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Parallel Fuel Rail Mod

Current Problem

The Subaru "Series to Parallel" fuel rail mod is nothing new. You can find countless posts on how to do this. But, for some reason, they seemed over complex to me. Except for one on that uses nothing more than a pair of Ts and a chunk of hose with the stock FPR. Now that's a simple setup. But, I'll be using a new FPR, so I sat down, thought about it, and came up with what I believe is by far the easiest version of this popular mod for my needs. The reason I'm doing this is because I'll be needing a better and adjustable FPR for the higher fuel pressures I'll be running. Why? Boost! What else? Since I'll be changing the FPR, might as well do this Parallel mod too.

Subaru decided to route the fuel to the injectors in series. This results in a bit of a pressure drop by the time the fuel gets to the last injector. The routing order goes 4 -> 2 -> 1 -> 3 . Which means that cyl #4 gets the most pressure, while #3 gets the least. I can't confirm the actual pressure differences though. I'm basing this on what I've read on both NASIOC and The stock FPR is located right at the end of the fuel rail at #3.


   FPR ----> Return ---->
   |                     Tank
   3    4 <- Filter <----
   |    |
   1 <- 2



Layout the system so fuel is run through both fuel rails in parallel, this all but eliminates the huge pressure drop on #3. Basically, I want fuel to be fed equally into one side of each fuel rail, then returned equally from the other end of the fuel rail.

Current Solutions

There are a few different ideas on how to do this. And, they all seem to work just fine. But, I find that the more common one seems to require a lot more work, a lot more parts, and is really just way overkill. If you look on, the solution they have involves pretty much replacing all the hard lines with rubber hose, and using a few T-Adapters.

My Solution

What's different in my idea? Why not use the hard lines to my advantage? They are already run nice and neat between rails. It's just a matter of routing fuel to each of them in a different way. The key here is that both rails are linked to the hard lines with short pieces of rubber hose. This is how I will re-engineer the system. In fact, I did not even have to cut a single piece of existing hard line.

I'm going to be using an SX Performance adjustable pressure regulator. It has 2 return ports. I'll be using both in my setup, but a simple T-Fitting or Y-Block could allow for a single return line too.

Stock, the fuel feeds into, and returns from a pair of hard lines that stick up between the intake manifold on the drivers side. The fuel feed hard line curves around the corner towards the firewal and starts into cyl #4. Then after going through #2 then #1 then #3, the fuel is returned via the 2nd hard line just below where the fuel is fed in from the filter. When done with this mod, there will be 2 feed lines, and 2 return lines. In fact, the current feed hard line, and current return hard lines will both become feed lines. The 2 return lines will be completely new lengths of rubber hose.

Here is the short version of how the fuel will now run. Yes, it's really just this simple.

  • The feed from the pump will split off, and run to the current feed hard line, and the current return hard line under the manifold.
  • The rubber feed joint between the 2 rails (cyl #2 -> #1) will be removed, and 2 hoses attached to each end, resulting in a new pair of return lines. This joint is visible between the intake runners on the drivers side. These lines feed into the new FPR.
  • The stock FPR is removed and replaced with the Weapon-R adapter and a piece of hose.

And, I managed to do this whole mod without removing the Intake Manifold, or the fuel rails.

Parts Needed

This is almost an EXACT list of parts I u/sed. I got some my stuff from PepBoys, and some from EBay. The links I've provided are really just for reference. I just tried to keep the ID of all the parts as close to 7mm or 5/16" as I could.


  • 7mm < 5/16" (it's actually just a hair under. More like 4.4/16)
  • 8mm = 5/16" (not exact, but very close)
  • Subaru seems to use 8mm OD hard fuel line.
  • -6AN = 6/16" = 3/8" ID

Alternative Parts: * SS barbed Y-Fitting

Step 1

  • I start by removing some of the bits that will change. There is actually not that much. This will result in both rails being isolated from each other. Remember, the system could still be under pressure, so work slow, and be ready for any fuel spillage.
  • Disconnect the hose running from the fuel filter to the main feed hard line going under the manifold on the drivers side.
  • Disconnect the return hose running from the hard line under the manifold on the drivers side.
    • If you look in the photo below, you can see the 3 hard lines I'm referring to.
      • The top line is the stock feed line from the tank.
      • The middle line is the stock return line back to the tank.
      • The bottom line is the vacuum line that also goes to the tank.

3 lines

  • Remove the short rubber fuel line that joins the drivers side rail to the hard line passing to the passenger side rail. This is found at the end of the drivers side rail, @ cyl #2 right between the dirvers side intake runners. It's difficult to see in the photo, but it's the blurry bit of hose deep down that runs vertical in the photo and between the intake runners. You can also see the hose clamp on it has a bit of blue paint on the screw head.

3 lines

  • Remove the stock FPR, vacuum hose, and rubber fuel hose.

stock fpr

Step 2

Install the fuel rail adapter at the passenger fuel rail cyl #3.

  • Then, using some of of the new 5/16" hose, and clamps, I connected the adapter barb end to the return hard line that the stock FPR was connected to.
  • I've eliminated the FPR from the system now. And with the connection still severed at cyl 2->1, both rails are still isolated.

connect fpr

Step 3

Use the Y-Block, some of the 5/16" hose, and clamps.

Next, I connected the pump feed line coming from the fuel filter, to the Y-Block inlet. With more hose & clamps, I connected the other 2 ends of the Y to each of the hard lines sticking up between the intake manifold. Remember, one was the original feed line, and the other was the original return line. This will cause fuel to be fed to cyl #3 & #4 at the same time. This Y-Block comes with a 1/8npt port for a fuel pressure gauge fitting. I'll be using it for my pressure gauge in place of my existing T-Adapter. The stock hard lines had to be bent up just a little too keep the hose from kinking too much. You can also see the fuel pressure sensor sticking out the side.


Step 4

Using 2 lengths of rubber hose, I connected the 2 hard line ends that were left open near cyl #2. One hose goes to the end of the rail at #2. The other hose goes to the hard line running under the manifold and over to cyl #1. Each of these become the pair of return lines. The 3 red circles in the following image show the return lines. The circle on the left shows where the 2 return lines are attached. Looking at the 2 hoses visible in the left circle, the left line runs down and curves right to attach to the right input of the FPR. The right hose runs up and under the manifold, then around to the right and attaches to the left input of the FPR.


This is a close up of where the 2 return lines connect to the OE hard lines.


I then connected the 2 return lines to the 2 return inputs on the FPR. If I was using a single port FPR, I could would need a 2nd Y-Adapter or perhaps a T-adapter.

return lines

Finally, the return line from the bottom of the FPR attaches to the factory hard return line just behind the strut tower.

Step 5

Once I have my TMIC installed, the FPR will be mounted to the side of it. But, until then, the FPR will be mounted to the side of the factory "Torque Box" with an aluminum bracket. The FPR I'm using has dual inputs. So I connected each rail return hose to the inputs of the FPR. If an FPR with a single was used, then a simple T-fitting would be needed to combine the 2 returns into one. Now I just attached the return from the FPR to the tank return line.


Now, I took the return port from the FPR, and connect it to the return hard line heading back to the tank with a length of 5/16" EFI hose. I had to use a piece of 3/8" hose since my FPR had a -6AN return barb. I just happened to have enough 3/8" hose lying around.

The Final Result

And that's it. Easy as pie, and very little modifications actually needed. No need to run a lot of lines, cut any hard lines, nor even remove the manifold. Now the fuel feeds in parallel to 4->2 and 3->1 at the same time.


      Y <-- Filter <--
    /   \
   3     4
   |     |
   1     2
    \   /
     FPR -> Return


All Photos

Here is a collection of all the photos I took during this mod.