SX FPR install

The stock Fuel Pressure Regulator (FPR) is designed to increase or decrease fuel pressure 1psi for every 1psi air pressure change. 1G/2G NT FPRs are rated at 47.6psi at zero psi (meaning with the car idling, disconnect the vacuum hose to the FPR, and plug it with a golf tee, bolt, or your finger), the 1G Turbo FPRs are rated at 36.3psi at zero psi, the 2G Turbo FPRs are rated at 43.6psi at zero psi. What this means is that at idle (assuming a healthy engine with stock cams, which should have roughly 18-20" of vacuum), 1G/2G NT fuel pressure should be around 39psi, the 1G turbos should be around 27psi, and 2G turbos should be around 35pi (because 2" vacuum equals roughly 1psi). On a 1G turbo, you should have roughly 41psi at 5psi boost, and 51psi at 15psi boost.

When installing a very high flow fuel pump, the stock FPR can easily be overrun, leading to a fuel curve that is no longer linear. On my car, it runs about 40psi of fuel pressure at idle all the way to about 5psi boost. This makes it harder for the ECU to keep the AF ratio "correct", as the nonlinear fuel curve makes the ECU have to dial down the injector pulsewidth (IPW) at idle and low boost, and then raise it back up where it belongs at higher boost. If you have really large injectors installed, the ECU may not be able to dial down the IPW enough at idle to have a smooth idle, as the high pressure combined with the large injectors leads to a rich condition.

The problem isn't really the large pump, it's the stock FPR. The return hole on the valveseat in the stock FPR is very small, 1/8" or smaller - it's just not designed to flow a lot of fuel. The fix is to install an aftermarket adjustable FPR (AFPR) that has a much larger passage in it, and will allow you to dial the fuel pressure back down where it belongs at idle. NOTE - make sure you get the right kind of AFPR. The B&M CommandFlo is not what you want. It is basically parts you add to your stock FPR to modify it for higher fuel pressures, it does not let you lower them, as it still has the same stock restrictive outlet hole. You want a Paxton, SX or similar "rcing" AFPR with really large outlets. However, make sure you buy an EFI AFPR, as they do have ones for carbureted cars that have an adjustment range of about 2-15psi - you need an EFI AFPR with an adjustment range of 25-60 or so psi.

There are several ways you can plumb this setup, many people cut the end of the stock fuel rail off and weld an aluminum 8AN fitting directly to it. I am going with the complete bolt-on solution, where you cut up a stock regulator, weld part of a steel 8AN fitting to it, and then everything else bolts to it. The advantage of this is that it is very easy to swap between the AFPR and a stock FPR for troubleshooting or if you sell the car. You can get stock FPRs off of Hyundais in boneyards pretty cheaply.

The following list of parts has the Summit Racing part #s next to each part - note that you can get these AN fitting from many different shops, so go by the part description, not the #, if you are ordering from some other place. The parts are listed in order of installation, from the fuel rail to the stock fuel return line.

Note that some of these fittings will require modification in one way or another. You can find fittings that will work without modifications, but I was working with what was ordered, which in one case was not quite what I was expecting. If you want to order parts that require no mods at all (short of the welding to the fuel rail or what's left of a stock FPR), do some research and find out what parts you really need. Note also that Summit Racing sells a minimum of 3 feet of the braided line, you don't need anywhere near this. I bought the 3ft pieces because I plan on doing at least 2 cars, and will also be replacing the line from the fuel filter to the rail later, so the extra will come in handy. Many local shops buy huge rolls of SS line, so you can often buy exactly the length you need. If you do this, do make sure to measure the length you need, then get several inches more than that. If the install of a hose end goes wrong, you'll have to cut the end of the hose off, and it will then be too short if you bought the exact length needed.

Here's a listing of fittings that will not require modifications from AN, using the 10AN output Essex Regulator.

So why use 8AN instead of 10AN from the fuel rail to the AFPR? Because ;-) Actually, the fuel rail is about 8AN size internally, you won't really gain anything by using 10AN hose. Others have asked why even use 8AN when the return off of the AFPR is 6AN - because the AFPR has 10AN outlets, and 10/6AN reducers are less common than 10/8AN reducers. Use whatever you want, the price difference between the sizes of hoses and fittings really isn't that much.

Here's a listing of fittings that will not require modifications from AN, using the 6AN output Essex Regulator.

    The first thing you need to do if you are going to do the bolt-on install is get a spare stock FPR and chop it up. Cut the entire top of the FPR from the base, and grind/file/sand the base smooth. Notice the hole in the base is pretty small - about 15/64". Well, the hole in the actual seat inside the top of the FPR is only a little larger than 1/16" in diameter! This is a pic of the 8AN fitting cut in half, sitting on top of the stock FPR base, with the stock FPR outlet sitting above it. Notice how much larger the hole in the 8AN fitting is than the hole in the stock FPR base. I bored the hole in the stock FPR base out further after this picture, taking it to 5/16". I probably could have gone a bit further, but I wanted to leave a decent amount of metal under the O-ring seat.


    Cut the 8AN steel union fitting in half as pictured.


    Here's another view of the 8AN steel union fitting half next to the stock FPR base (before drilling it out). Note that I cut the hex on the 8AN fitting down, so that the weld wouldn't go over the edge of the FPR base.


    Weld half of the 8AN to the stock FPR base, and bolt it to the fuel rail (it would probably be a good idea to install a new O-ring).

    NOTE: pressure test the fitting after welding to make sure there are no leaks. The welds on mine looked perfect, but there were microcracks even after several attempts to seal them. Evidently the welds were microcracking as they cooled. The shop ground most of the weld back out, grabbed some high-tech alloy rod, and rewelded it, and that took care of it. Also, don't even consider MIG welding this fitting unless you are an absolute expert. This should be TIG welded if at all possible. This is not a weld that you want to leak!

    Mount one of the 8AN 90deg fittings to one end of the 8AN hose. The install directions for the ends varies depending on the style of fitting you are using, follow your directions. In my case, you slide the outside part (red) of the end over the end of the hose until it is just under the threads in the end, mark the hose (or use tape like I did)...


    ...oil the threads of the inside fitting (blue), the barbs of the inside fitting, and the inside of the hose. Push the inside half of the fitting into the hose until it meets the outside half. Then hold the outside half stationary, and screw the inside half into it as far as you can by hand...


    ...then use AN wrenches (or flare wrenches, like I did), hold the outside half stationary, and screw the inside half into it...


    ...until the two halves almost touch. Make sure the hose does not push out of the outside half more than about 1/16" while doing this (that's what the mark/tape is for). Again, the directions for the fittings you have may be different - use your directions.

    The key point to the install is the length of 8AN hose, so at this point, ignore the other end of the hose and work on the rest of the install.

    Install the fittings into the FPR, and decide how you are going to mount it. The SX I used has an 1/8" NPT tap on it for a fuel pressure gauge, so that decided the orientation of the AFPR, since I am going to mount a large electrical sender in that port for the oil pressure gauge I use as a fuel pressure gauge. Here's a view of the FPR with the NPT port showing.


    Here's a bottom view of the FPR with the 6AN return port showing. The 10/8AN reducer is on the left, the 10AN plug is on the right.


    NOTE - at this point, I needed to modify the fittings I had ordered. The 10AN plug could be left alone most likely, but I cut the flare off of the end to make sure that the O-ring fully seated before the plug hit the bottom of the passage in the FPR. The 10/8AN reducer I had ordered had 37deg flares on both ends - not good - the 10AN flare bottomed out in the AFPR passage, which would have cut off the return flow to the tank. I cut the flare off to make sure it would flow well.


    Here's a view of the 10/8AN reducer with the flare end I cut off. Note the O-ring, that is necessary to seal the fitting in the AFPR.


    This is the 6AN O-ringed to 6AN 37deg flared union - not how it does NOT have a flare on one end - the unflared end is the one that goes into the AFPR, so it does not bottom out and block off flow. This is what the 10/8AN fitting should have looked like, with the 10AN end flat. Note the O-ring to seal the connection to the AFPR.


    Here's a view of the FPR with all the various fittings installed, including the vacuum line port.


    Figure out where you are going to mount the AFPR. I installed the 8AN hose with the installed 90deg end to the FPR for clearance checks, and worked on finding good mounting point. It would probably be best to actually bolt it down while working on the rest, to make sure all the lines go where you want without rubbing anything, but I was able to continue the install without bolting it down, the stiffness of the AN lines held the FPR where I wanted it (and it's a tight fit, it almost stays put without bolting it down without the lines attached, and does stay put with the lines in place). Here you can see the AFPR placed in front of the stock solenoid bank that has the FPR solenoid. The 6AN line goes straight down past the brake hoses feeding the brake master cylinder reservoir to the master cylinder. You can see the rest of the 6AN line between the brake and clutch master cylinders, close to the stock metal fuel return line. The tape on the 6AN line is centered over the cut line.

    Click on the pic for a larger version.


    Here's a top view of the AFPR. You can see where the AFPR mounting bracket sits on top of the stock solenoid mounting bracket. I pushed down on the solenoid mounting bracket to bend it down a bit to make sure the top of the AFPR did not hit the hood.


    Figure out how long the 6AN line needs to be to fit over the stock return line. Wrap some tape over the cut line - it should be about 1" wide, as you are going to cut through the middle of this tape - this tape is there to keep the SS braiding from fraying during the cut. Use a hacksaw or Dremel (if you use a Dremel and are using Teflon lined hose, it would be best to only cut down to the Teflon liner with the Dremel, then use a knife or saw to cut the Teflon liner, as the high speed of the Dremel may melt/distort the liner) to cut the hose. Then remove the tape, push the SS line onto the stock return line until it hits the second bead and use a high pressure hose clamp to seal it.

    Here's a pic of the FPR with the 6AN clamped to the return line, the welded fitting bolted to the fuel rail, and the 8AN line removed for further work. At this point, I mounted the second 8AN fitting to the FPR, so I could tighten the 8AN line to the fuel rail and check for the cut line on the hose.


    At this point, all that needs to be done is to figure out the length of the 8AN hose. Screw the other 90deg 8AN fitting onto the AFPR, run the hose to it, wrap tape over the cut line, and cut the hose. Then remove the tape, slide the outside half of the fitting over the end of the hose (again, follow the directions for your type of hose) to the right depth, and wrap some tape around at the base of the outside fitting for the depth check. Then install the 90deg fitting. When tightening it down, you should try to make sure the 90deg end ends up facing the right direction to screw onto the fitting on the AFPR, so you don't have to twist the line to fit. (If you get Swivel fittings, you won't have to worry about this).


    Screw all the fittings together (AN fittings do not require Teflon tape, and it should not be used) and make sure they are on tight.

    Note the upper timing belt cover is removed in this pic - the 8AN 90deg fitting was rubbing the cover, so I had to remove it during the install. I slightly notched the cover where it hit the AN fitting to avoid it rubbing too hard.

    I've unbolted the strut bar from my car and swung it up out of the way so you can see the install better. Click the pic for a larger version.

    After finishing the install but not bolting the AFPR down, I realized I shouldn't have made the 8AN line so straight - it should have a slight arc in it to allow for engine movement. So I'm going to redo the line with an arc in it (estimated line shown), then bolt the AFPR down.


    Use the fuelpump check terminal next to the battery to pressurize the system and make sure you have no leaks. All you have to do it get a wire with alligator clips, attach it to the fuelpump check terminal and the positive battery post, and it will power up the pump. If there are no leaks (you might want to let it run a good 30-60 seconds and carefully check every connection), that's it!, you're done.

    It is absolutely essential that you run this check! I found a pinhole leak in the weld of the FPR base to 8AN fitting during my check, and had to get it rewelded.

Set the AFPR for the proper pressure - see the top of the VFAQ for the proper levels. Leave the vacuum line off of the AFPR and use the fuelpump check terminal to pressurize the system, and set the pressure. Now would be a good time to reset the ECU if you have been running a large pump on the stock FPR - the ECU has been dialing back the pulsewidth on the injectors due to the higher pressure, and if you don't reset the ECU, it's going to run lean at idle and low cruise for a while as the ECU raises the fuel values to match the new lower pressure. Resetting the ECU will probably get the values set correctly faster than not resetting it.


    Here is a pic of a 1G and 2G fuel rail. The filter side of the rails is the same, so you can use either rail on either Gen car, as long as you use the same FPR base, as the bolt patterns differ between 1G and 2Gs. The 2G rail is on the bottom, note that it is about 1/4" longer than the 1G rail on the FPR end. If you use a 1G rail on a 2G (with a 1G FPR base), you will have more room to work with.

    Here's a pic of a Low Profile AN fitting. If you use this, you may not have to trim the top timingbelt cover at all. Check the AN Plumbing site for the part # for whatever AN size you are using.

Future Mods

    Future addition to the VFAQ - using AN hose for the filter to fuel rail, using the same "bolt-on technology", welding a 6AN weld fitting to the fuel rail end of the stock fuel feed line, and an AN banjo fitting on the stock fuel filter. 6AN is used here because I have been unable to find an 8AN banjo fitting that will fit the stock filter. After that, I may add another section on replacing the factory style filter with a high-flow filter with AN ends.

    Here's a listing of fittings that will not require modifications from AN

      Part #Description
      9671066AN 37deg flared steel weld fitting (pretty sure this is the right one, no picture)
      8045066AN 37deg flared 45deg LowProfile Swivel-Seal fitting
      3000066AN Auto-flex braided hose, per ft (get 2-3 ft)
      8076916AN banjo fitting to fuel filter
      230106optional 6AN filter to replace the stock filter (To mate properly to the stock line, you should probably get a steel male inverted flare fitting that mates to the stock line, weld a 6AN 37deg flared steel weld fitting to it, and use a 915106 Straight Female AN Swivel Coupling to mount the filter).
      8091066AN 37deg flared 90deg Swivel-Seal fitting
      AT809106 - optional 8AN 37deg flared 90deg Swivel-Seal fitting (BLACK hard anodized finish)
      Order this in place of the 807691 banjo fitting if using the 230106 filter.


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Last modified: Jul 16, 2001
Copyright 2001, Tom Stangl