8 stacks with two tanks

[Bob Putnam]

I'm sure I'm confused (one of those days). This is also a test to see if I'm thinking about this right.

I assumed the "overflow" tank you referenced was the "right tank" in the diagram.

Either tank will do.

If so, then the LP pumps are going to flow as much fuel as they can at 2psi regardless of engine demands as they are oblivious to anything but the 2psi regulator. Because the line behind the 2psi regulator is "free flow" the regulator is going to flow continuously. There is no way to dead-head it like you would with carburetors when the float bowls are full.

The swirl pot is completely sealed except for a very restricted return line to one of the tanks. The LP fuel pumps will only be able to flow what is necessary to take up what is used by the engine.

The EFI is going to scavenge from the swirl pot what the engine consumes.

The excess (if any) is going to flow to the "overflow" tank.

No. The flow from the HP pump is returned to the swirl pot. The only fuel lost is what's consumed by the engine and the small amount passed by the bleed system.

 

[Ron Scarboro]

The swirl pot is completely sealed except for a very restricted return line to one of the tanks. The LP fuel pumps will only be able to flow what is necessary to take up what is used by the engine.

You then need to make sure that the LP pumps you use have an internal bypass and regulator. I don't then see the need for a 2psi regulator in the model because the psi in a "very restrictive" return line will be much higher as any non-regulated pump will pressurize until the flow rate matches the back pressure through the return line at maximum flow rate. Even a regulated pump is likely to be at 5psi or so.

No. The flow from the HP pump is returned to the swirl pot. The only fuel lost is what's consumed by the engine and the small amount passed by the bleed system.

I had that. Given the size of the return line from the swirl pot to the "overflow" tank you can calculate the pressure in the swirl pot plotting that against the flow curve of the pump. The downside to this system is that there isn't much opportunity for the fuel to cool.

A larger return line and a lower presssure in the swirl pot would allow for better fuel temperatures and less aerated fuel (better injector modulation) for the HP pump.

You could also lower the pressure by having a weaker LP pump, but then racing applications could overwhelm the LP pump and "very restrictive" return line meaning a loss of pressure across the rails.

 

[Terry Oxandale]

So based on the very informative comments thus far, IF by chance the LP pump(s) is insufficient to meet consumption demands with the regulator on the very free flowing -10 return line (and subsequently producing a negative pressure in the pot), then I'd be better off with a "very restrictive" but open return line that would allow air to be drawn back into the swirl pot, thus allowing the HP pump to work efficiently. The "very restrictive" return line would provide some pressure rise in the swirl pot duirng times of low consumption.

Not sure here, but I believe the LP Holley pumps will allow fuel to be drawn (or pushed) through the pump, so this may not be an issue, but a good idea (restrictive by open return line) to keep for a rainy day.

 

[Ron Scarboro]

So based on the very informative comments thus far, IF by chance the LP pump(s) is insufficient to meet consumption demands with the regulator on the very free flowing -10 return line (and subsequently producing a negative pressure in the pot), then I'd be better off with a "very restrictive" but open return line that would allow air to be drawn back into the swirl pot, thus allowing the HP pump to work efficiently. The "very restrictive" return line would provide some pressure rise in the swirl pot duirng times of low consumption.

IMHO a regulator on the return line between the swirl pot and the "overflow" tank is a bad idea. The restrictive return line would be a better idea than that. However, a free flow solution with a switch valve has the advantage of lower fuel temp and better fuel consistency and is worth the bit of added routing and complexity IMHO.

Not sure here, but I believe the LP Holley pumps will allow fuel to be drawn (or pushed) through the pump, so this may not be an issue, but a good idea (restrictive by open return line) to keep for a rainy day.

Any pump with an internal regulator and bypass would be a great pump for that application.

Here are the maximum GPH for various return lines...

• Up to 45 GPH = 5/16" or -04 AN
• Up to 90 GPH = 3/8" or -06 AN
• Up to 250 GPH = 1/2" or -08 AN
• Up to 450 GPH = 5/8" or -10 AN
• Up to 900 GPH = 3/4"or -12 AN