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Dual Fuel Pumps and Dropping Fuel Pressure

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We dyno'd our car a week ago. The car is 430 inch windsor, F1R procharger, holley efi running on E85. The car made a few pulls but we had issues with it leaning out after 5500 rpm. The best pull made 900 hp at the tires at around 5500 rpm, 16 psi boost. We run two fuel pumps and Aeromotive A1000 primary and Aeromotive Eliminator as the secondary pump. The secondary pump is set to switch on at 50% throttle or a set manifold pressure by the ECU. Both pumps would come and when tested independently would run and bring the system up to pressure, around 45 psi. The issue is that after 5500 rpm with both pump running the fuel pressure would start to drop.The regulator should raise fuel pressure 1:1 with boost. Our best guess as to what is going on is the alternator is not supplying enough amperage for both pump to keep up. Lines and filters are not clogged.

Has anyone else run into similar situation that may have suggestions?

How much does it drop? Is there a chance you are really overrunning the reg or returnline at lower load so that the pressure drops to what its REALLY is set to when it starts to eat away fuel enough?

As the alternator is maintaining the voltage, I would agree it is effectively powering the pumps - what is the actual voltage across the pumps? I would like to see within 0.2V of the voltage across the alternator (output to housing) and if it is less, might be a good idea to run down the voltage drops. Remember, power is proportional to the square of voltage, and flow and/or pressure can drop off rapidly with voltage loss - you will see that if you reference the pump's output curves which should be on the manufacturer's site.

You say the fuel lines and filters are not clogged, but are they actually large enough for the volume of fuel you are passing through them? In any fluid transfer system there is a pressure gradient through it (otherwise the fluid won't flow) and as the flow rate increases through a restriction, like the filters, lines and, often overlooked, the fittings the pressure drop increases.

If I were you, I would check the voltage first and improve wiring, etc, as required, then review the flow ratings of the various parts of the plumbing, starting from the fuel tank pickup(s), then the filters and then the pipes/hoses and fittings. NOTE, when I say fittings, some are based on high pressure (5k PSI working pressure is common) hydraulic parts and have small internal bores for strength but there are lower pressure-high flow versions of many available which have extra large internal bores.

IIRC, a rough guide for a petroleum fuel is 1/2 pound/hp/hr in a NA application which I would estimate to be roughlydouble for E85 - so maybe aim for 15lbs/min minimum for E85 with the extra over-heads and losses? I think that would be roughly 2 US gallons/min?

What if it's just a transient effect of waiting for the rail to fill as the pump switches on? Try running the 2nd pump continuously and see if it still occurs.

Good point, and why a damp(en)er should be used.

What bore of fuel lines are you running?

Have you measured the voltages during a run at each pump?

Have you checked the bore of all your fittings? it wouldn't be the first time I've seen fittings narrow internally causing restrictions.

Have you got check valves installed?

The lines are -10 (5/8" or 16 mm) ID an feeding from fuel cell to pump and from the pump to the regulator. From regulator to the rails, they are -8 (1/2" or 13 mm) ID. We did not measure voltages during each run, it is something we will try to do in the future. I may look at connecting a voltage meter across each pump so they both can be viewed during running. There is a check valve installed in the system, we followed Aeromotive's guidelines for system, filter placement, check valve location , y location and line sizes for feed and return.

So you're running it as a dead end system?

What bore size is your fuel rails?

The internal bore of the fittings, not that of the hoses - some are very restrictive.

For example - https://www.xrp.com/product/big-bore-xtreme-flow-fitting/

Chris, he said "and return".

What FPR are you using? To me it sounds like the FPR pressure has been set with too high volume of fuel and cant keep up when injector duty cycle increases. Try lower pump voltage at idle and low loads, make sure to reset the base pressure at the lower voltage.

Andy from adaptronic has a good video of FPR slopes.

A second fuel pressure gauge just after the pump outlet will tell you how restrictive your line and filter are.