably be fine.
If you are building something with 500hp or more or using nitrous, you might want to consider a larger fuel line.

The basic older fuel systems for carbs have a mechanical pump on the cars engine. Fuel flows from the tank to the pump by gravity and then gets pressurized and pushed to the carbs float bowl. Still a 3/8 fuel line should satisfy most engines, but if you have really high HP I would use an electric pump. Also, its best to use an in tank pump rather than the ones that just get inserted into the rear area of the fuel line and outside the tank. One thing you don't want is too much pressure when using an electric pump with a carb because it will push the float needle off the seat continuously.
So now you need to consider the output of the pump compared to the size of the fuel line. If the pump is capable of say 60 psi and 100 gallons per hour....the size of the line CAN affect both flow volume and pressure. It is possible to have too large a fuel line. If the fuel line is too large, you will get the maximum 100 gph but your pressure may never build up because there is nothing to resist the flow......unless you have a pressure regulator.
On the other hand, if you have too small a fuel line, it can be restrictive just like a pressure regulator and cause some pressure build up...but you will never achieve the maximum flow rate.
Experience demonstrates that a 3/8 line will satisfy most fuel requirements for 450 HP or less without being too big.

 

I find this asinine nincompoopery often.
1/2 line with with fittings down to 1/4 actually 7/32 .


Wonder how that shit is supposed to work ?
Lots of non thinkers with a tool box

 

You never have pressure unless there is a restriction.

 

How about running a 110 g.p.h. pump through 1/2" line to a Holley reg to 3/8 line(s) when the reg has a .220 orofice ?

 

Here's the thing ......The whole system will be regulated by the smallest orifice in the complete system. Any source of resistance will cause resistance to flow. The regulator WILL and SHOULD be a source of resistance because it's job is to resist just enough to provide the needed pressure. It has two orifices to do this function. One is the inlet and needs to have both pressure and volume. It gets volume because the supplying fuel line has more capacity than the regulator can bypass thru it's second orifice....the outlet or return line. Return lines are usually smaller than supply lines. The point here is that if the supply line is too small it will also resist flow and may keep the pressure regulator from being able to do it's job properly.
I just finished building a home built paint booth/ storage building. When setting up the airlines I went to great lengths to run long air piping to allow the air to cool after being compressed. Because I know the long cooling run of pipe will also cause friction and resist air flow, I used 3/4" pipe. Now I know when the air gets to the water separtor and pressure regulator it will pass thru a somewhat smaller orifice.....so I bought one that had 3/4" pipe connections. This way I kept volume up because I know I want lots of airflow at low pressure for the HVLP spray gun. Now I know the hose I will be using is smaller than 3/4.....but it's proven to be capable of meeting the HVLP guns needs. Somewhat like 31 Vicky mentioned above....one of the things that many people do is to use a small orifice automotive type hose connection. If I had done that I would have totally negated all the previous effort to insure unrestricted volume. Look at industrial style connectors which have a different shape and much larger orifices. They are physically about the same size as the cheap automotive ones, but the hole thru the middle is about twice as large and flows about 3 times more air volume. And, if your other air tools don't perform as well as you would like, switch to the industrial size fittings and watch them.