HOHN

-- Higher pressure to the VP will not increase flow through it. 2psi, 10psi, 30psi-- it matters not. The VP will only flow so much, and you can't force it to flow more no matter how much you try. 20psi doesn't cool it or lube it any better than 12psi. Enough is enough, and more than enough doesn't get you anything.

-- How much is enough? No one really knows, but I think 7psi is plenty (as Chrysler and Cummins also thought).

-- How much is too much? No one really knows, but Bosch limits pretty to 14psi internally, so why not use that number?

-- Cavitation happens. It happens more at low pressure.

-- Fuel temp matters. A sure path to hot fuel is pumping way more fuel than you need to when the engine's under load.

-- A CTD making 500hp is only burning about 28gph to do so. How much more do you need for lube and cooling? I claim that 40gph is plenty for any CTD 500hp or less.


BTW-- my original OEM lift pump (yes, that junky Federal Mogul pump designed during WW2) is still going strong at almost 75K miles and 6 years of age.

JH

Dinner

I agree with everything you have said, however if you are reading 2psi at the IP during idle lets say. Wouldn't this starve the IP once acceleration, engine RPM increase? Since the LP really isn't helping to flow any fuel, the IP would have to "Pull" fuel through the LP, which would create heated fuel? Or is the IP able to stay healthy when it is "pulling" it's own source of fuel?

dkenny

so if the lift pump/pumps could maintain 10 psi under all conditions. that would be good? but since the filter is after the stock LP..what happens when the filter plugs..not saying that a after market would help in this case, this is what a fuel pressure gauge is for.

but what about cold fuel?
but what about biodiesel? cold biodiesel..its thicker?
but what about WVO? I don't/won't run this but other do?

-dkenny

i think a few extra psi but not too much psi would be good.

HOHN

No one knows. The problem is that we cannot measure fuel pressure at the only point where it really matters-- the entry into the high-pressure section of the VP. You need enough fuel pressure so that there's no cavitation in the fuel as the VP tries to fill those pumping chambers rapidly.

You're not going to heat the fuel by "pulling" through the LP.

The point at which the fuel cavitates inside the pump is unknown, but higher pressure definitely makes internal pump cavitation less likely. But cavitation as also a function of flow, localized fluid velocity and lots of other complicated fluid dynamics stuff. Suffice it to say, we just have no certainty about how much FP is need for a given rpm at a given hp.

I'm personally comfortable running 7psi all day because Bosch uses the VP in many applications where it must suck its own fuel through a filter-- no lift pump and no gravity feed.

Still, the VP isn't a favorite. Internally here at Cummins, most of the engineers I've talked to sort of roll their eyes at the VP. The VP was used basically as an act of desperation because Bosch didn't have the HPCR ready and Cummins couldn't hit emissions targets required on 1 Jan 98 without electronic controls.

JH

nitrousn

Good sound information. To many myths and unfounded theories. Why people want all this un-needed fuel pressure and big A$$ pumps that could fuel a shuttle to the moon is beyond me.

Mike D

I can answer that, the carter pump is a POS and at over $160 a whack people start looking for a better more reliable option. The other reason is that not everyone runs a stock truck with stock injectors. Sucking the fuel pressure down to zero doesn't give me the warm fuzzys.

ado2b1

I've got two carb pumps in series with a static regulator at 14-15psi. WOT never goes below 7psi no matter what. Best of all the pumps were free. Scavenged from gas projects. Didn't figure they would last long, but one year and still going strong.

I'm just hoping this really does add a measure of protection for a VP.

Fishin2Deep4U

Hohn, the only issue I have is that pressure is not measures in the VP44. Since we are measuring the restriction at the last fitting typically, the pump body has less pressure in it.

I have modified a VP44 inlet to accept a 14mm inlet. Pressures vary far more widely now. And WOT pressures can sink much lower. Makes me wonder how much pressure we really DO need!

BTW, there was a HP and economy increase for me above 30psi.

Dave

HOHN

I agree-- we have no idea what pressure is INSIDE the vp. That's really the only pressure that matters. I measure it *at* the VP inlet, which is probably somewhat close-- but how close? We can't know.

I would expect that the mpg and power gains you found at 30psi have to do with eliminating the last traces of cavitation and entrained air. Nice to have, for sure. But this must be balanced against VP wear and fuel heating. If your temps are in line, wear is the only issue-- and it's one you cannot quantify. We simply have absolutely no data how fuel inlet pressure vs VP wear--- never mind electronic durability. We suspect that negative pressure is bad. But we have no clue of how much positive is needed, what the effects of various pressures are, and how to put valid numbers on them.

Your pressure sink lower with the modified VP inlet because the inlet isn't as restrictive-- hence more flow and less restriction which would show up as FP on the guage. This works both ways....

jmo

Fishin2Deep4U

Yes, the increase in the inlet of the VP diameter allows for easier flow. I want to get a gauge inside the case one day to see just what is happening inside.

I don't know the long term effects of the high pressure, or the massive 12V pump pulsations on the VP. But now at about 80K, I am hopeful it'll keep on ticking.

Dave

bent valves

HOHN, I’m glad you started this thread. The folklore about the VP44 was getting a little too thick for me.

To add to your bullets;
-- The spillback valve used on the VP44 has no relationship to the external lift pump pressure.

“The vane -type supply pump in the low pressure stage pumps fuel from the tank (In the case of the Dodge the VP44 accepts pressurized fuel from the external lift pump) and produces a pressure of 8 …22 bar inside the fuel-injection pump depending on pump type and speed.” Page 212, Diesel-Engine Management
Translating bar to psi; 8 to 22 bar is the equivalent of 118 to 323 psi. Thinking that 20 or more psi of lift pump pressure adds more cooling and lube to the VP44 is a fallacy.

MeanGrn

maybe you guys havent seen this...?

http://www.bluechipdiesel.com/vp44diagnostichelp.html

nitrousn

Or this.



http://www.bluechipdiesel.com/diagnoseliftpump.html

67HotRod

All I know is not to run low on fuel, especially during the summer. And keep decent fuel pressure, even under WOT. ALL I can do....unless I swap in a P-pump.

Mike D

One thing you got to say about blue chip, creative writing skills.

The special X pump is a beautiful piece of fiction!!! LOL Gee, wonder why his special X pump does nothing to increase mileage HO trucks?

Is his shop even a Bosch Certified pump shop?

http://www.boschservice.com/ServiceL...p=DF&cntry=USA