Subaru Fuel Pump System Upgrades – Two Pumps are Better Than One
Updated: Sep 8
This post based on old knowledge. We have leaned a lot since this post. For the most current information regarding this please take a look at this post.
In this blog post we talk about our suggestions for how to best upgrade the fuel system of a Subaru from a wiring perspective. For all fuel setups we make the wiring simple with our Fuel Pump Controller Hardwire Kit so you get maximum power to the pump while leaving the Fuel Pump Controller (FPC) in place to regulate how hard the pump works. However, we have come to find out that when installing single large pumps like the Walbro 485 or 525, the pump is so big it tends to overrun the unmodified portions of the fuel system. Obviously, every car is set up a bit differently so these are generalizations based on our experience but our suggestion is to use two smaller or medium size pumps instead of one large pump.
Why not eliminate the FPC?
The reason the FPC is important is that full fuel pressure is not always desirable. With the FPC still in place the FPC can adjust for low, medium, and high pressure. That way when the car is at idle, the fuel pressure regulator isn't being over run due to a large pump running at full power all of the time. Additionally, with the not working is hard when less fuel is needed the pump will last longer. One other consideration is fuel temperature. When the pump is running full blast all of the time, the pump gets hot and heats up the fuel leading to increased fuel temps.
So the pump is too big for the FPC to handle. What’s the solution?
Eliminate the FPC for regulating the pump itself and use it just as a method to trigger a relay that runs the pump. The downside is losing the low, medium, high settings. Most tuners are going to say that’s not a good thing. We want to keep our tuners happy so having the different settings is probably a good idea. If the tuner is OK with this (assuming the wiring is taken care of) it's simply a matter of adjusting the ECU calibration to request 100% duty from the fuel pump controller anytime the pump is activated so the relay that runs the pump is triggered properly. If this is not done the relay will turn on/off rapidly and fry the pump and/or relay.
The better option is to switch to a two-pump setup with a smaller primary pump (340 would be good) that is still controlled by the FPC. That way there is enough fuel for cruising around and when more fuel is needed the larger secondary pump can handle the increased fueling needs at that point. The secondary pump would not involve the FPC and is set up on a pressure switch. By removing the stock components, the large secondary pump wiring can handle whatever is thrown at it. With a two pump setup we can retain the best of both worlds. The FPC can stay in place without worrying about failure, but there is also a large pump that can handle the requirements for when the engine needs more fuel.
Recommendations for a two pump setup?
In our opinion the best double pump hanger is the Radium but there are other companies that do make them as well as DIY options for those wanting to explore alternatives. iWire makes a plug and play hardwire kit for the Radium specifically but it could be adjusted for just about any application.