I know that fuel line size has been beat to death here and everyone SWEARS you HAVE to run 1/2" line to get any performance at all. I am wondering why that is so. Think about this for a minute. Assuming you get 10 MPG fuel mileage, most of you get at least that much. At 10 MPG you use 6 gallons of fuel at 60 MPH. At 5 MPG you would use 12 gallons per hour and at 2.5 MPG!!! you would use 24 gallons per hour.Now, has anyone ever taken the line off of the VP-44 and put it into a gallon container and bypassed the pump for one minute? I am guessing that it would about fill the container in a minute. I have not tried it yet but I will once I relocate my pump. That would be 60 gallons per hour out of the stock lines. This, by the way is based on an upgraded pump, in my case a DDRP. I have not done this exact test yet but I will when I relocate my pump. I have forgotten to tighten the banjo fitting on the outlet of the filter and had the truck still start all the while blowing fuel EVERYWHERE! I know what you are going to say, "well it can't pump that much under pressure". This is true up to a point. Relief valve is set at 14# I believe and anything over that is bypassing back to the tank so yes it can if it can maintain the pressure. Also the pressure would only affect the pump output and not the line flow established in the previous test.So basically here is what I am saying. You have a pump rated at 70 GPH, lets cut that by 50% for the pressure loss, no way are you losing that much but just the hell of it. That leaves 35#. Let's also say you are getting 2.5 MPG. (You need a new/bigger truck!). That STILL leaves 11 more lbs. than you actually need. Note that these are very drastic figures and no way are you ever going to lose that much to line loss or are you going to get that kind of mileage. But the mileage would compensate for the WOT blasts. I see no real reason for needing 1/2 inch line other than peer pressure.Opinions?Rick

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.

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.

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.

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, 20195 yr by JAG1

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)