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Mike,I agree with what you are saying.Here is something else to think about on fuel pressures.The overflow valve is not a binary valve (just open or closed), it is a spring loaded ball check valve.It starts to open at 14.5 lbs pressure. The more pressure the more it opens.At 15psi it is open a little at 17 psi it is open a lot more.I don't think the volume of fuel flowing through at 15 psi is much at all.I'm thinking that 16-18 is better than14-15 (more volume).It would be interesting to test this theory with an adjustable fuel pump, pressure gauge, and the overflow check valve measuring the volume of fuel escaping from the valve at different pressures for 60 seconds.My :2cents: worth.Jim

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Ok so for the sake of debate. If you have a fuel system that will supply enough volume, and pressure. Why run the valve at all? Could it be removed and run full flow all the time? If my FASS, AD, MITUSA, ect. Supplies in excess of 90gph (1.5gpm). I would still have more than adequate fuel supply to cool the pump and feed the injectors.

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Ok so for the sake of debate. If you have a fuel system that will supply enough volume, and pressure. Why run the valve at all? Could it be removed and run full flow all the time? If my FASS, AD, MITUSA, ect. Supplies in excess of 90gph (1.5gpm). I would still have more than adequate fuel supply to cool the pump and feed the injectors.

Well you would have enough fuel flow to keep it cool that's for sure.

The question is would there be enough pressure to force feed the VP44 or would it just run out the return line? If you look at the overflow check valve there are two holes in the output side, one small and one larger. the small one flows fuel all the time, valve open or closed.

The larger one only flows fuel when the check ball is pushed off the seat by a pressure of 14.5 lbs or greater. There is a limit as to how much fuel can go through the holes at the max of 20 psi. The restricted holes ensure there is positive pressure in the VP44. I suspect that without a restriction in the return line it would be difficult to maintain adequate pressure in the VP44.

Kind of like a garden hose with and without without the nozzle. With the nozzle the line pressure is up without it the line pressure is lower.

But I really don't know without someone (not me) trying it on their truck and reporting back with the results.

Anyone want to volunteer? :lol:

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Mike, I agree with what you are saying. Here is something else to think about on fuel pressures. The overflow valve is not a binary valve (just open or closed), it is a spring loaded ball check valve. It starts to open at 14.5 lbs pressure. The more pressure the more it opens. At 15psi it is open a little at 17 psi it is open a lot more. I don't think the volume of fuel flowing through at 15 psi is much at all. I'm thinking that 16-18 is better than14-15 (more volume). It would be interesting to test this theory with an adjustable fuel pump, pressure gauge, and the overflow check valve measuring the volume of fuel escaping from the valve at different pressures for 60 seconds. My :2cents: worth. Jim

Yeah... i agree so far with you... But if your flow rate through the pump is too much wouldn't you think it would be like taking a thermostat out of the block. The fuel might race through so quick that it doesn't pull heat away from the pump and electronics. Your right about the valve not being binary... So true... When I was playing with the one I got here I was playing with the air gun and regulated air. But what I saw was a check ball that vibrated even at pressures as low as 15 PSI. Now the overflow valve I got is used so the characteristics could be a weak spring. Being that air isn't very dense it could be the cause of the vibration. But now put diesel thorugh it it should be steady (for the most part). But with compressed air it appeared that the overflow valve is complete open by 20 PSI... But now if your idle pressure is like mine its at 17-17.5 PSI. So that valve should be open and flowing fuel through the pump. But at WOT now the fuel is flowing out towards the injectors at a high rate (>15 GPH) so WOT isn't a really big issues with cooling and lubing that I can see. I would assume somewhere below 10 PSI is where the diaphragm damage comes in. So operating a VP44 down to 10 PSI isn't bad that I can see but the pump has to be flowing a good rate all the time to kepp the fuel moving for cooling lubing. So if you idle/coasting pressure is above 14 I can't see how the VP44 can get damaged in theory... Because after watching several thread on numerous forums you always see "Common Joe" stock fuel system and it idles at 12-14 PSI and WOT dips below 10 rather easy... Typically this person would be looking at a VP44 replacement in under 100K miles. But now you look at even the home brew setups where the owner threw a Holley Blue (stretched the spring) and added a Big Line Kit... No problems. The idle pressure is typically in the 15-18 bracket and the WOT will fall to as low as 8 PSI... But I've seen several of these guys go 100-200K miles and no problems... Might change pumps frequently but VP44 lives on...

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But if your flow rate through the pump is too much wouldn't you think it would be like taking a thermostat out of the block. The fuel might race through so quick that it doesn't pull heat away from the pump and electronics.

Hmmm... if the thermostat is removed from the block, does the engine overheat or does it over cool? I would think the fuel racing through the pump would still keep it cool, perhaps even better because of a greater temp differential between fuel and pump therefor better heat transfer.

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It is an illusion of being overcooled. I did it once and the gauge got to 140 after about 10 miles.. but never moved much over that either. So you would think it was cooling that good, BUT, the real fact is, it is going through the engine so fast, it can't grab heat. So then you just get hot spots and an engine that is overheated but you won't know it by your gauges.Run a copper water line through a 150F swimming pool really fast and the water on the other side will still be cold. Run it through slow, it might get up towards 150..It has to be the perfect speed, too slow and the water will get up to 230+ before it even reaches the radiator, too fast and you got the illusion effect. You notice a thermostat has a BIG limit on flow, look at how much flow goes through it when it's fully open compared to if you just took it out.. Your talking almost 3 times the flow now. The thermostat must stay in to control the flow to keep it perfect. So even a stuck open thermostat is much better than taking it out until you get a new one. Different story if it's stuck closed :lol:

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It is an illusion of being overcooled. I did it once and the gauge got to 140 after about 10 miles.. but never moved much over that either. So you would think it was cooling that good, BUT, the real fact is, it is going through the engine so fast, it can't grab heat. So then you just get hot spots and an engine that is overheated but you won't know it by your gauges. Run a water line through a 150F swimming pool really fast and the water on the other side will still be cold. Run it through slow, it might get up towards 150.. Think you know what I'm talking about now.

My thought exactly...

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It is an illusion of being overcooled. I did it once and the gauge got to 140 after about 10 miles.. but never moved much over that either. So you would think it was cooling that good, BUT, the real fact is, it is going through the engine so fast, it can't grab heat. So then you just get hot spots and an engine that is overheated but you won't know it by your gauges.

If you can't measure it with your gauges how do you know that's what's happening?

Run a copper water line through a 150F swimming pool really fast and the water on the other side will still be cold. Run it through slow, it might get up towards 150..

I think that in this illustration if you were to measure the water temp and volume of each example (fast line and slow line) and convert it to BTUs pulled from the pool you would find the # of BTUs would be nearly equal. The fast line feels colder because there are not as many BTUs transferred for the same volume of water because the heat transfer can only happen so fast, it is not instantaneous.

It has to be the perfect speed, too slow and the water will get up to 230+ before it even reaches the radiator, too fast and you got the illusion effect. You notice a thermostat has a BIG limit on flow, look at how much flow goes through it when it's fully open compared to if you just took it out.. Your talking almost 3 times the flow now. The thermostat must stay in to control the flow to keep it perfect. So even a stuck open thermostat is much better than taking it out until you get a new one. Different story if it's stuck closed :lol:

I agree with your reasoning here with the exception of what you call an illusion effect.

If the water goes through too fast it will cool the engine too much. Yes you will have hot spots but that is because the engine is running and the heat in the cyl (over 2,000 degrees on a diesel over 4,000 degrees in a gas engine) when it fires. These hot spots will be there whether the thermostat is in or out.

I'm not trying to be argumentative just trying to reason it out by the laws of physics.

To illustrate my point, the rocket engines used in the NASA space program have the nozzles cooled by running the fuel through tubing coiled around the nozzles to take the heat away so they don't melt. The fluid fuel is being pumped at the rate of over 6,000 gal per minute don't know the pressure but I'll bet it's higher than 20 psi, :lol: and it still takes the heat away. I must admit that the nozzle temps are a little above a 190 degree diesel engine block though. :lol:

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Hmmm... Valid thoughts... What did I stir up here??? :shrug: Well still in all the VP44 does have issues with either the electronics getting overheat and burning out or the mechanical part of it failing. I think the idea of the return line also plays a role in this too being that its stock old 6mm ID line and can only flow so much volume per time. So at some point or another you show hit a plateau where pressure increase doesn't cool any more because of the simple fact you can only get some much volume down that line. You got to remember the return rail for the injectors have got a small amount of fuel returning too. Maybe its a good debate happening and maybe we can figure out a better way to make these pumps last longer... :thumbup2:

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I'm not trying to be argumentative just trying to reason it out by the laws of physics. To illustrate my point, the rocket engines used in the NASA space program have the nozzles cooled by running the fuel through tubing coiled around the nozzles to take the heat away so they don't melt. The fluid fuel is being pumped at the rate of over 6,000 gal per minute don't know the pressure but I'll bet it's higher than 20 psi, :lol: and it still takes the heat away. I must admit that the nozzle temps are a little above a 190 degree diesel engine block though. :lol:

Hmmmmmm, your on to something. Keep arguing! I know results itself can be an illusion since it's not like I have an engine block temp gauge, only coolant temp. I am going to investigate this more and see what I can find out.

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These kinds of debates are always good because they provoke the thought process. There are only winners and no losers. :thumbup2:

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Well you would have enough fuel flow to keep it cool that's for sure.

The question is would there be enough pressure to force feed the VP44 or would it just run out the return line? If you look at the overflow check valve there are two holes in the output side, one small and one larger. the small one flows fuel all the time, valve open or closed.

The larger one only flows fuel when the check ball is pushed off the seat by a pressure of 14.5 lbs or greater. There is a limit as to how much fuel can go through the holes at the max of 20 psi. The restricted holes ensure there is positive pressure in the VP44. I suspect that without a restriction in the return line it would be difficult to maintain adequate pressure in the VP44.

Kind of like a garden hose with and without without the nozzle. With the nozzle the line pressure is up without it the line pressure is lower.

But I really don't know without someone (not me) trying it on their truck and reporting back with the results.

Anyone want to volunteer? :lol:

I like this discussion! Let's fuel this some more (pun intended). OK lets remember flow and pressure are NOT Synonymous. Using the garden hose analogy. You burn you finger and run to the hose and run the water on it. I have just cracked the faucet and have a high delivery pressure at the opening, but no flow. Your finger still hurts. I open the faucet wide open my line pressure drops dramatically BUT I have tremendous flow and my finger is cooled. Now instead of a high pressure 3'rd degree burn I have a high flow 1'st degree burn.

I fully agree with the point that Velocity of the cooling media is important to absorbing the latent heat stored in the material requiring the absence of heat. Looking back at our VP. Is the reason for the check valve to ensure adequate pressure in a system that has an inadequate supply system? Remember cost is the #1 reason components are chosen at the mass production level. So I supply a $15 lift pump and make it work with a $3 check valve (mass production costs) or build it to last and eliminate almost all failures with a $400 pump? I multiply this cost over 400K units produced. HUGE money so as a manufacturer I choose to roll the dice play limited warranty and give ya what we got. Only point I'm raising here is. Absolute requirement because it will work no other way, or engineered solution to have acceptable (debatable function)?

Does the VP pump really care what its supply pressure is? Or is it happy with having amply supply of fuel to never starve it? I would venture to say the outlet pressure will not change regardless of inlet pressure as long as I have not starved the inlet to the pump. So back to the original question If I supply 3-5X more flow than the system was originally supplied to the customer with does that change all the original operating parameter assumptions? Check valve used to keep supply at pump inlet because my original lift pump and barely supply adequate flow to the system that it indeed would starve inlet to pump due to return line having the path of least resistance?

Again all my points are to stir discussion. I can't say these points are correct or incorrect.

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Well hells bells!It's because the heat transfer is so good that the coolant is taking heat away so easily it keeps the engine extra cool.There is a somewhat counterintuitive principle at work here, which is probably why you are asking the question: The relationship between flow and delta-T (or heat removed) is not linear. A doubling of the flow rate does not result in a halving of the delta-T, it results in slightly more than half the delta-T and thus slightly more heat transfer than with the lower flow rate. This is because the approach temperature, the difference betwen the water temperature and the air temperature, is larger when the flow is higher and thus the heat transfer is more effective.

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I like this discussion! Let's fuel this some more (pun intended).

Good Punny laugh! :lmao2:

OK lets remember flow and pressure are NOT Synonymous.

True... But you can't have volume without pressure (pump). Then volume is limited to the size of plumbing too... (This one I know from fire fighting).

Using the garden hose analogy. You burn you finger and run to the hose and run the water on it. I have just cracked the faucet and have a high delivery pressure at the opening, but no flow. Your finger still hurts. I open the faucet wide open my line pressure drops dramatically BUT I have tremendous flow and my finger is cooled. Now instead of a high pressure 3'rd degree burn I have a high flow 1'st degree burn.

Another version... Go to a house fire and grab the owner garden hose (60-80 PSI) and walk up to the burning building and fight fire it's like peeing on camp fire. But now grab 1 1/2" fire hose (100 PSI) and fight fire to might just hold the growth but won't put it out... Now kick it up again... Team up with two guys on a 2 1/2" line (50 PSI this will about put you on your arse by yourself!) and you knock the fire right down. size of plumbing has a direct bearing in volume vs. pressure...

I fully agree with the point that Velocity of the cooling media is important to absorbing the latent heat stored in the material requiring the absence of heat.

Kind of like get a piece a metal red hot with torch then pouring water on it to cool it The water flow over so fast that the metal does cool a bit but its still hot. But now take that piece of metal and stick in a bucket of water and the metal has time to transfer heat to the water. (A bit extreme in my example from pouring to flooding) This think no matter what heat can only transfer to a liquid only so fast...

Looking back at our VP. Is the reason for the check valve to ensure adequate pressure in a system that has an inadequate supply system? Remember cost is the #1 reason components are chosen at the mass production level. So I supply a $15 lift pump and make it work with a $3 check valve (mass production costs) or build it to last and eliminate almost all failures with a $400 pump? I multiply this cost over 400K units produced. HUGE money so as a manufacturer I choose to roll the dice play limited warranty and give ya what we got. Only point I'm raising here is. Absolute requirement because it will work no other way, or engineered solution to have acceptable (debatable function)?

Does the VP pump really care what its supply pressure is?

As for pressure I've seen people claim everything from 5 to 45 PSI will work on a VP44...

Or is it happy with having amply supply of fuel to never starve it?

At some point there is a limit to how low you can get because of the diaphragm rip problem.

I would venture to say the outlet pressure will not change regardless of inlet pressure as long as I have not starved the inlet to the pump. So back to the original question If I supply 3-5X more flow than the system was originally supplied to the customer with does that change all the original operating parameter assumptions?

Well... Maybe... Plumbing will limit the volume of flow vs pressure. Like some people putting FASS DDRP pump on They are a 45 GPH pump which is 10 GPH over the stock assuming its a stock fuel system yet it sould be in good shape right? But its not... The pressure falls off rapidly because of the limitation of the plumbing from idle to WOT operation. But now add a big line kit and this problem fades slightly but still the demand might be greater than the pump able to keep stable.

Check valve used to keep supply at pump inlet because my original lift pump and barely supply adequate flow to the system that it indeed would starve inlet to pump due to return line having the path of least resistance?

That's my thought too... The check valve is installed ot keep the pressure at a least a minimum level... Other than that yes the fuel would circle back to the tank which is the path of least resistance.

Again all my points are to stir discussion. I can't say these points are correct or incorrect.

The other factor is how is the pump measured for volume isn't that done at full flow open line.

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Looks like everyone is seeing things pretty much the same way now, so If I can make my original point again.Since the overflow valve starts to open at 14.5 psi I think that 14-15 psi fuel pressure is marginal for pushing enough excess fuel to cool the pump while going down hill for long periods with the engine idling because it will barely open the overflow valve. I think that 16-18 is better than 14-15 (more volume).I have my FASS95 set at 17-17.5 and haven't seen it drop below 13 at WOT.After this debate I'm considering stretching the spring a little to get it up around 19 psi.Jim

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I have my FASS95 set at 17-17.5 and haven't seen it drop below 13 at WOT. After this debate I'm considering stretching the spring a little to get it up around 19 psi. Jim

:lmao: Yeah... I might stuff a extra shim washer in my AirDog this weekend... Seriously... It might not be a bad idea to be above 14 PSI all the time... It might be worth kicking up towards the 18-20 PSI mark more so... But as for exceeding 20 PSI I got no long term information on it... I heard of several going above this but as for life of the VP44 I don't know.. My current standing is 17-17.5 @ idle and 15 @ WOT... I seen a few cold morning and having to jump on throttle hard and seeing 13-14 PSI with thick fuel. (yeah I know better... :nono:)

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This is a very interesting thread. It's good to see so many minds at work. Just to add another thought into this, I believe we are all overlooking some of the design of the VP44~~like the internal low pressure pump INSIDE the VP. By design, this pump should try to keep the pressure above the 14 psi set point of the overflow valve; thus, as long as there is adequate flow to the VP, the overflow valve will always be open somewhat to maintain the 14 PSI. I believe (there's that word again), the 10 PSI minimum suggested by the FSM is to ensure adequate flow, mostly through the overflow valve, to cool and lubricate the VP. Looking at the picture of the VP, it looks like the overflow valve is very near the inlet fuel inlet; in reality, as far as operation of the VP, it is actually in another chamber downstream of the internal low pressure pump.Just my thoughts.Jimmy

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This is a very interesting thread. It's good to see so many minds at work. Just to add another thought into this, I believe we are all overlooking some of the design of the VP44~~like the internal low pressure pump INSIDE the VP. By design, this pump should try to keep the pressure above the 14 psi set point of the overflow valve; thus, as long as there is adequate flow to the VP, the overflow valve will always be open somewhat to maintain the 14 PSI. I believe (there's that word again), the 10 PSI minimum suggested by the FSM is to ensure adequate flow, mostly through the overflow valve, to cool and lubricate the VP. Looking at the picture of the VP, it looks like the overflow valve is very near the inlet fuel inlet; in reality, as far as operation of the VP, it is actually in another chamber downstream of the internal low pressure pump. Just my thoughts. Jimmy

I am not familiar with the internal design of the VP44. Do you have any drawings illustrating the chambers and pump? Are you saying that the pump is between the inlet and overflow fittings, or between the chambers and the internal high pressure side inlet? Are you implying that the additional inlet pressure makes no difference in opening the overflow valve because of the pump? Please explain more. Jim

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I guess what I'm saying is that the internal low-pressure pump is between the inlet port and just about everything else in the VP. I believe (I love that word) it's purpose, along with the overflow valve, is to maintain a fairly constant pressure (with adequate flow) on the inlet to the high-pressure pump. I guess I'm also saying that the inlet pressure doesn't make any difference as long as it maintains adequate flow to the injection pump. To do this, adequate pressure on the inlet must be maintained (FSM says 10 pounds, I'll go along with that). We also know that too little pressure to the VP can cause diaphragm damage and make for hard starting, as well as other bad things. We also know that too much inlet pressure is also not good for the pump (causes hard starts, etc). Jimmy

post-167-138698164628_thumb.jpg

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I guess what I'm saying is that the internal low-pressure pump is between the inlet port and just about everything else in the VP. I believe (I love that word) it's purpose, along with the overflow valve, is to maintain a fairly constant pressure (with adequate flow) on the inlet to the high-pressure pump. I guess I'm also saying that the inlet pressure doesn't make any difference as long as it maintains adequate flow to the injection pump. To do this, adequate pressure on the inlet must be maintained (FSM says 10 pounds, I'll go along with that). We also know that too little pressure to the VP can cause diaphragm damage and make for hard starting, as well as other bad things. We also know that too much inlet pressure is also not good for the pump (causes hard starts, etc). Jimmy

Are you sure this diagram pertains specifically to the VP44 or is it a generic diagram for most solenoid-valve controlled radial-piston distributor injection pumps like it is labeled? Which part on the diagram is the infamous diaphragm? This is labeled as the high pressure stage, is this implying that the overflow valve is venting the high pressure side of the pump? I'm just a little confused by the drawing perhaps you could clear it up for us?. Jim

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Are you sure this diagram pertains specifically to the VP44 or is it a generic diagram for most solenoid-valve controlled radial-piston distributor injection pumps like it is labeled? Which part on the diagram is the infamous diaphragm? This is labeled as the high pressure stage, is this implying that the overflow valve is venting the high pressure side of the pump? I'm just a little confused by the drawing perhaps you could clear it up for us?. Jim

Although this diagram shows a four lobe HP pump, I am sure it's operation is the same as our beloved VP-44. It's a little difficult to see, but if you look closely, you'll see three different shades of blue. The dark blue is the high pressure Pump out to the injectors; the medium blue is downstream of the internal pump (about 14 PSI mostly controlled by the overflow valve); the light blue on the left is from the lift pump. As you see, there are a lot more solenoids, valves, etc it there which are controlled by electrical signals and medium pressure. There is also a relief valve bypassing the LP pump back to it's inlet if pressure gets too high. The overflow valve is shown on the far right. I'm not too sure where the famous (infamous) diaphragm lives; I'll have to do a little more looking. It's been a couple of years since I really dug into this. I'll see what else I can find that might be of help. humbly jimmy
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Here's a link from TDR with some excellent pictures of a torn town VP44. http://www.turbodieselregister.com/user_gallery/displayalbum.php?&albumid=17275

WOW, great display pictures of all the parts of the VP44.:thumbup2: You are right the only thing that higher pressure will do is provide a greater available volume if the supply line is larger than stock. It cannot open the overflow valve further due to the pump's internal design. I'm not going to reduce the pressure on my lift pump though, I'm comfortable with the 17-17.5 at idle. Thanks for the information, never too old to learn. Jim

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