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Thread: Fuel hose size affects rail pressure?

  1. #1
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    Fuel hose size affects rail pressure?

    Hi everyone thank you for helping me out. M50b25 non vanos engine I've owned for awhile now. There's a fuel leak from one of the rubber hoses near the starter. The hose size is 7.5mm. Unfortunately, I couldn't get that but the parts guy said 8mm hoses will be fine. I bought those, intend to tape up and double clamp the metal pipes they fit over. Intend to replace either the feed fuel hose or the return fuel hose, depending on which one is actually leaking. Will be getting the manifold off to check that, have some other stuff to do there anyway so thought i might as well.

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    Now I'm wondering if the larger diameter will lower fuel pressure. And lower fuel pressure means lower rail pressure and the ecu not pumping out gas like it thinks it is. Is this a problem? The metal pipes and the fuel rail pipes are def 7.5mm or smaller. Will larger diameter rubber hoses before that affect the rails pressure ? The fuel pressure regulator is fixed at the end of the metal fuel rail, right up against the firewall.

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    Sorry if this is a bad question but I'm tired of thinking it through and I remembered bfc. Thank you gentlemen.DRJ
    Last edited by DrJBM; 03-18-2017 at 12:20 PM.

  2. #2
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    It shouldn't affect pressure at all

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    Hey appreciate the reply, what a relief. Would you mind explaining how it works out that way despite different hose sizes? I can't wrap my head around it I'm missing something important. Thanks.

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    The pressure regulator​ will hold it at the same pressure regardless of the tube size. The line itself will contain slightly​ more fuel given a slightly larger size, but it's really a nominal amount larger.
    It's been too long since I've taken a physics course, and I don't remember all the exact science behind it, but the fluid pressure will not change because of that.

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    The fuel goes from the pump to the engine and back to the tank. There is a regulator that opens and closes to keep the pressure on the rail. The size of the hoses should not matter because the fuel flow is constricted by the regulator. The regulator can be physically located at the fuel rail -- exit end -- or anywhere along the return side. The pump delivers a fixed pressure, let's say 60 psi for the sake of discussion. At idle, the vacuum on the engine is high and the fuel demand is low, so the fuel pressure regulator is held open so that 5 psi is on the injectors and the remaining fuel returns to the gas tank. When you go on the freeway, vacuum is low and fuel demand is high, the pressure regulator closes so that all of the fuel is at the injectors. Using a 0.5mm larger hose than stock will not be a problem. Indeed, you might have a hose that is really 8mm, but has shrunk over time. It should be all but impossible to get an actual measurement with the tools that most of us have at home. And, it's not rocket science, it just a car.

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    Your logic is right in thinking a larger diameter hose would result in less pressure. Bernoulli's principle explains this phenomenon. But the fuel pressure regulator will keep the fuel pressure constant, regardless of the hose diameter.

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    Quote Originally Posted by XnWarden View Post
    Your logic is right in thinking a larger diameter hose would result in less pressure. Bernoulli's principle explains this phenomenon. But the fuel pressure regulator will keep the fuel pressure constant, regardless of the hose diameter.
    So that should mean the regulator's pressure limit is MUCH lower than even the reduced pressure of a larger fuel hose. Even if that larger hose is somewhere else in the fuel circuit. Am I correct here ?
    Last edited by DrJBM; 03-18-2017 at 09:12 PM.

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    Quote Originally Posted by JDStrickland View Post
    The fuel goes from the pump to the engine and back to the tank. There is a regulator that opens and closes to keep the pressure on the rail. The size of the hoses should not matter because the fuel flow is constricted by the regulator. The regulator can be physically located at the fuel rail -- exit end -- or anywhere along the return side. The pump delivers a fixed pressure, let's say 60 psi for the sake of discussion. At idle, the vacuum on the engine is high and the fuel demand is low, so the fuel pressure regulator is held open so that 5 psi is on the injectors and the remaining fuel returns to the gas tank. When you go on the freeway, vacuum is low and fuel demand is high, the pressure regulator closes so that all of the fuel is at the injectors. Using a 0.5mm larger hose than stock will not be a problem. Indeed, you might have a hose that is really 8mm, but has shrunk over time. It should be all but impossible to get an actual measurement with the tools that most of us have at home. And, it's not rocket science, it just a car.
    Thank you for that comprehensive writeup. I love that, it helps me understand what is actually going on. I hate to do something important and have it work but not know why it worked even though the problem is solved.

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    Mr JDS this is the thing I couldn't work out. Is pressure throughout a fluid circuit of various hose sizes, determined by the smallest hose, or the largest hose? And that pressure reading, is it only used for calculations or is it the actual pressure at every point in the circuit ? Or are actual pressures different at every point ? This is for a situation where the pump puts out constant force but the pipes after that are of various diameters. I hope i've put that clearly although I probably haven't. Thank you.

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    In a 'no-flow' situation, the fuel pressure should be pretty much the same everywhere. Once gas starts to flow, there are flow losses in the system, with a higher pressure at the fuel pump outlet than at the fuel pressure regulator, again due to system loses.

    Regarding using a larger hose, theoretically, it just takes longer to fill and pressurize, but the larger hose has lower flow loses. In your case, the difference is minor, but just make sure you bought high pressure fuel pump hose.

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    Quote Originally Posted by DrJBM View Post
    So that should mean the regulator's pressure limit is MUCH lower than even the reduced pressure of a larger fuel hose. Even if that larger hose is somewhere else in the fuel circuit. Am I correct here ?
    Yeah i wouldnt worry about it honestly. I think the regulator is capable of pressurizing any size hose that will fit on it. There arent that many options.

  11. #11
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    I consider my doubts on this issue resolved. My conclusion is that the fuel pressure regulator steps DOWN the pressure MUCH MORE than the initial output pressure of the fuel pump. Thus even a larger fuel hose inserted somewhere in the circuit would have much higher fluid pressure than what the regulator would step it down to. And thank you everyone for your input it was very illuminating.

    DRJ
    p.s. Yes I bought dedicated fuel hoses so they will hold pressure and won't degrade in gasoline.

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    Quote Originally Posted by DrJBM View Post
    Thank you for that comprehensive writeup. I love that, it helps me understand what is actually going on. I hate to do something important and have it work but not know why it worked even though the problem is solved.

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    Mr JDS this is the thing I couldn't work out. Is pressure throughout a fluid circuit of various hose sizes, determined by the smallest hose, or the largest hose? And that pressure reading, is it only used for calculations or is it the actual pressure at every point in the circuit ? Or are actual pressures different at every point ? This is for a situation where the pump puts out constant force but the pipes after that are of various diameters. I hope i've put that clearly although I probably haven't. Thank you.


    Fluid pressure is the same throughout, the flow rate will change with the size of the hose. If the hose is small leading to the point where the actual demand takes place, then the flow to replenish after the demand is made can be insufficient -- future pressure might be low -- if the hose is too small. If the hose is large and demand is made, then the replenish rate will be met. If the injector rail makes a fuel demand and the hose leading to the rail is not sufficient to refill the rail -- try to look at this as a binary process, demand when an injector fires, then refill until the next injector fires -- then the pressure at the injector will fall until such time as the rail fills again. If the hose is large, then the rail fills faster. All of this is more than you were asking about though. Deviations happen if the regulator leaks and does not provide the restriction on the return line to keep the injector rail filled, and if the fuel pump is weak so that the flow rate and/or fuel pressure falls.

    I suggested the fuel pressure at 60 psi to illustrate the idea, not to state that this is in fact what the pressure should be. The idea is that at idle, there is far more fuel sent to the injector rail than the rail will consume in the normal operation of the engine. In this case the pressure regulator will open so that the excess fuel can be returned to the tank. As the demand for fuel increases, the supply from the pump is constant, therefore the regulator has to close to hold more fuel on the injector rail to meet the demand. The M50 engine needs the fuel pressure to be 3.0 bar (43.5 psi), the M52 uses 3.5 bar (51 psi). With the engine running, the vacuum acting upon the fuel pressure regulator will reduce the fuel pressure by about 0.5 bar. The fuel pressure regulator can give poor idle symptoms, and there is a check valve on the pump that can feed into these kinds of problems.

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    It doesnt matter what size the hose is. The only thing that matters is fuel pump able to keep up with demand and needed pressure, the fpr regulates to its set range by vacuem and the injectors allow the proper amount of fuel through when opened. The fuel line simply needs to be big enough to allow the fuel to pass easily without causeing reatriction to flow but not so big that too much fluid is contained putting access stress on the pressure regulator and fuel pump. Less work is nominal.

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    Quote Originally Posted by M3blitkrieg View Post
    The fuel line simply needs to be big enough to allow the fuel to pass easily without causeing reatriction to flow but not so big that too much fluid is contained putting access stress on the pressure regulator and fuel pump. Less work is nominal.
    Quantity of fuel contained doesn't change the quantity of work done by the pump or regulator at all. Flow rate is flow rate, and fuel isn't compressible so there aren't even losses to account for there.
    Caprica Junkie

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    If fuel isnt compressable then how does a fuel rail get its whatever lbs of pressure? The amount of fluid a fuel pump has to squeeze before it gets released is directly related to amount of work the pump must do. Think before posting attackish responses pls.

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    It takes more energy to compress more fluid to a set pressure. Size directly relates to flow rate if the pump is a set force. Juat for a simple expirement. Tak your faucet. Turn it on full blast. Now make the hole smaller with your finger by trying to close the end with your finger. How fast does the water come out? ... Lesson. Smaller hose =faster liquid=same flow rate. Now go ten miles away from the same water plant. Considerably longer distance away. (Much more water inbetween pump soarce and faucet). Turn that faucet on and you will see immediatly that the initial flow rate is slower... ..

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    Quote Originally Posted by M3blitkrieg View Post
    It takes more energy to compress more fluid to a set pressure
    that's wrong.

    the fluid just acts as the transporter of the pressure. just like brake fluid.

    fluids can't be compressed. (they can be compressed a bit, but not with pressures we deal with here, not even commonrail diesel injection pressure is enough. i'm just saying so you don't go google it and then come back and say they can be compressed and say i'm wrong because i said they can't)

    anyway, as far as i understand that hose is somewhere more or less in the middle of the whole fuel system, so it doesn't matter in any way. if you have a garden hose and cut it and put a 2 feet concrete pipe in between then at the end of the hose still the same happens as before.

  18. #18
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    Quote Originally Posted by samy01 View Post
    that's wrong.

    the fluid just acts as the transporter of the pressure. just like brake fluid.

    fluids can't be compressed. (they can be compressed a bit, but not with pressures we deal with here, not even commonrail diesel injection pressure is enough. i'm just saying so you don't go google it and then come back and say they can be compressed and say i'm wrong because i said they can't)

    anyway, as far as i understand that hose is somewhere more or less in the middle of the whole fuel system, so it doesn't matter in any way. if you have a garden hose and cut it and put a 2 feet concrete pipe in between then at the end of the hose still the same happens as before.
    It's like pushing a box along the ground, are you compressing it? No, but it is moving and if there was something in its way it would feel the force transported through the box.

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  19. #19
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    Its the same as a brake system for example. Your foot is the pump the master is the fpr and the caliper is the rail. If you elongated or made the brake line large enough to hold a gallon of brake fluid. Your foot would now have to push the gallon of brake fluid instead of only a few ounces. You may not have to push it further or longer than before. But you are indeed pushing more fluid. What is harder. Pushing 4ounces of brake fluid or pushing a gallon of brake fluid?
    Last edited by M3blitkrieg; 03-26-2017 at 08:58 AM. Reason: Punctuation

  20. #20
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    While it does affect it, it does not noticeably affect it. The diameter of the hose is increasing by 1/15 of it's width. We'll call it a 1/8 ratio increase in volume just because I'm lazy for the math. The car takes a fraction of a second to pressurize fuel, that small of an increase in pressurization time would not be noticeable at all to the driver, and although I'm sure there's a way, I dont believe op probably has reliable and precise enough equipment to measure that slight difference in the time it takes to pressurize the fuel where the difference wont be written off.

    Can we end this thread now? Op got the question answered, and we're arguing nothing. Yes it changes it on paper, but when the change is so slight as to not be able to notice it, it is negligible at best.

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