Fuel line size, is bigger really better?

[Mopar1973Man]

Less demand volume. I've got more fueling mods than can demand more volume than your truck so of course I'll see a bit more pressure drop. (Edge Comp and RV275's)

[JAG1]

Second time running up to Vulcan we put on the draw straw and I gained about 1 psi but lost the 1 psi when I put on the 30 micron pre-filter.

[rickf]

If you want to cool the VP more with fuel you are going to have to open the return line hole up in that return valve. Now you get into the can of worms, how much pressure Does the pump itself actually require to run and how much is being used for cooling. The original specifications were written around the factory pump. We are mostly running much higher volume, positive displacement pumps. Pressure is not needed to achieve the volume like it was before. We can get very large volumes of fuel at lower pressures. I would be willing to bet that with a 100 GPH pump you could double the size of that hole in the return valve and still maintain 14 lbs. of pressure, plus have a massive amount of return flow to cool the pump. Now, let's make that cab of worms a little deeper, is there anything in the pump that is designed to actually transfer the heat from the computer to the fuel? A heat sink grid? I doubt it. Just smooth surfaces. You will reach a point of diminishing return at some point when you are moving enough fuel that it is not absorbing the heat that is there because the fuel is not staying there long enough to absorb heat. This will also happen in an engine which is run without a thermostat. The water flows through the radiator so fast it does not have time to give up its heat and therefor the engine overheats.I will make a concession to the 1/2" lines in one respect. If you want to keep the pressure low but still run the same volume then the bigger lines will make sense but as has been stated everything in the system has to be upgraded to the larger size, not just the lines. Keep in mind the inlet size of the VP-44, if it is 3/8" then it is all moot.

[diesel4life]

I dont want to speak for anybody, but I do believe Mike has never said running 1/2" lines is a MUST. Hes merely recommended it.Heat sink has been studied countless hours by certified bosch rebuilders and its been concluded long ago that is the number 1 killer of the vp. Now theres not many ways to keep them cool as they are attached to the engine and becoming hot is inevitable. They performed many tests over the years and could not come up with a viable and cost effective solution other than adding more volume. This isnt something that one person came up with overnight, its been studied and tested tirelessly over the years. The failure rate of the vp has dropped drastically since they were new, now what other contributing factors are there other than the fact that nearly everyone (yourself included) is running higher volume pumps. I can think of one, that being upgraded materials in the ip itself to prevent mechanical failures, but thats not the majority cause of failure. Are 1/2" lines a necessity? No theyre not, but it does make sense to me to increase the size enough to increase the volume to minimize pressure drop which will in turn allow me to dial my lp back a notch or 2 which can only prolong its life. Bottom line, if you feel the stock lines are adequate for your needs, than stick with them.

[JAG1]

rickf said:

If you want to cool the VP more with fuel you are going to have to open the return line hole up in that return valve. Now you get into the can of worms, how much pressure Does the pump itself actually require to run and how much is being used for cooling. The original specifications were written around the factory pump. We are mostly running much higher volume, positive displacement pumps. Pressure is not needed to achieve the volume like it was before. We can get very large volumes of fuel at lower pressures. I would be willing to bet that with a 100 GPH pump you could double the size of that hole in the return valve and still maintain 14 lbs. of pressure, plus have a massive amount of return flow to cool the pump. Now, let's make that cab of worms a little deeper, is there anything in the pump that is designed to actually transfer the heat from the computer to the fuel? A heat sink grid? I doubt it. Just smooth surfaces. You will reach a point of diminishing return at some point when you are moving enough fuel that it is not absorbing the heat that is there because the fuel is not staying there long enough to absorb heat. This will also happen in an engine which is run without a thermostat. The water flows through the radiator so fast it does not have time to give up its heat and therefor the engine overheats. I will make a concession to the 1/2" lines in one respect. If you want to keep the pressure low but still run the same volume then the bigger lines will make sense but as has been stated everything in the system has to be upgraded to the larger size, not just the lines. Keep in mind the inlet size of the VP-44, if it is 3/8" then it is all moot.

If moot.... then how do you explain the major improvement that Mopar Mike and I both saw after changing over to big lines? Both improvements were dead on the same using the stock pump. I can answer that........ just look at the size of the banjo bolts..... them things belong in the scrap heap.

Edited September 13, 2019 by JAG1

[Mopar1973Man]

Once again it not about volume. Its about pressure stability.

Like when I jump on a fire truck and hook up a 2 1/2 inch hose I put a nozzle on the end that might neck down to 1 1/2 inch internally. the reason for the larger hose is to keep pressure stability at the far end which its does even though the nozzle can't even consume the 2 1/2 inch line volume rate.

Another example I've got my irrigation system. Starts at 8 inch line at the dam wall and necks down to 4" as it travels to the splitter manifold. Then its divided in 1" lines to 3 yards. But in my yard I've got a high water demand because of the pelton wheel. Since the line neck down to 1" and all the demand volume required I had to install pressure gauge so I could measure my losses to the 1" inch line. So from a static pressure I've got roughly 116-118 PSI with everything running pelton wheel and all sprinklers I can drop to about 20-22 PSI. The pelton wheel is a loss at that point and doesn't produce. In this example the demand volume is greater than the supply volume and so pressure stability is lost.

So as you you can see larger lines give better pressure stability since the line volume out weighs the pump consumption volume so the pressure drop is minimized.

1/2" Line at 15 foot at 15 PSI can flow 570 GPH... I hardly doubt we could consume that... But the pressure is stable...

6mm stock lines without banjos can only flow 75 GPH with the same length and pressure. (Calculated flow rates using 3.0 cSt fuel)