Deleting stock FPR
 

I am going to redo the fuel system with a surge tank and a return with an external FPR.

Now I know what I need and how to do that. But one thing I dont seem to find, despite reading most of the return fuel system topics and trying different searches.

How do you delete/block the stock FPR in the fuel pump assembly?

Well there are several ways to look at this.

With a return system, if it’s just being a lift pump to the surge tank the OE pump module will never see anywhere near the pressure needed to engage the integral FPR. Which this assumes the overflow return line from the surge tank back to the fuel tank is sufficiently sized so that lift pump pressure never exceeds 50 psig or so. Then it’s totally a non-issue to leave it alone. You should rework the electrical power to the OE module so it always runs in the low voltage/speed mode through the fuel pump resister. There’s no need to run it at full voltage/speed when there’s little/no operating pressure pumping to the surge tank. Instead you would use a Radium venturi-siphon and run your return flow through it to siphon fuel over from the other side of saddlebag.

Alternatively, if you were using the Radium FST-R surge tank with built in FPR then there’s no return fuel line since it drops the return fuel back directly into the surge tank. Without some backpressure the OE pump module won’t engage the FPR. In that case you’d need to add a restriction/regulator into the fuel feed line between the OE pump and the surge tank to force the pump into bypass mode so that it will siphon fuel from the other side of the tank.

Alternatively in either situation you can put the Radium venturi/siphon in the fuel feed line between the lift pump and the surge tank, but then you need to plumb the siphon hose outside the tank. Or you could rework the discharge hose between the pump and the module top to flow through the Radium venturi/siphon inside the tank first instead and keep it all internal. Either way you have to some work involved for plumbing lines, mounting things, etc.

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Thx for the info, this helps alot.

I have looked into the radium fst-r. It's a nice solution to keep it rather simple.

How would the restricion work for the bypass mode? Might be a stupid question, but when it comes to this, I don't have much imagination to see what would needed to be done.

The plan was to use the radium venturi pump in either path I would take.


And yes i know, it will be quite the effort for getting it all plumbed nice and without issues. The fst-r has my attention now, as it would be the best way to get a decent fuel system, without drawing too much attention at a tech control.

​​​​​​​I live in Belgium and they aren't very keen on modified Cars here.

The OE pump module runs bypass fuel from the integral FPR through a siphon venturi. There is an internal fuel tank line with pickup from the other side of the fuel tank that connects to the body of the siphon venturi. When the bypass fuel flows through the siphon venturi it also then pulls/siphons fuel from the other side of the tank into the side where the fuel pump module is. Otherwise there’s no way for fuel to readily flow from one side into the other than what may splash or slosh over during cornering. Some people have installed another fuel pump module so that both sides have one with the far one pumping over to the side where the engine supply pump is. The potential issue is that pumps don’t like to run dry for very long. So if you get below 1/3-1/2 tank and the far pump has pushed all the fuel into the other side it’s sitting there running empty. Now if you have the resources it may be possible to use the level float signal on that side to determine when the pump should run or not, but to date I’m not aware of anyone doing that. Some people have a manual switch that they can turn it on/off by gut feel. I’m not sure that’s the best way, but as you may be starting to grasp, the situation is a bit complicated to say the least.

the ultimate solution is a OE fit replacement pump module with a pump, filter, FPR, and siphon just like factory except with the capacity to support the needed power level. Nobody is really offering anything like that, but I’m working on a possible solution myself. Not my highest priority right now though.

I read your first reaction again, a bit more clear headed now.

If you should put the radium venturi in the feed line for the Surge tank, doesnt that mean the flow from the passenger side of the tank, gets pulled to the Surge tank aswell instead of dumping in the drivers side?

Yeah I have seen ppl do 2 pumps in either side, but I am not too keen about that. I prefer to keep it as it is, just the fpr and the siphon system have been bothering me. If possible, i'd prefer not to run an extra fuel line under the car to the front, for said reasons about modified cars above.

I’m just throwing different scenarios out. I’m not sure that’s the best one. The decision tree is likely determined by the need. What power level on what fuel is your goal, i.e. have you determined what pump size is going to be needed yet? Because if your need is above what the OE pump can provide even as a low pressure lift pump then that’s going to influence whether the module assy needs to be gutted/reworked or not.

I did have an OE S2 fuel pump tested at Deatschwerks and on 13.5V it was discharging 269 LPH @ 30 psig, which was the lowest test pressure. The PRV doesn’t open until around 50 psig. Back then I didn’t understand why it opened so early when the factory fuel pressure rating is 58 psig. Now I understand that the pump is oversized to intentionally force the PRV to bypass and always be siphoning fuel from the other saddle. It’s not until you cut the actual pump discharge so far back it will eventually reach 58 psig output because even though the PRV opens at 50 psig, once the flow rate through exceeds a certain level it backpressures up to the 58 psig OE rating. I don’t recall the exact number at the moment, but even the best NA Renesis engine only needs around 95lph on E10 gasoline.

If you were to put the siphon on the pump discharge line (2nd diagram below, requires silver orifice in Radium venturi, loses ~40lph output @40 psig) it could be the line from the surge tank to the engine too, but again in that scenario you will have to get creative with plumbing all this stuff in/out of the tank

https://cimg6.ibsrv.net/gimg/www.rx8...9f7a1f0ce.jpeg

https://cimg7.ibsrv.net/gimg/www.rx8...6fac941f4.jpeg

Which also you can have the standard surge tank with the FPR on the discharge there rather than up on the fuel rail and run the return back through a short line to the tank with the venturi mounted in there too.

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The plan is 400whp (give or take, not aiming directly at that, but close would be nice), but wanted everything to be at least a bit overkill so I have room for more or E85 if they decide to make it available here.

If I understand correctly, the FST-R return to the fuel tank, is not under pressure, as its a normal overflow line instead of a return from the internal FPR?

So a better option would be a normal surgetank with the FPR mounted on the return from the surgetank to the fueltank (like you said in the last sentence), and return via the radium venturi into the stock siphon, in which case that line is under pressure as it comes directly from the FPR.

Doing it like this, is a good option and pressure on the feed is still as it should be?

If you’re going to have a return line from the engine bay then what difference does it make?

well you seem to at least get the picture, so good luck with whatever you do.

I would do that as a last resort if I cant do it without. It would have to look like stock, since any form of engine tuning isn't allowed. I do have the luck of the car being extremely rare here, so they don't really know the details of how everything looks.

But still, that doesnt mean I can go all out with mods. To give you an example, I have to switch my vented hood and exhaust because it is illegal.


If I am correct, removing the FUEL relay from the fuse box, makes it run in the low pressure mode all the time?
Also something else about the siphon, since the stock FPR is still in place, it will hold some pressure, isnt that enough to make the oem system continue to work?

For 400 whp you need at least 200 lph fuel pump flow + bypass flow. The OE pump in low speed mode isn’t going to make that. However, to keep it in low speed mode the high speed relay just needs to be removed from the front fuse box. When it engages the two wires through the low speed resistor are joined together to bypass it (path of least resistance). So removing the high speed pump relay prevents that.

I know I need enough flow, but the OEM pump will only need to fill the surge tank, so it's better to keep it at low speed all the time?

In the Surge tank, I'll probably go for a walbro 450.