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

everyone SWEARS you HAVE to run 1/2" line to get any performance at all.

Not true at all.

Without reading all this... Simply put the reason why you want to run 1/2" lines isn't for performance reasons. The reason for 1/2" lines is because of volume stability. With 1/2" lines you'll only see a 2-3 PSI drop from Idle to WOT on the highway under load. Where 3/8" lines is more like 3-5 PSI drop and stock lines can be as much as 5-8 PSI drop. All them will perform just fine but the life span of the VP44 is at risk if the pressure is falling below 14 PSI. More about the VP44 requirements... http://articles.mopar1973man.com/2nd-generation-24v-dodge-cummins/25-fuel-system/415-bosch-vp44-injection-pump-requirements

What Mike states is true it is all about available volume!! The more volume you have the steadier the pressure. I run a FASS DDRP this is a 73 gph pump, 15 psi at idle, 12 at WOT pulling a 6% grade with 11,000# in tow, that is why I have 1/2 lines!

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

Bring on the tests! I have my own opinions on this matter but 1 good test is worth 1000 expert opinions. I'm sure this topic has been debated for years with little more than pressure differences so the more tests you can do the better. Not trying to stomp on anyones toes but we don't exactly have flow rate proof from the return line so this could be interesting. Make sure to list every variable you know so it can be a good legit test that others can derive ideas from

I'm a landscaper. I've studied irrigation and the effect on volume of water available through pipe. Concepts apply here too, just not able to say how big is big enough. You are dealing with two factors. Friction loss through pipe and through angle fittings. When you reach a certain point, you can't force any more liquid through pipe without dropping pressure. So bigger is better. Bigger means retaining more of the initial pressure. There is static and working pressure. Pressure measured standing still will be higher than when it's moving due to friction loss. Our trucks have a return line so the fuel is always moving. I'd wonder about increasing the size of the return line too. Right angles also will cause volume loss. Limited restrictions such as a 4" chunk of 3/8 line in the middle of a 1/2" line have a limited effect on volume vs larger restrictions. For this reason, I'd guess that the short run between the fuel filter and the VP is a low priority concern, except for the right angles and banjo bolts. Changing out the fittings will have a higher effect than increasing line size. Rambling thoughts.

A 3 gallon pressure accumulator tank between the filter and the vp44 would also increase available voluum but where do you draw the line. 3/8 banjos , with drilled out banjo bolts and 3/8 line from the tank forward is a 250% increase in voluum over stock.

Bring on the tests!

Done... Stock fuel pump with stock 6mm lines and 1/2" lines. http://articles.mopar1973man.com/general-cummins/36-fuel-system/137-vulcan-performance-big-line-kit [TABLE=align: center] [TR] [TD=class: moparnote, align: center]Running Mode[/TD] [TD=class: moparnote, align: center] Factory Lines[/TD] [TD=class: moparnote, width: 34%, align: center] Vulcan Performance[/TD] [/TR] [TR] [TD=width: 33%] Starter Bump[/TD] [TD=width: 33%, align: center] 14.0 PSI[/TD] [TD=width: 34%, align: center] 15.0 PSI[/TD] [/TR] [TR] [TD=width: 33%] Idling[/TD] [TD=width: 33%, align: center] 13.0 PSI[/TD] [TD=width: 34%, align: center] 14.0 PSI[/TD] [/TR] [TR] [TD=width: 33%] Cruise 65 MPH[/TD] [TD=width: 33%, align: center] 10.0-11.0 PSI[/TD] [TD=width: 34%, align: center] 12.5-13.0 PSI[/TD] [/TR] [TR] [TD=width: 33%] WOT[/TD] [TD=width: 33%, align: center] 7.5 PSI[/TD] [TD=width: 34%, align: center] 12.0 PSI[/TD] [/TR] [/TABLE]

Yes, done, but with a stock pump. Mine is putting out 23 lbs. at idle and 18 at WOT with the Quad on. 5 lb. drop but well over the safe limit, on all stock lines and fittings and filter.I don't think you can compare rotary vane pumps with gear rotor pumps. Totally different principals.

Beat to death :banghead: and 3/8 is more than enough on our rigs unless you are making well over 600 hp and drag racing on a regular basis where inertia plays factor on things in the equation.The only place I ever used 1/2 inch was between the filter and VP and that was just the ease of adding other things like 2 gauge ports ect.Research some of my past posts on this subject and what I see run on 1/2 inch hose daily, a 69 L 4210 Cubic inch 3516 V16 2000 hp Cat engines in 200 ton cat 789c end dump haul trucks that drive just like any other automatic tranny vehicle.And our little 5.9 l 359 cubic inch engine HAS TO HAVE 1/2 inch lines????

So we got Mike's test saying one thing, W&F saying another, and Rick contradicting everything. In any case, it is the flow that needs to be tested.. Mike told me that in the Bosch manual or something that the VP44 needs twice the flow rate recycling through the pump as it pumps in the injectors for cooling purposes. Meaning if the pump is shooting 5GPH to the injectors, then 10GPH needs to be recycling through the pump, so you need a flow rate of 15GPH total. Mike said the stock lift pump is rated at 32GPH? and those engines were rated at 235HP which is 13.9GPH x3 is 41.7GPH. This means the stock lift pump will sacrifice recycle flow in order to feed the injectors, which means the pump will heat up and you know the rest.. With that in mind, it is not the pressure that matters. Rick's overflow valve could be very strong for all I know and keeping the pressure up. It doesn't matter though, what matters is are these scenarios recycling twice the fuel? The ideal test would be to take the ball out of the overflow valve and put the return line in a bucket and bump the starter and time it filling up. Then put the 1/2" lines on it and do the same thing. I have issues with the overflow valve so this would put it out of the equation and purely test line restrictions. The reason we beat this to death is because such a test hasn't been done, or someone isn't sharing their results.

Mike told me that in the Bosch manual or something that the VP44 needs twice the flow rate recycling through the pump as it pumps in the injectors for cooling purposes.

Approximately 70% of supplied fuel is returned to the fuel tank. The valve opens at approximately 97 kPa (14 psi).

3/8" line might be fine. But it still comes back to pressure stability... Kind of why fire fighter don't put out house fires with garden hoses but pull a 2 1/2" line. It's about reserve volume and pressure stability.

So every fitting along the way of your 1/2 inch line is the same inside diameter of the line itself.................NOT!!!!You are only flowing as much as the fittings you use along the way will allow which I would bet are 3/8 at best if that. So unless you have #10 fittings at every connection you are wasting time on 1/2 inch line.There is a common sense factor here that people throw to the wind in a lot of cases.

Approximately 70% of supplied fuel is returned to the fuel tank. The valve opens at approximately 97 kPa (14 psi).

You told me it was for heat. That says nothing about heat, it's just stating a generalization. I know it needs to cool the pump off but that doesn't say it needs 70% to do it, its just yeah, a statement. Dig up more info relating to the heat. Do you got a pic of this 14psi valve? I can't figure out if there is an internal 14psi overflow in the VP44 AND an overflow valve somewhere else or if there is only the one. I know I know I don't pay attention to the 24V's. Alll the pieces are slowly coming together

You told me it was for heat. That says nothing about heat, it's just stating a generalization. I know it needs to cool the pump off but that doesn't say it needs 70% to do it, its just yeah, a statement. Dig up more info relating to the heat.

Do you got a pic of this 14psi valve? I can't figure out if there is an internal 14psi overflow in the VP44 AND an overflow valve somewhere else or if there is only the one. I know I know I don't pay attention to the 24V's.

Alll the pieces are slowly coming together

It is for heat and lubrication. The heat is the electronics that is the problem. The circuit board is heat sinked to the fuel side of the case.

Here is the full paragraph...

[h=2]Overflow Valve Operation[/h] Fuel volume from the fuel transfer (lift) pump will always provide more fuel than the fuel injection pump requires. The overflow valve (a check valve) is used to route excess fuel through the fuel return line and back to the fuel tank. Approximately 70% of supplied fuel is returned to the fuel tank. The valve opens at approximately 97 kPa (14 psi). If the check valve within the assembly is sticking open, fuel drainage of the Bosch VP44 injection pump could cause hard starting. If a Diagnostic Trouble Code (DTC) has been stored for “P0168 decreased engine performance due to high injection pump fuel temperature”, the overflow valve may be stuck in closed position.

Yes. Here is the valve...

The Overflow valve...

x10 mag of the lower bled hole.

Interesting how when you guys have low fuel pressure you blame it on the pump and 12V guys blame it on the overflow. Clamp off the overflow and you will get your 20psi irregardless of the pump. 400HP only takes 22GPH so even the factory pump could support that. You said its for heat and lube, and we know that, but the FSM doesn't and it only states the 70% generalization. I want something that says "10GPH x RPM = return line flow rate". Because nothing actually says it NEEDS return flow, and THAT is the culprit to this whole thread because anyone can get all the pressure in the world with the right overflow valve. We all know it needs it for cooling and lube, but how much flow does it really need? That 70% could be at idle for all we know.We need solid proof to stomp out this thread. Nobody is going to let it rest until we have a solid answer, why else would it have already come up again.

ISX, You had mentioned that I might have a stronger spring in my overflow valve and that is why the higher pressure. That could be a reason but not in my case, here's why. When I first got the DDRP pump my pressures were 8lbs. at idle and 0 at WOT! Turns out that they sent them out with the wrong relief valve springs so Geno's sent me a new spring. With the new spring in the pump the pressures are now 23lbs. and 18 lbs.. The pressure in this case is being regulated by the bypass on the pump. The DDRP, as are most aftermarket pumps, is a gear rotor pump. If it is rated at 100 GPH it WILL flow 100GPH regardless of the pressure. Pressure is a byproduct if restriction in the line. In this case it is the overflow valve in the DDRP, you just have that little hole to force fuel through. A gear rotor pump is a positive displacement pump so if you deadhead the line you will get thousands of pounds of pressure! (Or something will break, more likely) So my pump is rated at 75 GPH and that is what it is flowing. Now the issue is how much is flowing through the VP-44 and how much is flowing through the bypass on the DDRP to maintain the set pressure. Without a flowmeter it will be hard to figure it out exactly but if you put a hose on the return line and run it into a gallon container and start the truck and measure how much you get in a minute and multiply by 60 you will have the GPH of the return line. At idle the engine use will be negligible. that will give a pretty good idea of how much is bypassing internally in the pump to maintain pressure. Very low tech but workable.RickP.S. I love throwing the contradictory monkey wrench into these formulas.:evilgrin:

As I mentioned before. I am pretty knowledgable on flow rates through pipes. Limited restrictions (3/8" net) stuck in 1/2" pipe have only a small effect on volume and pressure. By adding a fitting that's say net 3/8" in 1/2" pipe does NOT make the half inch pipe have 3/8 flow. I don't have the friction loss tables for these fittings and hose, but I've calculated many times for PVC pipe. I'm just suggesting that from my knowledge of irrigation friction loss is relevant to this issue. Major restriction in the banjo fittings does make a big difference in available volume.

Interesting how when you guys have low fuel pressure you blame it on the pump and 12V guys blame it on the overflow. Clamp off the overflow and you will get your 20psi irregardless of the pump. 400HP only takes 22GPH so even the factory pump could support that. You said its for heat and lube, and we know that, but the FSM doesn't and it only states the 70% generalization. I want something that says "10GPH x RPM = return line flow rate". Because nothing actually says it NEEDS return flow, and THAT is the culprit to this whole thread because anyone can get all the pressure in the world with the right overflow valve. We all know it needs it for cooling and lube, but how much flow does it really need? That 70% could be at idle for all we know. We need solid proof to stomp out this thread. Nobody is going to let it rest until we have a solid answer, why else would it have already come up again.

You probably wont find anything from the FSM that says anything about return flow, when the vp trucks first came out it was stated by Dodge (and initially believed) that 5-7 psi as all that was needed for the vp. Thousands of failed vp's later, everybody lost faith and started doing their own tests. The vps are a different animal and cant really be compared to other fuel systems. Some injection pumps dont even require a lp so saying "this engine only needs x amount" is really comparing apples to oranges. My truck currently has the stock fuel lines, and Im running an AD FRRp pump. I do notice the pressure pulls down about 3 to maybe 4 psi under heavy loads, so Ive adjusted the pump up the same amount to compensate to ensure it doesnt drop below 14 psi. So by turning up my pump Im running it harder to compensate. The use of larger lines does seem to make sense, even if its to put less of a strain on the lp and to keep the vp cool. Not to mention Fuel lines are much cheaper than pumps

1/2'' lines allows you more latitude to put on a pre-filter which all diesel engines benefit from.Cooler running lift pump too. And Mopar Mike, your numbers are dead nuts on with what my truck did with the stock pump going to half inch lines.Except I was zero pressure drop. I was a steady 14 all the time for some unknown reason.

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)