Diesel Lift Pumps – What is More Important…Pressure or Flow?

Written by Matt Gilmore

There seems to be a lot of confusion on this subject.  You must have adequate flow to support the needs of the engine.  What about pressure?  Is pressure just a product caused by the resistance to flow?  Is pressure necessary?  Does a drop in pressure at wide open throttle mean I don’t have enough flow?

Sizing a lift pump with adequate flow is the first step.  You need enough flow to support the type of injection system you are running.  Sizing by horsepower will get you in the ballpark, but ultimately your injection system has to be taken into consideration.  Going off horsepower alone is not enough information.

Pressure exists due to a resistance in flow.  When it is put that way, it sounds like a bad thing.  However, pressure is necessary for a fuel system.  Many fuel injection pumps rely on fuel pressure to operate the timing circuit.  All fuel systems benefit from pressure during high demand situations.  A diesel engine running at 3000 RPM has very little time to fill a pumping element before the next cycle…milliseconds, literally.  Pressure helps to fill the pumping element.  If pressure drops too far, the pumping element will pull a vacuum.  It will live with some vacuum, but too much will cause cavitation and vaporization.  This condition can cause more damage than dirty fuel.  In layman’s terms, cavitation causes the fuel to vaporize.  The vapor fills the pump cavity; then the pump pressurizes the vapor.  When the vapor is compressed, it implodes.  The implosion erodes the metal away and leaves craters in the surface.

A quality fuel pressure gauge at the inlet of the injection pump should be used in all high performance applications.  This is a cheap and simple way to monitor fuel system needs.

What causes pressure drop at WOT?  The first thing that usually comes to mind is the lift pump won’t keep up.   That, however, is usually the incorrect answer.

Many things can cause low pressure.  The fuel pressure relief valve (what regulates the pressure) is one that is often overlooked.  Not all relief valves are the same.  Even if they look identical, there can be minor differences that have a major impact on valve performance.  Spring selection is one common mistake with budget relief valves.  There is no “one size fits all” in this category.  You cannot take an 8 PSI relief valve and simply shim the spring to 18 PSI.  It can be set to run at 18 PSI, but it will be erratic and drop off in pressure, under wide open throttle.  A quality relief valve will run steady pressure at idle and maintain that pressure at cruising speeds.  A full throttle run should have a minimal amount of pressure drop.

It should be noted that fuel pressure relief valve poppet design can have a major impact on fuel pressure and flow. The Ball Shaped Poppet is commonly used, however, this is a flawed design as the ball can vibrate (also known as “valve chatter”), which inhibits smooth fuel flow and creates fuel pressure spikes. A Cylindrical Shaped Poppet provides a superior design, as the diameter of the upper portion of the poppet is stabilized within the bore of the valve housing. The poppet smoothly slides open and closed along the bore. This combined with the inlet ports on the side of the poppet smooths fuel flow, and valve chatter is virtually eliminated.  This reduces fuel pressure spikes and results in a much better fuel flow curve.

Relief Valve Comparison with Names

Pressure versus flow, they are both important.  While flow is a necessity, pressure with good regulation is vital to a quality fuel system.


Fuel Line Size vs. Pressure Drop

It is important to understand the relationship between fuel line size and fuel pressure when planning a fuel delivery system. Fuel systems can be incorrectly designed if the pressure loss attributed to the length of the fuel lines isn’t taken into account. Excessive pressure drop in the fuel lines feeding a carburetor or EFI system will inhibit their proper function, and in the case of a bypass or return style regulator, excessive pressure drop through a return line will squelch the regulator’s ability to operate correctly. To deliver fuel at the correct flow rate and pressure, careful consideration of fuel line diameter and length, as well as whether the application is carbureted or EFI, is very important.

Back in the early days of aftermarket EFI systems, the great veterans of carbureted fuel delivery systems would scoff at running a small EFI return fuel line saying “It will never work!” They would also look at the much smaller return orifice in the EFI regulator and say “It will not flow enough!” What they did not know was that Carbureted and EFI systems operate differently because of sensitivity to pressure drop.

The topic of determining actual pressure drop and how it can affect different systems is typically not addressed or performed.  We often apply the “what has worked” principle and have a general idea about what line size can work and what will be too restrictive.

This article examines an analysis of pressure drop for a supply line used in a fuel system.  The example uses experimentally derived data to show how pressure drop in the fuel line will have performance effects on the fuel system.  This examination will also show how this performance loss affects a low pressure carbureted pump versus an EFI pump. While this article does not show an analysis over an entire fuel system, it does discuss how much pressure drop exists in some typical fuel lines, how can this information can be used to see how performance is affected, and how some components behave differently than others.

Let’s start by examining the source of pressure loss: Friction. Fluids experience friction as they pass through pipes and hoses, and this drag reduces the pressure under which the fluid is being delivered. In the case of fuel lines, two influences increase friction and therefore fuel pressure drop. The first is the length of the fuel line – the longer it is the more friction will be incurred. The second is flow rate – as the rate is increased so is friction.

Graph A. Shows the relationship between measured pressure drop as a function of flow rate, for four different fuel lines (two different sizes at two different lengths).  

Graph A copy

Every fuel line experiences pressure loss depending on fuel line length, and fuel flow rate. It should be noted that fuel lines which feed pressure gauges experience the least amount of loss, as the flow rate through the gauge line plummets to near zero flow levels. This is why gauges typically use 3/16″ or 1/4″ line size, as they will not affect performance due to pressure loss.

Now, we’ll examine how fuel flow rate is affected by fuel pressure drop in a carbureted system. Let’s take the maximum flow rate variable into consideration. Let’s say we are dealing with a carbureted fuel pump that has the following flow rate/outlet pressure characteristics:

Graph B.  

Graph B copy

The graph shows the pump’s maximum possible flow rate is at free-flow (160 GPH), yet we know the maximum flow rate while under actual operation is going to be much less.  If the minimum required fuel system pressure for our example is 6 PSI (not accounting for other losses), the graph shows us the maximum flow rate to be considered is 110 GPH.

Taking this information into consideration, let’s examine how different fuel line sizes can affect pressure drop. This example compares -6AN and -8AN supply fuel lines, both are 14′ long. Referring to Graph A. – the amount of pressure drop we can expect for 14′ fuel lines at 110 GPH is:

  • -6AN Fuel Line:  nearly 4 PSID
  • -8AN Fuel Line:  nearly 1.5 PSID

It seems that a “couple of PSI” will not change much. However, depending on how much pressure loss can be tolerated by the system before it affects performance, it could prove to be a significant problem.  Graph B. shows that by adding just two PSI of line drop to this example pump, we can lower the available flow rate from 110 GPH to 80 GPH (over a fourth of the total capacity). This example shows carbureted systems are very sensitive to fuel line size.

However, it is important to know that things are a bit different when determining the appropriate fuel line size for an EFI vehicle. EFI fuel pumps don’t experience the same pressure loss issues as carburetor fuel pumps. Take a look at Graph C which describes an example of the performance of an EFI fuel pump versus a carbureted fuel pump.

Graph C.

Graph C copy

Obviously, the fuel flow rate for the carbureted pump is much lower than that of the EFI pump when at higher pressure. So, let’s examine Graph D that compares the performance over a span of 3 PSI, near the lower end of the pressure range.

Graph D.

Graph D copy

When we take a closer look, we can see that the two curves have different slopes. Compared to the carbureted fuel pump, the graph for EFI fuel pump seems to be nearly strait across.  If we add 3 PSI in difference regarding pressure, the EFI pump flow rate performance may have dropped off less than 3%, yet the carbureted pump flow rate loss is much more dramatic at greater than 33%. Showing EFI systems are not nearly as sensitive to fuel line size as carbureted systems.

These examples go to show that when planning a fuel system not only is flow rate and line length important when determining fuel line size, but more importantly:  Whether the fuel delivery system is carburetor or EFI based.