In your Subaru, your fuel pump is typically controlled by the factory fuel pump controller, and power is sent to the pump through the factory fuel pump relay.
But if you have installed a 320 liter per hour (LPH) pump or larger, to get that maximum flow rate that those pumps advertise, you will need to send more power to the pump than the factory controller and relay can. This is where a fuel pump hard wire kit comes in.
This kit will use the factory relay to trigger a second relay that has a much higher capacity, that is connected directly to your battery so that you can send the maximum voltage needed to your fuel pump. The advantage of using the factory relay in this way is that they stock ECU still has the same control over the pump as it usually would, so the pump will not be running any more than it has to. But when it does it will be delivering as much fuel as it was rated for so your car will run properly.
If this sounds like a bit more wiring than you would want to take no yourself, fear not, a few aftermarket companies have come up with kits to make this much easier for you.
Deatschwerks has a very affordable kit, which is basically a universal one.
Radium Engineering also makes a universal kit.
iWire has the most precise kits that we have seen, and they have custom tailored their kits to the exact car and fuel pump that you are running. They will be the most plug and play option, but it will also take a little time for them to be built because of the custom options.
As a general rule, you should install this kink of kit if you are running a 320 LPH pump or larger, and you need that maximum capacity out of the pump, if you are running 1000cc injectors and you need to get close to 100% duty cycle on them, or if you are running any injectors larger than 1000cc.
It is also worth mentioning that if you do install a pump larger than 320 LPH, it is possible to damage the pump and shorten its life if you are not powering is sufficiently. So if you are putting any large fuel pump a direct wiring kit is strongly recommended.
- Jon Cooley