FASS DDRP Lift Pump and relocation kit questions.

[Joey.G]

Hi, I know the DDRP is not really a performance upgrade but it is new and came with the truck. It is mounted already but I plan to relocate it near the tank. The kit comes with 3/8 line, At what point do I want to go to 1/2 inch line? I am going 1/2 from Filter housing to Injection pump with better fittings. My plan at this point is just a Quadzilla tuner, Will DDRP keep up at max level? I will upgrade Lift pump at some point.



Thanks

 

[petersonj]

Relocating the lift pump onto the frame close to the fuel tank is a very good modification. It allows the lift pump to operate more like a pusher pump. When the fuel tank is full, the lift pump will actually have a flooded inlet.

An in-cab fuel pressure gauge (or a low fuel pressure warning light) for the lift pump is also very good to have for VP44 trucks.

As far as increasing line size, I personally would not do it unless a lack of fuel pressure at the VP44 inlet on a properly operating fuel supply system dictated doing so. At engine idle, the VP44 sends about 18 gph of fuel back to the tank via the overflow valve. The injectors consume less than 1 gph of fuel. At wide open throttle the VP44 will send about 30 gph back to the tank. The injector could consume up to 10 gph (just a guess) on a truck with large injectors and a tuner. The combined flow would be 40 gph. As you can see, even a low volume lift pump (65 gph) and stock fuel lines would be more than adequate to supply fuel to the VP44.

My truck has RV275 hp injectors and a Smarty tuner set on a mild tune. I have stock fuel lines and I have been operating a used frame-mounted FASS DDRP lift pump for the last 130,000 miles. I have no performance issues.

There are many well established myths and a lot of misconceptions regarding the operating relationship between the lift pump and the VP44 fuel injection pump that have been written in the diesel forums over many years. Do your research and avoid being drawn in.

- John

 

[Joey.G]

Does the DDRP have different springs to adjust pressure? I got the fuel pressure set up with the Big line kit and quadzilla.

 

[petersonj]

Does the DDRP have different springs to adjust pressure?

I think some pumps do and some don't. Why do you want to adjust pressure?

- John

 

[Joey.G]

I was just curious, this way if I did have an issue I'd know ahead of time.

 

[petersonj]

This is where some of the misconceptions surface. The sole function of the lift pump is to provide more fuel to the VP44 injection pump than the VP44 could ever use. As a result there will always be a positive pressure at the inlet of the VP44. A specific pressure is not that important. Flow is important.

In my previous post I mentioned that the maximum fuel the VP44 and injectors could use would be about 40 gph. Let's raise it to 50 gph. A 65 gph lift pump would provide 15 gph more than the VP44 could possibly use, consequently the fuel pressure would rise and 15 gph of fuel would return from the discharge of the lift pump back to the lift pump's own inlet. If the lift pump's relief valve is set at 10 psi, 15 psi, or 5 psi - it doesn't matter, the same amount of fuel (15 gph) would return to the inlet of the lift pump and the VP44 would be happy. So, raising the pressure of a lift pump does nothing to improve the performance of the VP44 fuel injection pump.

If, for some reason the VP44 and injectors consumed a maximum 80 gph with the engine under full power, then the pressure setting of the lift pump would be irrelevant because the fuel flow (65 gph) would insufficient and the VP44 would starve for fuel. And, of course, the fuel pressure would drop to zero on the gauge due to lack of flow.

- John

 

[Joey.G]

Makes sense John, I just keep reading not to let pressure fall below 10 psi or so as the extra fuel is used for cooling the VP.

 

[NIsaacs]

The best way to keep your lift pump and injector pump cool, is to run on the top half of your fuel tank. Low fuel is hot fuel.

I am not a fan of modifying lift pump locations and changing fuel lines. The factory steel lines are super tidy and hard to match. The more you keep them close to oem the easier it is to repair on the road. Parts are hard to come by on a complicated system.

I am running the Raptor 150, block mounted. If need be, the oem Carter can take its place.

 

[Ozymandias]

The VP44 has an internal low pressure pump, the only thing important is to keep a positive pressure for longevity of the unit.
Actually the VP44 was designed to run without any lift pump at all*, just real life proved that it didn't work out the way engineers thought it would.

*and was installed in many applications without one.

 

[petersonj]

Makes sense John, I just keep reading not to let pressure fall below 10 psi or so as the extra fuel is used for cooling the VP.

You are reading the many, many posts that absolutely believe in that very myth.

Any properly operating lift pump has NOTHING to do with how well the VP44 is cooled. There is a fixed displacement vane pump inside the VP44 that is regulated at above 100 psi. The lift pump feeds directly into the inlet of this vane pump. A 14 psi overflow valve fitted to the side of the VP44 controls the fuel returning to the fuel tank.

It is amazing how long some of the myths last. There were extensive fuel return flow tests done by a group of people way back in 2001. These tests should have debunked any theory that lift pump pressure is important to VP44 fuel return flow..., but, it didn't.

Apr 24, 2001
Apr 25, 2001


I have personally performed fuel return flow tests using my truck. I even went one step further and eliminated the lift pump completely for test purposes. The VP44 return flow remained the same as when the lift pump was included in the circuit - almost identical to the results of Mopar-muscle's test results.

By the way, my VP44 was replaced under warranty (along with the in-tank lift pump conversion) in 2005 at only 87,000 miles. The re-manufactured VP44 has now logged 296,000 miles (over half of its life with lift pump pressure showing 6 psi at idle and 3 psi at wide open throttle with the OEM in-tank lift pump.

Start with what you have and monitor lift pump pressure. If there is always a positive pressure under the most demanding engine load, then the VP44 will be happy.

Actually the VP44 was designed to run without any lift pump at all

It is for this reason that I have a fuel line (with a one-way check valve) routed around my frame-mounted lift pump. If the lift pump fails for any reason and its passageways are blocked, I will be able to continue driving without harming the VP44.

- John

 

[Joey.G]

Thank you for the information it does make sense.

 

[Joey.G]

If I frame mount the DDRP will I be able to use the same holes or at least some of them if I move to a Fass 165 or an Airdog down the road? I would rather not keep drilling holes in the frame.

 

[SAndreasen]

I wouldn’t waste the time or the money it’s perfectly fine right where it’s at .

 

[Joey.G]

Most things I have read said it should be moved, Not saying you are wrong I am not really sure.

 

[BigPapa]

Why do “they” say it should be moved? It was designed to mount where it is and functions well there. If you want something better, go to the in tank pump.

 

[Joey.G]

Pretty much everything I have read says the Lift pumps are better pushers then pullers and should be mounted as close to the tank as possible, A qucik google search turns up lots about why its better to move it.

 

[Ozymandias]

That's why the in-tank pump is the greatest solution as BigPapa said.