Fuel Pressure

[Soupy]

Reading here that anything over 12psi to the CP3 can be harmful. On my AD100 I have my fuel pressure gauge hooked up the fuel pressure port on the AD pump which always shows 19-20 psi. I take it this would be prior to the filters, hard to say if the fuel goes by the gauge sending unit first before the filters or not? Is this reading of 19-20 an accurate reading at the CP3 and if not how much of a drop would there be if any? Thanks.



Soupy

 

[steved]

Reading here that anything over 12psi to the CP3 can be harmful. On my AD100 I have my fuel pressure gauge hooked up the fuel pressure port on the AD pump which always shows 19-20 psi. I take it this would be prior to the filters, hard to say if the fuel goes by the gauge sending unit first before the filters or not? Is this reading of 19-20 an accurate reading at the CP3 and if not how much of a drop would there be if any? Thanks.



Soupy

The CP3 can take a wide range of pressures... I think I remember from anything above zero to more than 25 psi. I have been running between 15 and 20 psi on my FASS for around 160k.



If your AD is similar to my FASS, the pressure port is downstream of the filters to show a plugging filter.

 

[AH64ID]

Bosch specs are -5 to +15 psi, 20 won't hurt anything, but they don't need any more psi than that, unlike a VP44 they aren't pressure lubed.

 

[CKelley1]

you need to move your gauge to the cp3 and do not change the filters until the pressure drops under load. the dirtier the filter get's the better it filters.

 

[cerberusiam]

Reading here that anything over 12psi to the CP3 can be harmful. On my AD100 I have my fuel pressure gauge hooked up the fuel pressure port on the AD pump which always shows 19-20 psi.

Its really quite interesting to read how too much pressure is bad for a gear rotor pump when it pretty much defies the conventional hydraulics experiences. Granted these CP-3's are not quite the same critter but they don't operate on different theories. :-laf



Rule of thumb on pump input pressure is 1/4 to 1/3 of output pressure. If you haven't changed the COV from stock then your target output is 75 psi. Input pressure falls into 20-25 psi for best results. If you are using an performance COV that can go to 30-40 psi range. Effectively, the CP-3 can take 100 psi input pressurer without issues but it doesn't gain enough to be needed unless you are looking for every pony possible.



The detriemntal effects of to little pressure are much more damaging than too much pressure. Always over size the input for best results. Cavitation is the largest problem the gear rotor pump faces and thats strictly a function of input flow. Lack of hea dpressure to the supply sides causes the gears to slap in between the pressure pulses and transfers to lateral movements that wear the end plates. Stop the cavitation and the major wear is stopped. This becomes obvious with the Dmax pumps that don't use a lift pump and is one the major failure points in that system. Killing the gear rotor pump contaminates the whole fuel system so you want to minimize ANY possibility of that.



The Dodge implementation is much more effective in this area when the LP works correctly and provides adequate flow, but, a weak lift pump will kill the system as experience indicates.



Obviously the best place to measure accurate pressure is the CP-3 input as thats the critical point. It also tells you when you filters are getting close to change points.



That said, the system is not going to fail if you only run 15 psi. In fact, the fuel qualifty is probably much more of a factor than pressure in these systems. However, if you are going to set pressures then set them to eliminate the most possible adverse conditions and treat your fuel with additives. Best bang for the buck.

 

[AH64ID]

That said, the system is not going to fail if you only run 15 psi. In fact, the fuel qualifty is probably much more of a factor than pressure in these systems. However, if you are going to set pressures then set them to eliminate the most possible adverse conditions and treat your fuel with additives. Best bang for the buck.

Yeah, on a CP3 flow and quality is much more important than pressure.

 

[Soupy]

Thanks guys. Is my fuel pressure port before or after filters? Hard to tell by just looking at the system. Thanks again.



Soupy

 

[cerberusiam]

Yeah, on a CP3 flow and quality is much more important than pressure.

Yes, but, keep in mind we CANNOT talk flow rate with any consistency when it comes to these fuel systems. We have ZERO flow rates to correlate with Bosch requirements. AFAIK, there is no official or unofficial specs about flow requirements to provide adequate fuel to the CP-3. All we have to work with are an unofficial minimum spec for STOCK hp ratings.



All we have is pressure to provide an adequate measure of how much fuel is needed. If we go down the flow rat hole then then we go into the gray area a dealer will use to justify NOT performing warranty on these systems. If we stay with pressure we have absolute number that can be used for comparisons.

 

[cerberusiam]

Thanks guys. Is my fuel pressure port before or after filters? Hard to tell by just looking at the system. Thanks again.



Soupy

Pressure is a measure of restriction in a system so wherever you are taking the measurement its the resistance from the source to that point. If your gauge is sourcing from the AD pump housing thats the restriction of the flow out of the pump and does not account for filters and line size between there and the CP-3. In the stock system that can amount to 6-8 psi thru a plugged filter, more if there are more filters.

 

[steved]

If your gauge is sourcing from the AD pump housing thats the restriction of the flow out of the pump and does not account for filters and line size between there and the CP-3.

If the AD is similar to the FASS, the port is basically tee'd into the outlet of the pump housing that goes to the CP3... it is downstream of the filters.

 

[cerberusiam]

If the AD is similar to the FASS, the port is basically tee'd into the outlet of the pump housing that goes to the CP3... it is downstream of the filters.

. . but not any filter(s) between the AD and CP-3 which they should be some. Nor does it account for the restrictions in anything else towards the CP-3.

 

[AH64ID]

Yes, but, keep in mind we CANNOT talk flow rate with any consistency when it comes to these fuel systems. We have ZERO flow rates to correlate with Bosch requirements. AFAIK, there is no official or unofficial specs about flow requirements to provide adequate fuel to the CP-3. All we have to work with are an unofficial minimum spec for STOCK hp ratings.



All we have is pressure to provide an adequate measure of how much fuel is needed. If we go down the flow rat hole then then we go into the gray area a dealer will use to justify NOT performing warranty on these systems. If we stay with pressure we have absolute number that can be used for comparisons.

But Bosch has put out the spec of -5 to +15 psi of fuel, which allows for proper flow as if you decrease resistance (more demand from the CP3) you have to increase flow to maintain the proper pressure, so if you keep the pressure in those specs you will meet the flow requirements.

 

[cerberusiam]

But Bosch has put out the spec of -5 to +15 psi of fuel, which allows for proper flow as if you decrease resistance (more demand from the CP3) you have to increase flow to maintain the proper pressure, so if you keep the pressure in those specs you will meet the flow requirements.

AFAIK Bosch never has and never will publish a spec for required pressure per implementation of their fuel systems. Any Bosch specs for pressure have always reference to test case and that is all they are, reference points not to be used in actual implementations. Bosch will never open themselves up to that kind of liability.



I have no idea where this -5 to +15 psi has the feel of a misconception conjured out thin air to satisfy a proposed rationale. It simply doesn't fly when you look at the Dodge fuel system and what is known around it.



There neve rhas been any concise information or discussion around maximum pressures and any potential damage. This again is conured out of misconceptions given the the testing that has been done by a lot people on pressures. Quite likely could be a hold over from the VP and confusion around exactly what somebody was saying.



The only available info on minimum pressures is unoffical specs from Dodge\Cummins engineers at +3 psi in the Dodge system for best results and thats only for stock fueling. Throw that number out when you increase the fuel demand with injectors and\or programmers.



Anything that suggests negative pressures are acceptable is highly suspect. The gear rotor pump on the CP-3 does not tolerate cavitation well, plenty of pumps shelled out proving that theory on the Dmax's.

 

[AH64ID]

I was given that info from Bosch when I contacted them several years back, as well as 5um absolute is the minimum they recommend for the system. I also asked some Dodge/Cummins engineers a while back and was given the same spec. The spec makes sense based on the psi of the stock system, and the Dmax's lack of a LP, of which it still lacks a LP.

I have heard +3, +2, +10, etc but the data I was given is what I go by. I usually recommend +2 to people becuase gauges aren't as accurate at their min or max readings, so +2 gives a safety buffer for factual monitoring. +10 is probably a holdover from the VP44 days where Dodge had a min psi in the service manual, there is no min psi in the service manual for a CP3.

I also don't think fueling levels has as much of an impact on a stock CP3 as some do. Becuase if you look at Boyle's law then as the resistance drops (increase in demand from the CP3) the flow would have to increase to maintain the same pressure or the psi drops. This is true regardless of demand, so if you don't have to up flow with a fueling mod if your pressure stays in the safe area, the required flow is being met.

 

[rbattelle]

I have no idea where this -5 to +15 psi has the feel of a misconception conjured out thin air to satisfy a proposed rationale.

Generally speaking, I would agree.



The only official specification I've ever seen comes from the 2003 PDM, which indicates minimum LP fuel system flow rate is 38 GPH. This, of course, is for stock engines.



I've heard other people say that so long as you have positive pressure at the CP-3 inlet, then you have enough flow. Seems reasonable.



-Ryan

 

[AH64ID]

38GPH is about what I figure the in-tank does on the low side, based on circumstantial evidence and nothing more. I have never been able to find an actual rate, but a GPH rating indicates that flow is more important.