Project [Rotary] FD RX-7: Part 3 – Fuel System (Chassis Side)

Project [Rotary] FD RX-7: Part 3 – Fuel System (Chassis Side)

by M-P Spierer

Fuel systems upgrades vary drastically and sometimes only a simple pump or injector change will suffice. Other times, like in this project, it seems like every component in the entire system needs to be changedRecently, in Part 2 of Project RX-7, we covered the engine side of the fuel system, where the injectors, rail, and pressure regulator were upgraded. Now it's time to discuss the remaining fuel system upgrades that we are so eloquently dubbing the “chassis side.”

In Part 1 of this project, we talked about how to prevent oil starvation during cornering and acceleration by adding baffling to the oil pan. Similarly this can happen in the fuel system as well, just on a significantly larger scale. The difference in scale and the fact that gasoline is much less viscous than engine oil means that simply adding some trap door baffling around the fuel pump isn't a viable solution. Adding an external fuel surge tank however, should do the trick.
 


Radium Engineering specializes in fuel surge tanks that are designed for both internal and external fuel pumps. Many of the tanks designed for internal fuel pumps can accommodate two pumps. When configured with a single pump our surge tank has a capacity of 1.5 liters. Two pumps would decrease that capacity to 1.38 liters.

A fuel surge tank (commonly referred to as a “FST”) acts as a secondary fuel reservoir that is usually tall and narrow to decrease the effects of fuel sloshing generated by cornering and acceleration g's. To get a better understanding of how the Radium FST works and how they are plumbed, check out this video produced by Radium Engineering.

 


The top of the Radium fuel surge tank houses three black fittings that are used for surge tank feed, engine return, and primary tank return. The sole green fitting is a bulkhead fitting attached to the fuel pump inside and is reserved for engine feed. The Radium FST also features a 4-pin mil-spec sealed electrical connector for providing power to the fuel pump(s) contained inside.
Here are the guts of the Radium FST. The green bracket on the bottom cap holds one or two fuel pumps to feed the engine. The green bulkhead fitting attaches to a piece of 5/16″ ID fuel submersible hose that attaches to the outlet of the fuel pump contained inside. The top and bottom caps of the fuel surge tank, as well as each mounting bolt, are o-ringed to prevent leaks.

We turned to Aeromotive for the next step this build, choosing a fuel pump. Since the surge tank requires an internal pump and because we need to upgrade the fuel pump inside the RX-7's factory fuel tank as well, we needed look no further than a set of Aeromotive's 340 Stealth fuel pumps.
 

The Aeromotive 340 Stealth fuel pump is a drop-in pump that delivers an impressive 340 L/hr of fuel flow at 13.5V and 40 psi base fuel pressure. At 60 psi (20 psi boost pressure) it still flows 280 L/hr, which would be enough fuel for over 500 horsepower to the rear wheels of a rotary powered RX-7.

For this build we are using two Aeromotive 340 Stealth fuel pumps. One is for the low pressure in-tank pump that feeds the Radium FST (from here on out we will refer to it as the “lift pump”) and the other will reside inside the surge tank to feed the engine (the “feed pump”). The benefit of using the same pump for both locations is that it guarantees that fuel will enter the surge tank faster than it exits. This is because fuel pumps have a higher flow rate at lower operating pressures, so the low-pressure lift pump will always be outflowing the high-pressure feed pump, keeping the surge tank full.
 


The two Aeromotive 340 fuel pumps used for this project.

We are also using an Aeromotive 10-micron fuel filter and billet aluminum filter bracket. This will be placed inline between the feed pump in the FST and the fuel rails on the engine.

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