Fuel Pumps: How to determine your needs
Fuel pump type and flow rate are important considerations when developing your fuel system.
There are two main fuel pumps when building an EFI fuel system: inline and in-tank.
There’s also a third option: mechanical.
Mechanical pumps in EFI systems are generally reserved for very high horsepower. They are used with fuels like methanol that require large amounts of fuel flow. These pumps are located near the engine and have an entirely different set of requirements that we will not be covering.
Fuel Pumps: Sizing
The first thing to figure out about a fuel pump is your fuel flow requirement. We need to know how much fuel the pump needs to supply to meet the horsepower goal.
Modern in-tank and inline fuel pumps can meet most high horsepower demands. Determining your flow requirements will allow you to consider which style of pump is most beneficial.
The first thing we need is our brake specific fuel consumption or BSFC. Later, we’ll also use this rate to size the fuel injectors.
Here are some general V8 BSFC numbers, most engines BSFC’s are available with a simple online search. Custom engines excluded; however, your motor builder should know the approximate BSFC for your build.
Gasoline: .45 - .50
E85: .63 - .70
Boosted (turbo or supercharger):
Gasoline:. .60 - .65
E85: .84 - .91
After choosing a BSFC you’ll determine your horsepower goal. It's a good idea to assume a little on the higher side.
Example: a near stock, naturally aspirated truck motor, running pump gasoline with stock cylinder heads will have the lowest bsfc: .45. Assume the horsepower goal is 400hp. The equation is simple:
.45 x 400 (BSFC x horsepower) = 180 pounds of fuel per hour.
Now, we take our 180lbs of fuel per hour and divide it by 1.6 to get our liter per hour fuel requirement.
So, our lph fuel requirement is 112.
For gallons per hour we divide our 180 by 6 to get 30 GPH.
Once we know our liter per hour and gallon per hour fuel requirements, we can start to shop for fuel pumps.
Here are a few considerations to keep in mind when choosing a pump:
Most pumps are rated at their “free flow” state which means they are flowing at 0 psi. Since pump flow reduces as pressure requirements rise, this means that free flow is the pump's highest flowing state. It's also the least useful. Fuel pump manufacturers will often include charts that show flow rate vs. pressure. You need to determine if you will run 43 or 58 psi in your fuel system and then consult these charts to get an accurate assessment of the pump flow at the actual system pressure you intend to run.
The chart above shows the SEP 415 (rated 415 LPH @ 43psi) compared to two competitor Pumps rated at free flow which are then listed as higher flowing pumps (450 and 525lph). The chart shows that while it appears the 415 is the smallest pump it is actually the highest flowing for boosted pressure ranges and equal in naturally aspirated ranges.
Inline Fuel Pumps
Inline fuel pumps are extremely common and at one time dominated the aftermarket.
These types of pumps can be larger and have a higher rate of flow because they do not need to fit into the fuel tank. They work with nearly any application, and with fuel cells or fuel tanks. These pumps can be paired or even tripled together and wired separately with various forms of switches to activate them at different times depending on fuel requirement.
Inline fuel pumps use incoming fuel to cool the internal components. Because of this, they can overheat in certain situations like stop and go traffic where low air movement is unable to cool fuel and road surface temperatures are high.
Inline fuel pumps must be mounted as close to the fuel tank or cell as possible. These pumps are pushing pumps and can’t efficiently draw fuel in from long distances away. Mounting them too far away will result in poor performance, sometimes known as “Starving” the pump.
In-Tank Fuel Pumps
In-tank fuel pumps are the most common fuel pumps in the OEM market. Their size is limited to the opening on the fuel tank, so their horsepower capabilities are limited. However, modern brushless motors and turbine technology have produced some incredible in-tank pumps that can support massive power.
The fact that these pumps are in the fuel tank means they are well insulated. This is great for sound reduction and cooling. These pumps are nearly impervious to external temperature changes since they are surrounded by gallons of fuel.
They can be paired or even tripled together and wired separately. Using switches, this allows them to activate them at different times depending on fuel requirement, like inline pumps. Modern pulse width modulation capable fuel pumps can have flow reduced or raised on command from the ECU. Many aftermarket ECUs can support this feature making it a great option for any horsepower range.
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