Mexstan

Have read most of the threads re fuel pumps and fuel pressures but have not yet seen the answer to a question I have. Why is it necessary to have a higher fuel pressure at the injector pump? The injector nozzles on any injector can only allow a certain volume of fuel thru for a single pulse and raising lift pump fuel pressure from say, 5 to 15 psi does nothing to change that. Presuming that there is sufficient fuel volume to supply the requirements of the injector pump, what does raising the inlet fuel pressure do? For the sake of this discussion I am going to presume that ANY positive fuel pressure, even 0.25 psi is enough to keep the injector pump supplied, even at WOT. If that is true, then what good does raising the pressure to 10 or 15 psi do? The only common thing I see on these threads is that there is a possibility that the supply lines cannot handle the volume requirements at WOT and the higher pressure compensates to some extent. Hence the necessity for the larger holes in banjo bolts?<br>OK you experts, what am I missing here? I really would appreciate a good technical answer to this.

Iron Horse

I suppose somthing that is almost more important then psi is how many gallons per minute you are flowing to the pump. Hi psi with low gpm does nothing because it cannot flow fast enough to carry the heat away from the pump. <br><br>That is one thing I have noticed with bigger truck engines. For instance, the old Cat 3406A runs about 60-80 psi of oil at idle, but flows about half as much oil as the newer 3406E that runs half the oil psi. <br><br>I will do a little research monday and see if I can find some fuel flow requirements for the ISB.

Mexstan

dd4x4, thanks, but higher pressure cannot mean more flow unless the flow has somewhere to go. And to follow this logic, then the higher pressure will also translate into increased fuel consumption which we all know does not happen. <br>Iron Horse, if the pump (presume you mean the injector pump and not lift pump) is so touchy about temps then surely the manufacturer would have provided it's own radiator?

Push Rod

Hey Stan, I'm going to turn your question into a fuel system 101 ;D

The system on the 24 valve truck (VP44 and CP3 equipped) is composed of a lift pump, which pulls fuel from a tank and lifts it to the injection pump. This lift pump is capable of 95 GPH at 15 PSI in a 2nd gen RAM. The 3rd gen lift pump is capable of 75 GPH at 11.5 PSI. The injection pumps also have some fuel pumping capability. The sliding vein pump in the VP44 is capable of 7 PSI in a free flow situation with -6 (approx) lines. I am unsure of the CP3's capabilities, except to say in the Duramax application it is capable of generating enough flow for at least 400 HP without pump damage and without help from another fuel transfer pump. With the lift pump/fuel filter blocking flow on the Dodge system, all I can say is that at 340 HP (approx), I've been able to drive the system into vacuum and run the CP3 out of fuel during a hard run.

The fuel systems on these trucks is a closed, recirculating design. Enough fuel flow must be provided to the injection pump for both injection AND cooling. The fuel used for cooling is returned back to the tank, where it attempts to cool down before returning back to the lift/injection pump circuit.

The entire system is flow sensitive, however flow gauges are pretty spendy units to have permanently mounted in a truck. So we use fuel pressure gauges instead. We can be assured at a system pressure of 15PSI that the lift is attempting to push about 95 GPH (11.5 and 75 GPH on CP3 trucks). At WOT throttle, these pressures are often reduced down to 0 PSI, or even vacuum, depending on the modification level of the pickup truck. Since this is a closed system with a constant feed (the lift and the VP44/CP3 working in tandem) the reduction in pressure after the lift pump means that we must be experiencing a volume loss somewhere, otherwise pressure would remain at a constant 15 PSI (11.5), since the lift is generating approximately twice (estimated 30% more than required in the case of CP3) the flow as the injection pump can handle. How much reduction in pressure can the system withstand, before we drop below our magic 45 GPH (X GPH on CP3)? I'm not sure. I've never tried to hook up a flow meter on the run from the lift to injection pump, although I should have. At the very least, we'd want 7PSI of pressure, as this is what the VP44 is capable of generating on its own. At this pressure, with the stock pumps, we can be assured that we're giving the VP44 all the fuel it needs for injection and cooling. Again, we're not sure what we need in the case of CP3 pumps. I've been too chicken to bypass the filter/lift assembly to see how much fuel pressure we can generate. ;D

Is there a benefit to be derived from higher inlet pressure? Possibly, although its doubtful as the VP44/CP3 will only take in so much fuel. Higher pressure may mean additional fuel entering the internal chamber, although the outgoing pressure to the injectors is carefully regulated. Higher inlet pressure could mean that the VP44/CP3 would have less work to do to get pressures up to those required by the injection event. How much less work? Probably not a pile, but every little bit counts.

So what is the ideal fuel system? IMHO, lower pressure, high enough volume to satisfy the system's hunger for injection and cooling. Higher pressures = more heat which the system already has a tough time dissipating. In my opinion, the ideal system is a 15PSI system capable of generating enough flow to maintain pressure at ~15 PSI throughout the RPM range and load demands. Systems spitting fuel through at higher than 15 PSI are simply generating heat in order to gain volume. Choose a single fuel transfer pump with a large enough rotor to generate sufficient flow at sufficient PSI. Overrate the pumping capacity a bit, so it has the power to pull fuel as well. It is true that pumps prefer to push, however it is a fallacy that the pull from the fuel tank to the lift pump is causing any problems at all. I've seen these little Carter pumps working in far more demanding circumstances. Perhaps if we were in a free-flow situation, the long draw would be hurting the pump, however we require half the flow that the Carter is capable of generating. It spends a very large part of its time in bypass mode. At higher power levels, it appears as though the Carter isn't keeping pace, so an uprated pump will be required.

Ack, sorry for the monster post. I think I answered your question somewhere in there, Stan

Rod

Scotty

I'll add that the system is only as good as its weakest link.<br>The bottle necks [especially in a higher horsepower truck] that are created at the connections for the filter housing [in and out] and into the VP-44 would be the first thing to address. Banjo bolts would be fine for 300-345 HP tops.<br>Then refitting the system from lift pump to VP-44 and as you go up the ladder, complete refit from the tank. Keep in mind that its imperative to keep the inner diameter throughout the system as close as possible for optimum flow. <br><br>I am only referring to the VP-44 equipped trucks until I get my 03' and finish a system for them. ;D

BigBlue

So what kind of pressures are a good 03 cummins supplying from the lift pump? Do the injection pumps cost as much since the injectors themselves do the timing and not the pump? Or are they just as expensive?

Push Rod

[quote author=BigBlue link=board=8;threadid=15587;start=0#146892 date=1055025911]<br>So what kind of pressures are a good 03 cummins supplying from the lift pump? Do the injection pumps cost as much since the injectors themselves do the timing and not the pump? Or are they just as expensive?<br>[/quote]<br><br>Post filter, at idle, I was seeing 9.5 from a pump that should be putting out 11.5. WOT with the EZ on the 80 HP level, I could draw down pressure to 3 PSI. These days, I'm getting 8 PSI at idle, with drawdown to vacuum at WOT. This is with high flow banjos in place, so I don't think the high flow bolts are going to do a single thing on the 03's. I've got a -6 kit to test out tomorrow, and then we'll be swapping to a -8 kit and PE4100 pump.<br><br>I'm unsure of the cost of a CP3. I haven't priced one out yet.<br><br>Rod

Mexstan

Rod, thanks for the long but informative answer. Sorry it took me so long to get back but I have been busy. Also wanted time to think about this.<br> Correct me if I am wrong but the the bottom line to all of this is that I am correct in my original surmising that the reason for increased lift pump pressures is a deficiency in the supply system. Specially when one does mods that require more fuel.<br> There is one point I will challenge you one. Granted, increased fuel pressure can result in an increase in temperatures, but at the pressures we are talking about (20 psi max?) the temperature increase must be more theoretical than actual. So if the pump temps are so critical why is it not practical to increase the flow (by even more pressure?) but provide a way for the excess fuel to get back to the tank easier? I have yet to see any discussion on increased return line diameters or anything related to that. How about a small cooler such as a finned section of return line if the temps are that critical?<br>To take this a step further. The '03 has a lift pump which supplies the fuel pump/gear pump. This gear pump raises the LP pressure to 80-180 psi. Any fuel not being used by the injector pump goes thru the overflow valve. I have not yet seen this valve or the gear pump in pieces so bear with me as I could be talking nonsense here, but to decrease the critical temps in the injector pump, would it be practical to modify the overflow valve to allow more excess cooling fuel thru? Would it be possible or even practical to modify the gear pump to provide more flow for cooling purposes? Maybe the gear pump has enough volume and all that is necessary is to increase the LP pressure to about 50 psi and just modify the overflow valve? What about bypassing the gear pump and just install a lift pump that will supply 200 psi and modify the overflow valve to suit? Would this not satisfy all the demands of sufficient fuel to supply the injector pump no matter what, plus increased cooling?

Push Rod

[quote author=Mexstan link=board=8;threadid=15587;start=0#147297 date=1055161705]<br>1) Correct me if I am wrong but the the bottom line to all of this is that I am correct in my original surmising that the reason for increased lift pump pressures is a deficiency in the supply system. Specially when one does mods that require more fuel.<br><br>2) There is one point I will challenge you one. Granted, increased fuel pressure can result in an increase in temperatures, but at the pressures we are talking about (20 psi max?) the temperature increase must be more theoretical than actual.<br><br>3) but provide a way for the excess fuel to get back to the tank easier?<br>[/quote]<br><br>1) Yes. We're increasing pressure at WOT to ensure that there is enough volume. I don't think anyone has done the calculations to determine how much flow is really needed. The stock system dropping soooo much pressure at WOT means that there is a deficiency in flow. We've chosen 15 PSI as our target, since Cummins chose a 15 PSI pump to supply with on the 2nd gens and a 12 PSI pump on the 3rd gens. We don't believe that the VP44/CP3 are pressure sensitive, but I can't help but think that Cummins chose these pumps for a reason.<br><br>2) You're right, we honestly don't know what the temperature increase is, however there will be one, even if slight. And every little bit helps. There have been a few talking about putting a cooler on the lines, and I believe there may be some merit to this. I think the return line would be ideal.<br><br>3) The return line is not a choke point in the system at all. There is a max of about 30 GPH (on VP44 systems, unsure of CP3) being returned to the tank, and this is at low pressure. There isn't anything that needs changing out.<br><br>And your comments on modifying the pump are indeed interesting, however on the VP44 pumps, this overflow valve and cooling circuits are controlled electronically. You'd have to change the circuitry in the pump. I doubt there is much room left on internal passages to allow for much extra flow.<br><br>On the CP3 side, you do raise some interesting points, however the mechanical pump you speak of on the CP3 is part of the injection pump itself. The CP3 is a dual stage pump whose first stage raises pressure up to 900 PSI, while the second stage raises pressure up to 30,000 PSI (Bosch specs, not necessarily what we see on the Dodge trucks). I don't think we could split the pump, and use only the high pressure side. Besides, I don't think we'd want to. Mechanical pumps are alot more reliable than electrics in high pressure situations.<br><br>In either the VP44 or the CP3, we do need to fix the system up to the pump. I personally feel that the pump internals are probably just fine to 500 HP, and in the case of the CP3, probably higher as this pump is used in other trucks at higher flow demands.<br><br>Rod

Mexstan

Thanks again Rod. OK, I've got it. Just wanted to make sure I really understood what was going on in my truck and you have helped a lot. Now it will be interesting to see if there are any other opinions etc.

DF5152

I like the disscusion but there is one point i dissagree with. Maybe due to my ignorance but the return line can directly reflect flow and pressure. By restricting flow back you can increase pressure on the supply side. chevrolet vortec engines use this type of restriction on the return side due to a lack of an adjustable FP regulator. however this is only in supercharged application. Now as far as higher pressures causing more heat are we talking the lift pump or the injection pump?

Push Rod

[quote author=DF5152 link=board=8;threadid=15587;start=0#147413 date=1055180545]<br>1) Maybe due to my ignorance but the return line can directly reflect flow and pressure.<br><br>2) Now as far as higher pressures causing more heat are we talking the lift pump or the injection pump?<br>[/quote]<br><br>1) In the case of the VP44, it doesn't. The VP44's internal sliding vein pump changes pressure once it gets inside the pump, and its internally regulated from there. I am unsure of the CP3 return system, as I've never looked at an exploded view of one, and I'm skeered to explode my own <br><br>2) Fuel heat. Anytime you compress something, you generate heat. Plus you'd have heat soak from the lift pump, as a lower volume, high pressure pump is spinning faster than a lower pressure, high volume pump. Its probably not much heat, maybe only 1F or 2F, maybe 20F or 30F. Either way, if we don't need the pressure (which we don't), why deal with any added heat?<br><br>Rod

dodgeman01

Speaking of lift pump.... Today I was going down a hill with the truck in 3rd gear with the rpms around 3200rpms. When I floored it the truck started to spit and sputter a little. when I got to the bottom and started back up another hill everytime I floored it it would sputter when not pushing it she ran fine. funny thing was when I got back on level ground it ran fine also fuel tank was 3/4 of the way full ???<br>DM01

Robert Venable

What if you simply went to a different lift pump that had a higher GPH ratting with the same pressure as the factory one? And if the filters are the next limiting factor, what about putting 2 or more filters in parrellel in the system? Would this help any?

Push Rod

[quote author=Robert Venable link=board=8;threadid=15587;start=0#147533 date=1055201870]<br>1) What if you simply went to a different lift pump that had a higher GPH ratting with the same pressure as the factory one? <br><br>2) And if the filters are the next limiting factor, what about putting 2 or more filters in parrellel in the system? Would this help any?<br>[/quote]<br><br>1) At higher power levels (375 - 400+) this is necessary (past this power level, the stock Carter doesn't like drawing from the tank as much), but you do need to take care of the poor lines on the truck. Electric pumps are designed to be used with a certain line size, both on the inlet and outlet. The factory feed from the tank is 5/16&quot; ID, which is well suited for the Carter thats there (it wants 3/8&quot; inlet and outlet), however the outlet lines are much too small. If you went to a higher volume pump at the same pressure, without swapping lines, you simply make the pump's internal bypass work even more.<br><br>2) I don't actually think the filter is that much of a restriction, but if it does prove out to be that way, you wouldn't want to put two of them in-line. You'd have even more restriction as the micron size still needs to be fine enough to filter out contaminates.<br><br>Rod