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  1. #1

    Default Fuel pressure education

    Can someone educate me on fuel pressure? I installed a Carter ss series self regulating mechanical pump M60454 rated at 7 min 8 max psi to fuel a new 650 Holley 4150 double pumper that Holley says needs 5-7 Psi. I’m running a 3/8 steel line from the pump to the carb through a billet Holley 40 micron filter before a pressure gauge just in front of the first of two fuel inlets to the carb. The carb still has Holley filters in the inlets.

    The car runs great on acceleration and idle but the gage only reads 5 psi. Shouldn’t it be 7 as thats is that’s carters minimum? Shouldn’t the pressure be dictated by the pump, and only be higher with restrictions like the filter or the 90 degree /T finlet fittings at carb inlet? Wouldn’t it only be lower with a return system or a leak? Confused as to where 2 lbs pressure would be lost. Is it because the carb is dictating it as the last restriction in the chain?

    Given the car runs great I don’t see a need to change anything but I am bugged I don’t understand the dynamics here. I’ll probably change out the gage just to check it’s accuracy nut that seems like an easy out.

    thx steve
    Steve

    1983 Mustang GT
    Popular Hot Rodding Project Car '83/84

    1983 Capri 331 ci, TKO600

  2. #2
    FEP Power Member richpet's Avatar
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    Default

    Your filter is between the pump and the gauge. Hence the filter, as a slight restriction, can show a drop in pressure after it where your gauge is. Or...
    *The pump could be putting out slightly less than advertised.
    *The gauge could be reading a touch low
    *It could be just perfect

    Since it's running properly I would not overthink it.

    Almost as important as pressure is actual flow rate, and you seem to be flowing enough.

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    Last edited by richpet; 08-22-2022 at 08:19 PM.
    83 5.0 GT. Quicker than it looks! 10:1 (or just over) 306, Motorsport a332 cam, 140A alt, t5 conv, 8.8 w/ 3.27's, Edel rpm, alum rad, very worked e7's, Holley SA carb, etc... SOLD IT!!!!

    Now an 1981 Granada! .040 over 302, Edel E-street heads... Currently building a 347 because, why not?

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  3. #3
    FEP Senior Member gt4494's Avatar
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    [QUOTE=richpet;
    Since it's running properly I would not overthink it.

    Almost as important as pressure is actual flow rate, and you seem to be flowing enough.

    Sent from my Pixel 6 Pro using Tapatalk[/QUOTE]

    As he said. The Model A uses gravity to get fuel pressure never has an issue. As long as it runs right it must have enough fuel.
    "If you can't explain it simply, you don't understand it well enough."
    Albert Einstein

    1984 20th Anniversary GT350
    Almost "Stock"

  4. #4

    Default

    Like was said, if the car is running fine then don't overthink it. The pressure difference is not much at all, gauge could be a couple psi off.
    79 Zephyr, 4.6L 4v/4r70w swap, with team z front and rear suspension, 8.8 and upgraded brakes and coil overs. Running Holley Terminator X Max.

  5. #5

    Default

    I’m not so much worried as confused…hence my comment on education. Seems to me that a filter and a gauge should impede flow and therefore increase pressure. If a gauge supposedly reading 7 psi is off by 2 psi that’s a +30% error rate which is junk in terms of information. I’m leaving the set up as is and will try another gauge. Thx all.
    Steve

    1983 Mustang GT
    Popular Hot Rodding Project Car '83/84

    1983 Capri 331 ci, TKO600

  6. #6
    FEP Power Member richpet's Avatar
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    It looks like in your original post you said the gauge is after the filter. That means that might contribute to a slightly lower reading at the gauge. Any higher/pressure increase would be immediately after The pump and prior to it's first impediment to flow. The pump is putting out the pressure, the filter is allowing reduced pressure past to the gauge.
    Also, pumps are tested and rated with optimal/different conditions. I get your 30% error comment - I am quite a nerd. I honestly think your observation is 100% within normal actual real world reality.

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    Last edited by richpet; 08-24-2022 at 09:02 AM.
    83 5.0 GT. Quicker than it looks! 10:1 (or just over) 306, Motorsport a332 cam, 140A alt, t5 conv, 8.8 w/ 3.27's, Edel rpm, alum rad, very worked e7's, Holley SA carb, etc... SOLD IT!!!!

    Now an 1981 Granada! .040 over 302, Edel E-street heads... Currently building a 347 because, why not?

    "Never underestimate the power of stupid people in large groups"

  7. #7

    Default

    So with pipelines you loose pressure the longer the pipeline is (roughness of the pipe walls contributes to frictional pressure drop) the more the pressure drops, each fitting will contribute to pressure loss when equated to straight length pipe (again roughness of the pipe wall), and any restrictions (things that reduce the ID of the pipe) will have a pressure drop across them which is referred to as delta P). Realize that the flow rate will be the same from the discharge of the pump to the carb but there could be a pressure drop....blah blah blah nerd engineering stuff which is my world.

    As others have said the pump could be on the low end of what it is advertised to pump or the pressure gauge may not be 100% accurate. First thing I would look at is if the pickup in the tank has a sock on it you may want to replace that. From there what is the condition of the fuel lines from the pickup to the suction (inlet) side of the pump? You need to make sure it is in proper order so that the pump is not dealing with an NPSH (net positive suction head) issue which will affect discharge (outlet) pressure and volume (flow rate). When you have verified that you need to test the discharge pressure with a known gauge at the discharge, after the filter, and at the carb.

    This is just me but I would ditch the filters in the inlets of the carburetor and make sure the Holley filter you have flows the required gpm (preferably more) so there is not a big Delta P across it which is a pressure drop. Holley requires a 40 micron filter for carb setups and having both of these is redundant and causes a pressure drop you don't want.

    The fuel pressure gauge on my T-Bird is on the fuel pressure regulator which is less than 3' of fuel line from the carb and I do not have the filters in the inlets of the Holley 750 DP.
    Last edited by Aerocoupe; 08-25-2022 at 10:49 AM.
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    '83 351W TKO'd T-Bird on the bottle

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  8. #8
    FEP Super Member gr79's Avatar
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    Default

    Return line from pump to tank can reduce fuel pressure.
    Temp pinch the return hose closed then read fuel pressure.
    -2# no big deal if engine runs fine.

  9. #9

    Default

    Could just be the gauge as others have said. I work as a plumber and noticed that as you get to the smaller psi gauges, they tend to be more sensitive to things such as temperature or vibration and have a tendency to “settle” or read just a bit off. That is just my experience though.


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    1984 LTD LX, 160k mile Explorer 5.0, Comp XE264HR-14 cam, Alex’s Parts springs on stock GT40 3 bar heads, Unported Explorer intake, 1 5/8 shorty headers, off-road H-Pipe, Spintech 9000 mufflers, Holley Terminator X Max, J-Mod 4R70W, Mustang 8.8 w/ 3.73s, Tubular front and rear control arms, front coilovers, Turbocoupe rear coil springs

  10. #10

    Default

    I’ll keep playing with this to see what affects it. Some thoughts
    - put a different new gauge on and got the same 4-4.5 reading. Maybe both are off but unlikely as both new and are different manufacturers
    - no sock but a filter outside the tank on the rear bumper. No sediment, and a new filter and then running no filter he had no impact on psi
    - no return line
    - prior owner rand braided 5/8 hose from tank to bumper filter, and 1/2 braided to pump.
    - This winter I’ll replace all with steel line since I don’t know if it’s PTFE lined or how old.
    - filter is 40 microns 2 ft from pump and gage is less than 1 ft away from filter and gage is at the T to the first carb inlet
    - similar set up gets 6 lbs on my Mopar and Chev strokers although they both use return lines with regulators
    - Holley recommends using both 40 micron filter and internal carb filters but maybe I’ll try without just to get a reading

    happy car runs well but I’ll play with it over winter and let you all know if I the psi up.
    thanks steve


    -

    -
    Steve

    1983 Mustang GT
    Popular Hot Rodding Project Car '83/84

    1983 Capri 331 ci, TKO600

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