Geico266

With the recent Walbro threads consuming more server space than the infamous Conveyor Belt -v- Airplane thread I thought I would start a thread about why relocating the stock LP either near the tank helps. It has been alleged that the location of a pump (sucking or pushing) makes no difference.

If there is no difference in the pump pushing or sucking then why is there improvement in performance when guys move the stock LP near the the tank? Why is there an increase in pressure between a full tank of fuel and an empty tank? Why when I hit the brakes I can see a 1 PSI increase?

Ph4tty

Well there's really no question the pump will experience less vibration on the frame than on the block and that kind of vibration can't be good for the poor LP.
I don't think you'll see much discussion on this one. From what I've seen posted a relocated pump will offer better pressure at WOT than one in stock location even with upgraded lines/banjo bolts.

I haven't followed the walbro threads, but did you say performance gain? I thought most aftermarket lp's supply more fuel than the VP44 can use- so where's the gain?

Superduty

"Why is there an increase in pressure between a full tank of fuel and an empty tank?"

Because you have about an extra foot of static pressure head feeding the entire system. And that allows the feeble stock pump to supply more pressure.

If you put your fuel tank on your roof you would see even more pressure.

Geico266

So the location of the pump is important? So it doen't have to work so hard pulling right?

Superduty

NO. The pump is supplying the same pressure difference between its inlet and its outlet in both situations. The only difference is if you put the friggin fuel tank on the roof you the pressure at the pump inlet is +3 PSI (for example) whereas if you buried the tank in the ground the pressure at the ground the pressure at the inlet would be -3 PSI. When your tank is full of fuel (versus empty) you aren't moving the pump, you are moving the tank.

Lets say your pump is capable of generating 12 PSI from inlet to outlet. When you put the tank on the roof you get 3 PSI + 12 PSI = 15 PSI at the outlet. When you bury the tank in the ground you get -3 + 12 = 9 PSI.

See how it works ?

If you want to get really fancy, put the tank about 32 feet above the VP44, throw the lift pump away and you'll get 14.7 PSI of pressure. Then your pump really doesn't have to work.

I'm sorry I jumped in your thread, but I had to kill the "tank is full versus empty" myth before it caught fire.

Superduty

No, never. I am a non violent man. I meant it metaphorically.

Superduty

I give up.

kandgo

the recent Walbro threads consuming more server space than the infamous Conveyor belt -v- Airplane thread Thats just too funny!!!! Hang in there Geico!! Goodluck,,,Rick

csutton7

electric pumps are pushers not suckers and anytime you try and make them suck fuel you shorten their life, so the shorter they suck the better they like it, but it's still not the cure all--a tank on the roof is the cure all--next best thing is a truely 100% gravity fed pump, which can be done by punching a hole in the bottom of your tank like I did or installing a bed mounted tank and using it as the main tank and the OEM tank as the storage then pumping the fuel from the OEM tank up into the bed mounted tank when needed---chris

Geico266

So the location of a pump is important right?

Superduty

OK... lets tackle this pump placement myth.

Lets call the VP44 elevation zero feet.

If we throw away the lift pump and mount the fuel tank 32 feet in the air, we get 14.7 PSI at the VP44. Does everyone understand that ? Lets call that pressure 32 feet of head.

Now... lets say we mount the tank 10 feet in the air. Lets say the lift pump puts out 12 PSI from inlet to outlet.

Situation "a": the lift pump is mounted right by the VP44. What is the VP44 pressure ?
Answer: 10 feet of head + 12 PSI.

Situation "b": the lift pump is mounted 5 feet above the VP44. What is the VP44 pressure ? Well, we've got 5 feet of head feeding the inlet of the lift pump plus 12 PSI plus another 5 feet of head below the lift pump, so we get 10 feet of head plus 12 PSI.

Situation "c": the lift pump is mounted 5 feet ABOVE the fuel tank. What is the VP44 pressure ? Ans: Well we have an inlet pressure of minus 5 feet plus 12 PSI plus 15 feet. So the total is 10 feet of head plus 12 PSI.

Situation "d": the lift pump is mounted 5 feet BELOW the VP44. What is the VP44 pressure ? Ans: we have 15 feet of head feeding the lift pump plus 12 PSI, minus the 5 feet up we have to push the fuel to feed the VP44. Total is 15 feet + 12 PSI = 5 feet = 10 feet plus 12 PSI.

See ? 4 situations of different lift pump mounting and the same pressure each time.

Geico266

Man, SD I gotta call you on this it simple is not true. You are taking 14.7 (sea level pressure) and applying it as an answer to a head calculation. It simply is not done that way.

Superduty

As long as the inlet absolute pressure remains reasonable, a pump is just a pressure adder. It doesn't care if the inlet pressure is -5 PSI or +10 PSI. It just adds as much pressure as its motor will allow and that is it. The motor on the pump don't know and don't care if the inlet pressure is 10 PSIA or 20 PSIA. It just spins the vanes until it runs out of power and the net output is 12 PSI from the inlet of the pump to the outlet.

If the stock lift pump makes 12 PSI from inlet to outlet and you feed it a 5 PSI vacuum the outlet pressure will be 7 PSI. If you feed it 5PSI with another pump, like a Holley, you'll get 17 PSI.

kandgo

Yes Geico it does matter you are thinking right. ! I am not going to get into educating the ford guy but there is more to it than head pressure, there is the most overlooked criteria in figuring movement of a liquid and that is friction loss, and the pump being closer to the tank in your case would help as it is not pulling so far as it is pushing the fuel at a more even and consitant rate. Goodluck Rick Ps. I was also taught not to argue with a fence post.

Superduty

It sure is.

The pressure at any point in a fluid system is the sum of its static pressure, its velocity pressure and the elevation pressure.

If you really want to argue this out then I have to introduce something called fluid dynamics. Man am I sick and tired of fighting with people about friggin fuel pumps ! But what choice do I have ? Either I explain this whole thing and drive you in the <metaphorical> ground doing it or the myth that the placement of the fuel pump makes a difference is going to live.

So... which would you like ? Would you like to accept the fact that the placement of the pump doesn't matter (and didn't in the last example) or do I have to beat it into you <metaphorically>?

I have just never seen anything like this.