Subaru Surge Tank Hardwire Kit

Subaru Surge Tank Wiring

Aug 12, '22

In this blog, we'll discuss surge tank wiring and the differences between a standard in-tank pump hardwire setup. If you would like to have us wire up your surge tank, please contact us directly because each is built to the customer's specs.

To start, to see our most up to date information about hardwiring for an in-tank pump setup by clicking here.

Please be aware, to keep things simple for readers of any electrical skill set so terminology and details may not be described in a way that meets scientific muster.

Definition of terms

Surge Tank - An external fuel pump assembly (from 1 to 3 pumps depending on the setup) that draws fuel from the OEM fuel tank assembly.

Fuel Cell - Similar to a Surge Tank except that the fuel cell is a replacement for the stock fuel tank. The stock fuel tank would be removed if a fuel cell is installed. These generally have lift pumps (like the OEM tank feeding the surge tank) to fill the reservoir that the main pump feeds the engine so the wiring for them ends up matching the surge tank design.

In-Tank - Fuel pump and/or hanger inside the OEM fuel tank assembly.

Hardwire Kit - Wiring upgrades that relate to the OEM fuel tank assembly. Please click here to see the kits we offer. 

Cliff Notes

  • Hardwiring for an in-tank pump upgrade is different than a surge tank
  • Our hardwire kit can't be modified to run a surge tank
  • The in-tank pump wiring does not need to be upgraded with a surge tank
  • 2 fuel pump controllers are needed for a proper surge tank setup

Why Would You Run a Surge Tank?

Maybe the place to start is why someone would want to run a surge tank. The main reason is that even with upgraded fuel pump hangers like the Radium, the saddle design of the Subaru fuel tank has problems with fuel starvation in corners. If the car is driven enough on the track, fuel starvation can lead to serious issues. If you are not sure if a surge tank is appropriate for your project, consult your local shop and they can help you decide what the right fuel setup is for you.

Fuel Pressure and Pump Amperage Draw

The fuel pressure regulator needs to be fed enough fuel by the pump to supply the injectors with the correct amount of pressure. The amount of pressure required is based on the engine. If the engine is in vacuum, you would require less pressure which is easier since the engine is pulling fuel out of the injector. If the engine is in boost it requires more pressure since the engine is pushing against the injector. So as the pressure inside the engine increases the more fuel the pump has to supply if it wants to maintain that fuel pressure. The harder the pump works, the more amperage (electrical current) it draws.

A way to look at this would be to fill a bucket with water using a hose with a nozzle. If you wanted to fill a bucket at the same rate with the nozzle open all of the way as with the nozzle only slightly cracked open, you would need a lot more water pressure to fill the bucket in the same amount of time as if the nozzle was open all of the way. 

All of this to say, the wiring for the in-tank pump does not need to be upgraded. Without the pressure from the fuel system on the in-tank pump, it can easily keep up with the demands of the surge tank without having to work hard because it's just a matter of filling the bucket without a restrictive nozzle. This also means that you don't need an extremely large pump to keep up with even 2 surge tank pumps because the in-tank pump does not have the same restrictions as the surge pumps that are feeding the engine. 

ECU control is Vital - 2 Fuel Pump Controllers

With the above in mind, we don't have the same concerns about fuel pump controllers failing with the in-tank pump because the workload of the pump is much lower to flow the same amount of volume. So what we do is run 2 fuel pump controllers, one for the in-tank pump and another for the primary surge tank pump.

The reason for this is that you want to keep the ECU in control of the fuel pumps so that the primary surge tank retains the off, low, medium, and high of the factory pump. This will keep fuel pressures down at idle but also for the safety factor that the ECU can shut down both in the tank and surge pumps if the car isn't on or if an accident happens.

We've seen many people wire up the in-tank pump to run whenever the key is on. Not only can this be hard on your fuel system but is potentially dangerous because this pump may be supplying fuel to feed a fire in a worst-case scenario.

Just an additional note. A single fuel pump controller is not designed to handle the draw of 2 fuel pumps (even if one isn't drawing much) so the 2 controllers (one for each pump) are required to ensure the reliability of the system.

Multiple Surge Tank Pumps

The primary pump will always be paired with the in-tank pump but for additional pumps, we recommend that they be triggered via a manifold pressure switch similar to what we do with our Radium 2 pump hanger kits. With 3 a pump surge tank, usually, the 2 secondary pumps will be paired together with the manifold pressure switch. There have been times when we've run 3 fuel pump controllers (in-tank, primary surge, secondary surge) but only if there are a total of 4 pumps (3 in the surge) with the 3rd surge pump on a manifold pressure switch. It's pretty rare the fuel requirements call for 3 surge pumps so this would be a discussion to have for your specific project if you think you need this much fuel. 

Tags: Fuel System