Selecting Surge Tank Fuel Pumps

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
Click HERE for a tool to convert from lb/hr to cc/min

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.

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New Product Release: Subaru Fuel Rails

Click HERE to see all of the new Subaru products.

Following a successful launch into the AWD market with the Mitsubishi Evolution VIII / IX / X product line, it was only natural to progress to the EVO's arch nemesis, the Subaru Impreza WRX STi. We have been producing Subaru BRZ boxer fuel rails (shown below) for some time with great feedback, so we had a solid base to build on.

Radium Engineering has just released a family of fuel rails and related accessories for the popular Subaru boxer engines. Two fuel rail kits cover models from 2002 to 2015.


The first kit is targeted at the Subaru engines that are equipped with side-feed injectors from the factory. It is well known that tuners prefer top-feed injectors for the cost and vast selection. The Radium Engineering conversion fuel rail kit (shown above) utilizes specially machined injector seats that insert into the fuel injector bores on the TGV housings. These allow the use of top feed injectors in a clean and leak-free package. More information on these injector seats is shown below. Different height injectors can be accomodated simply by swapping out the insulating mounting boss spacers that are included with the kit. Injector compatibility is explained more below and also on the product page HERE.


The other fuel rail kit is a perfect bolt-in option for Subaru engines that originally came equipped with top feed injectors. It can also accomodate different height injectors by swapping out the insulating mounting boss spacers included in the kit. More information on this fuel rail kit can be found HERE.

Both kits utilize fuel rails that are machined from 6061 aluminum and followed up with bright dipped black anodizing and laser engraving. Each fuel rail is equipped with three 3/8" NPT ports (one on each end and one on the bottom).  This allows the rails to be plumbed in a wide variety of configurations. More information on plumbing the fuel rails can be found on the product pages in the "More Details" tab.

 

Why Radium Engineering Fuel Rails?

These fuel rail kits cover a large range of Subaru vehicles that go all the way back 13 years. There are dozens of aftermarket fuel rail kits currently available. So you ask, "Why spend time developing products in a saturated market?" Well quite honestly we found a few areas that can be improved.

More than any other engine, the Subaru's horizontally opposed 4 cylinder is extremely vulnerable to poor fuel delivery. The most common symptom is the lean spike stumble seen on many applications. Like all Radium Engineering fuel rail kits, we are the only to offer an optional integrated fuel pulse damper. Many manufacturers skip on this vital piece with the assumption that large bore rails are not suseptable to the issue. However, gas is not compressible. The opening and closing of the injectors creates pressure pulses in the fuel rail which can lead to unstable fuel pressure. In order to achieve a safe and consistent rail pressure, a pulse damper should be used (especially with high flow injectors). The internal diaphragm absorbs oscillating pulses and helps stabilize fuel pressure. This correction can be measured throughout the RPM range and can often help an erratic surging idle. The fuel pulse damper (shown below in center port) can be used in any of the three 3/8" NPT ports on the Subaru fuel rails.

The restrictive 0.19" (4.8mm) ID OEM cross-over fuel piping can also contribute to fuel pressure issues. With excess fuel demands from larger turbochargers, this problem only gets worse. The Radium kit comes with four -6AN fittings for large 3/8" (9.5mm) hose and can be configured in parallel (depicted below) which many Subaru tuners are convinced is essential for the Subaru fuel system. Many other plumbing methods can be used as well (series, parallel, dead end, etc).

Taking every last measure to reduce heat input into the fuel system, machined phenolic mounting spacers and washers are included to completely isolate the fuel rail from heat transferred from the TGV housings and mounting bolts.

Two sets of these press-fit phenolic spacers (8 total) and matching hardware are included in this kit to suit different height fuel injectors. The kit is designed for specific injector heights for proper fitment. Upper and lower O-ring spacing requirements are depicted below for the Top Feed Upgrade Kit.

and here for the Top Feed Conversion Kit

The 20-0168 Top Feed Conversion Kit also comes with 14mm injector bore billet injector seats. These actually snap into place and seal with Viton O-rings. They are anodized and laser etched for a long lasting surface finish. They can be purchased separately HERE .

For the owner of the less common Version 8/9 JDM STi intake manifold that does not use TGV housings, we have a 20-0172 Billet Adapter Kit that allows for the use of our top feed fuel rail upgrade kit.

As previously mentioned, all 3 ports of each fuel rail are tapped with 3/8" NPT. This allows for the use of our billet FPR adapter if an OEM fuel pressure regulator is used.

Click to see all of the new RADIUM products for the Subaru Impreza WRX, Impreza STi, Legacy GT, and Forester XT.

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