The purpose of this article is to walk through the process of fuel pump selection to understand what is involved when selecting a fuel pump. There are 4 main factors to consider; brand, price, flow, and fuel type.
Brand
Many tuners have very strong aligments with certain brands, personal and/or professional. Here at Radium, we rarely see fuel pump failures, so we are not biased for or against any particular brand. If you feel strongly about a particular brand, then stick to that. But dont completely ignore the options from the other brands, as they may have an offering that fits your application for a very reasonable price.
Price
Speaking of price, fuel pump prices can vary depending on pump model. For some customers, price is a concern and must be seriously considered. Know your budget and work within it.
Flow
Selecting Primary Pump(s)
Here is the fun part, numbers. We could talk forever about horsepower and brake specific fuel consumption (BSFC) and how that relates to fuel pump flow, but that tends to require many assumptions and approximations. We commonly use a simpler approach to size a fuel pump system, we base it off fuel injector flow. Essentially, we look at the max flow output of your injectors and make sure we have enough fuel pump energy to support the flow rate. This requires you to have the proper injectors selected for your engine. Visit the sites below for assistance in selecting fuel injectors for your vehicle. If you already have injectors, skip this step.
http://www.witchhunter.com/injectorcalc1.php
http://injectordynamics.com/injector-selector/
http://www.rceng.com/technical.aspx?UserID=25303407&SessionID=ySDo433ZAx4gdKkS9Y4m
http://www.deatschwerks.com/resources/fuel-calculators/fuel-injector-calculator
Once your injector flow rates are nailed down in cc/min, use the calculator below and input all the variables. Maximum injector duty cycle is the maximum amount of time the injector will be open (100% would be wide open). This value can be viewed and data logged using most modern engine management systems. If you are unsure, use 90% which will provide a safety margin. Number of injectors is pretty explanatory, just use the number of injectors this fuel system will be feeding (4, 6, 8, etc). Base fuel pressure is the static fuel pressure you set at idle, commonly 40-50psi. If you are using a turbocharger or supercharger, enter the max boost you plan to run in psi. Next, run the calculator.
There are two results shown, both show a flow rate in LPH and a pressure. If you run a vacuum line to your fuel pressure regulator, use the first line results (most common). If you are not using a vacuum line on the fuel pressure regulator (common with in-tank returnless systems), then use the results in the second line.
So now you have a target flow rate and pressure, the next step is to look for pumps that can meet that flow rate at that given pressure.
Flow vs. Pressure graphs are commonly published for all popular fuel pumps and if you are unsure, contact the pump manufacturer. We have assembled some flow numbers in the table below for some of the most popular pumps, all of which are compatible with most of our fuel surge tanks.
If the flow number you are looking for is higher than any of the pumps listed, you are going to need two or more fuel pumps. For this case, add each pump's flow rate together to get the total flow rate.
Selecting the Lift Pump
Fuel surge tanks require what is commonly called a "lift" or "fill" pump. This is the pump that moves fuel from the OEM fuel tank (or fuel cell) to the surge tank. This pump will operate at a very low pressure (less than 5psi) so it will draw low current and flow more than it's advertised flow rate, which is usually at 3 Bar (43.5 psi).
Selecting a lift pump is fairly easy. For most street applications, the OEM fuel pump will work well as the lift pump. For more demanding applications, a simple formula is used. The lift pump should have a rated flow rate of at least 50% of the primary pump(s) flow rate. For example, if a Walbro F90000262 (400 LPH) pump is chosen as the primary pump in the surge tank, you will need a lift pump that has a rated flow rate of at least 200 LPH (at 3 Bar).
Type of Fuel
Only two of the pumps listed in the table above were specifically designed fo use with E85, the Walbro F90000267 and the AEM 50-1200. However, other pumps listed have been shown to work well with E85 in most cases. For more information, contact the pump manufacturer.
EXAMPLE
Let's walk through an example: Nissan RB26DETT (6 injectors) using race gas, 1300cc/min injectors with 22psi boost pressure. Base fuel pressure is 43psi. The fuel pressure regulator is vacuum referenced to the intake manifold. Plugging these values into the calculator using 90% max duty cycle tells us that we need a primary pump that can flow 463 LPH at 65psi.
Primary Pump: Looking at the chart, we will use the values in the 72.5 psi column. The highest flowing pump shows 353 LPH, making it clear we are going to need a dual pump fuel surge tank. Since many of the pumps will meet our requirement when used in pairs, it is now a good time to take into consideration other factors such as the cost of the fuel pumps, and the packaging size.
For this example, we will assume the vehicle is extremely limited on space and must use the smallest surge tank package available. This leads us to the Radium standard fuel surge tank family. In order to fit two pumps into the standard surge tank, we are limited to 39mm pumps only (Bosch and Walbro F9000XXX pumps are too big). Looking at the chart we can see that two of the AEM 50-1000 pumps would have a combined flow of 520 LPH at 72.5 psi, which easily meets and exceeds our 463 LPH at 65psi requirement. The Deatchwerks DW300 and Aeromotive 340R would also work but are more expensive.
Selecting the lift pump for this application is as easy as taking half of the primary pump flow, so 1/2 of two pumps is one pump. So we will use a third AEM 50-1000 pump as the lift pump. Once again, other pumps can work as the lift pump, but for consistency many like to keep the same brand throughout the system.
ADDITIONAL CONSIDERATIONS
Here are some additional things to consider when designing your fuel system:
Staging - If using more than 1 primary pump and depending on the FPR's bypass capabilities, fuel pump staging is common. However, the pump(s) must be activated at the proper time or a pressure spike may occur. If you are using a programmable engine management system, it is very easy to program an output to trigger a relay for the staged pump(s). There are ways to do this as well using pressure switches, etc. You do not want to run all of the primary pumps all of the time. This leads to excess heating of the fuel and other negative effects.
Overkill - Grossly over-building the fuel system has drawbacks. For example, if you use the process above to size the fuel pumps, but your fuel injectors are oversized (i.e. injector duty cycle never exceeds 40%) for your engine, then you will end up with fuel capacity for three or four times what your engine requires. This leads to excessive electrical power consumption, and a lot of returned fuel from the regulator, which can cause the surge tank to build pressure, which is not ideal.
A good way to check, is to datalog your injector duty cycle while on the dyno and use the max value in the formula above.
Fuel Line Size - Testing and years of spec'ing and building fuel systems has shown that -6AN line is big enough to handle flow from any of the pumps listed and/or discussed on this page. If using two pumps, join them together into a -8AN line using one of these. Return line from the regulator should be -6AN for most applications and -8AN for triple-pump builds. These are general rules of thumb and there are alwas exceptions....
Voltage - Voltage has a huge effect on pump flow. Make sure your pumps are getting 13 Volts minimum.
Feel free to contact us at info@radiumauto.com for any questions.