Engine/Transmission (1998.5 - 2002) VP44 - the light dawns - maybe...

[Gary - K7GLD]

I think maybe I finally see the light on one piece of the VP44 puzzle, as to flow rates...



I have been looking at the VP44 as though it had 2 relatively independent flow paths - one, thru the injector pump portion directly supplying the injectors, and at a fixed rate due to pump piston/bore size per RPM - and the other that FEEDS that portion, but bypasses excess fuel thru the VP44 for cooling and elimination of unused fuel - the kicker being, I have been looking at that bypass as an *open path*, that could, to a degree be varied in flow rate by varying inlet pressure to the VP44 as any other open but fixed orifice would be.



BUT, I now realize (I think), that the bypass section is ALSO flow restricted and controlled by a FIXED condition created by the VP44's internal lift pump that supplies/controls 100% of the VP44 internal fuel flow. Put another way, I *assume* that if the VP44 is NOT physically rotating, NO, or precious little, fuel will flow thru the VP44 regardless of applied pressure - and what DOES flow during normal operation is STRICTLY controlled by the RPM of the pump, only so much volume of fuel per RPM can get past the rotors/vanes of the internal lift pump - regardless of whether there is 1 psi or 100 psi available at the pump inlet!



I understand that there IS a SMALL vent orifice AFTER the built-in lift pump incidental to the bypass pop-off valve designed to vent air in the fuel if need be - but apparently, that is AFTER the vanes or rotors of the pump, and other than (maybe) minor leakage past the pump rotors, not much incidental fuel can get past?



To summerize:



IF I am correct in my evaluation, then, ALL fuel flow thru the VP44 must FIRST get past the rotors/vanes of the built-in lift pump, which is strictly controlled by RPM - NOT pressure of fuel at the VP44 inlet. THAT being the case, in terms of volume, it really doesn't make any difference whether there is . 1 psi or 100 psi at the inlet - only SO MUCH fuel can physically pass thru the pump per revolution - and as I understand it, based upon the maximum operational design RPM of our Cummins, that amounts to about 45 GPH maximum - regardless of fuel pressure or the size of fuel lines and fittings.



OK - beat me up on this and tell me where I went wrong...



(other than not comprehending that in the first place... )

 

[Ncostello]

Gary, does 45 GPH account for injector fuel consumption and fuel returned to the tank at WOT (stock engine total maximum). I'm trying to find the absolute maximum volume per hour that the VP will flow through itself at WOT including the fuel necessary to cool and lubricate. There has to be a maximum amount of fuel the VP can pass through per hour. Maybe its 45GPH like you say. I have no idea.

 

[Groover]

Gary, I think you are on the right track with this line of thinking. Have thought the same thing but have no particular data/research to back it up. The VP 44 internal pump is definetely RPM dependant thus lift pump pressures drop as RPM increases.



Positive pressure to the VP-44 is good. How much is really required as you say flow is determined by the internal pump. So as long as you have some positive pressure from the lift pump everything should be cool.



The argument that higher lift pump pressure (like 15+psi) is doing MORE lubricating and cooling of pump parts than say 8-10psi -- I don't buy it! I think the limiter is the internal pump in the VP44 as you said.



Now having said that - when my lift pump was showing less than 5psi. I got it changed out at DC. Do I think my VP-44 was harmed by running for short period of time with less than 5psi (always 2-3psi under any circumstance) ? Not a bit!



Flame on

 

[Pit Bull]

Gary I think your right it is flow not psi that counts. The reason DC wants a certain amount of psi is to catch a failing lift pump before it might take out a VP44. JMHO

 

[Gary - K7GLD]

Neil, if I'm correct, the approximate MAXIMUM flow rate THRU the VP44 - that means ALL fuel supplied, both what is consumed AND what eventually gets bypassed, is 45 GPH - based upon the maximum recommended RPM of our Cummins engines - I would imagine the same pump on a higher reving engine would be rated differently - and these pumps ARE used on engines other than our Cummins.



An additional spec from Cummins states the need/design for 70% flow bypass for VP44 cooling - and tests done by Mopar Muscle revealed at WOT, there was a steady 30 GPH fuel bypass REGARDLESS of inlet pressure or supply (other than starvation... ). SO, if I'm right, and maximum flow rate provided thru the VP44 IS 45 GPH, what is 70% of that flow?



WHY, *gasp*, it's 31. 5 GPH - pretty much EXACTLY what MM found out in HIS tests!



I'm still left thinking that as long as anything NEAR a positive flow source is supplied to the VP44, all will be well - and radically higher pressures or firehose-size lines and fittings are largely overkill - what you DON'T want is NEGATIVE pressure, or a situation where the VP44 itself is forced to suck fuel from the source... NOT that higher pressures and larger lines are in any way BAD, just perhaps not as necessary as we might think - and I'll still stick with my pusher setup, and err on the "more is better"

theory...



I'm so far convinced that no matter HOW much pressure, or flow rate you provide at the inlet to the VP44, yer only gonna actually get 45 GPH MAXIMUM thru it at WOT, max RPM, period!



Another factor that then enters into the equation, is that if the VP44 is RPM-fixed in fuel low - and 30 GPH or so MUST be bypassed by design - that leaves 15 GPH max for actual engine consumption... How much power CAN you develop in our engines at 15 GPH?

 

[Ncostello]

MM's flow meter posts were really helpful with this kinda stuff. Seems like your numbers make perfect sense.



Out of curiosity, where does 45 GPH come from? Is that Cummins product data or a calculation from other data. I also have seen 55 GPH posted on 24 V forum too. Again, I dont know, I'm just curious.

 

[Gary - K7GLD]

The 45 GPH figure supposedly comes directly from Cummins techs - I've seen it quoted several times, seems to dovetail perfectly with other related info, maybe someone has a pointer to a reliable source?

 

[RKohn]

Hey Gary, I thought you left the building...

So that's what you were doing... . You climbed the mountain in search of enlightenment. Actually, it's not enlightment that gave you those visions, it's the lack of oxygen at altitude. Breath some diesel fumes and you'll be back to normal.

 

[Gary - K7GLD]

NAHHhh - I'll never be "normal" - and I refuse to grow up...



Youse guys will just have to accept me the way I am...



Suffer with it!

 

[Smills]

So really we should all have flow meters gauge's instead of psi gauge's to see whats going to are VP44 and if its enough!



or better yet the dealers should have flow testing equipment to test lift pumps might be a better diagnostic tool.

 

[Gary - K7GLD]

Well, actually, once you get past the flow bit, simply monitoring to make sure you have positive pressure AT the VP44 is about all that should be needed - if you do that, the rest SHOULD pretty well take care of itself - my primary point and final analysis is, you can provide all the PRESSURE and volume capabilities ya want at the VP44 inlet - but it's only gonna use just SOOOooo much...



But a little bit MORE to be on the safe side sure makes ME feel good!

 

[Smills]

So fuel boxes would definitely change the amount of fuel used by the VP44

 

[Gary - K7GLD]

They will change the percentage of fuel consumed vs the percentage returned to the tank... Up to the fixed limit of the pumps injector bores/pistons - while still maintaining bypass flow for VP44 cooling.



Is it possible with a fueling box to divert so much fuel to the injectors that the bypass percentage is endangered? I don't have a clue...

 

[GLASMITHS]

Gary

I think you are correct on this. It does not sound reasonable to supply high pressure fuel to another pump when all you need is enough to supply that pump, not try to ram it through. Somewhere some tech said 22psi or below, not over.



I have been tempted to put a pusher on mine. However, I think a new pump and prefilter by the tank would be a better set-up. That is supposed to be the major problem now, the location of the lift pump. Thanks for your post... ... ... ... ... ... ... .

 

[geusterman]

You guys could just do what I did. I plumbed my air system to the fuel tank. The automatic pressure regulator keeps the airtank charged and another regulator keeps 10 lbs air pressure on the fuel tank. I switch off the pump when I fuel and I have a little pressure release button I tapped into my fuel cap. Don't have to worry about the lift pump failing now. Ok, just kidding



But it could work! George

 

[GLASMITHS]

Geoooooooorge!!!!!

The only problem with THAT set up would be the return line

 

[Evan A. Beck]

Lets not forget about the fuel delivery rate... Using that figure (in cubic mm), you could extrapolate what is getting burnt versus what is getting returned to the tank.

 

[Big White Beast]

George,

Don't laugh! When I put my new filter set up, pre pusher, I thought I had cycled it enough times to fill it. Truck ran for a second and wouldn't restsart. I was kind of in a panic so I fired up the compressor and put a rag around the tank mouth and blew some air in. It must have been enough to fill the filter because I cycled the pumps once and it fired right up. Next time I pre fill the filter.



Garrett



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[GLASMITHS]

Garrett

I'm glad you brought that up. My air compressor and port gen is always with me on a trip. So if need be (in emergency)... ... ... ...



However, I also carry an unopened bottle of PS to fill the filter if need be. Fill up the filter upfront and the rest in tank. No problems starting.



Edit: Do you know the micron rating of the NAPA filter? That is the set-up that I want. Have looked at RACOR. Theirs is 30 or 40.

 

[Smills]

Garrett Where did you get that filter mount it looks like it works really good. Could I get the part # , brand, and where to find it.



Thanks Shane