Hoping one of you Subaru experts will know the answer to this.
A friend has a larger-than-stock aftermarket fuel pump he needs to install and because the factory fuel pump controller won't handle the amp load of the larger pump, he needs to hard-wire the bump, bypassing the fuel pump controller. I did this too, when I installed my surge tank and put a giant pump in there so I can run E85 on a big turbo down the road.
My friend is telling me that two tuners have told him that if he hard wires the fuel pump, he'll need to install an aftermarket fuel pressure regulator and get a tune. That didn't sound right to me. I know the fuel pump controller simply restricts voltage to the pump during low-demand use, in order to lessen the amount of heat the pump is dumping into the fuel system and to extend the life of the pump, but the fuel pressure regulator works by simply returning as much fuel to the tank as needed to keep the fuel pressure at the factory level. The only reason I can think of as to why you'd need an aftermarket regulator is if for some reason the factory regulator can't return enough fuel to the tank to keep the pressure under control when using a large pump that gets 12v at all times. I've googled it and haven't really come up with anything authoritative on what point the factory regulator can't keep up. Clearly it keeps up just fine when the factory pump is at full voltage because the fuel pump controller does send it 12v at WOT, etc.
I've always run an aftermarket regulator in my 818 so I really don't know.
My friend is telling me that two tuners have told him that if he hard wires the fuel pump, he'll need to install an aftermarket fuel pressure regulator and get a tune.
100% correct.
The OEM FP controller uses duty cycle to control the pump, 33/66/100%. By hard wiring the pump and bypassing this system the pump runs at 100% all the time (you have to manually set this in the tune). Without an aftermarket regulator this will put ~70psi to the rails all the time, that's WAY too much.
The only time the pump is at 100% is at WOT and start. You do not want all the pressure all the time (idle, part throttle, lift off, etc.) so you need to be able to bleed that back into the tank and keep the base static at ~43.5 psi.
The OEM FPR is a 1:1, so for every pound of boost it increases the fuel pressure 1#. Starting at 43.5 and running 15-20# of boost gets you the pump's max pressure of around ~65-70 psi. If you start at 70 psi you can't then add another 20 psi since the pump can't physically flow that.
I was wondering the same exact thing - when I hard wired my aeromotive 340 pump (on the stock FPR), my wideband was reading very rich so I went back to using the oem fuel controller. Recently I bought the aeromotive FPR because I thought the stock one couldn't keep up and this one would fix the issue, haven't installed it yet. It sounds like I still need to tune the fuel curve either way or find a high current controller (maybe the one redfogo is using?). Thanks for weighing in Keith.
Keith thanks for the reply. I'm still having trouble understanding it. I understand the duty cycle thing that the OEM controller uses, and I get that hard wiring makes the pump run at 100%. But I don't understand why the pump would control or have anything to do with fuel pressure; that is the job of the fuel pressure regulator. Unless Subaru is different from other cars I have worked on, the regulator has complete control of the fuel pressure. It keeps the pressure to 43.5 PSI by allowing excess fuel from the pump to be routed back to the tank. The pump always supplies more fuel than is required to make X-PSI. So whether the pump runs at 33% or 100%, the regulator automatically accommodates for that by letting more or less fuel return to the tank, keep the pressure constant. And of course, the regulator also allows the pressure to rise or fall based on manifold vacuum.
Does the factory Subaru regulator not work this way or something?
So Jeff, you're running 12V constant to the pump with aftermarket pump and FPR and don't have any issues? Or could they have been taken care of during your tune?
First setup: Aftermarket FPR, Walbro 255, Factory Fuel Pump Controller. Got a tune on this from the get-go.
Current setup: Aftermarket FPR, BIG Walbro pump for E85 (forget which one), hard-wired to 12v constant. Didn't NEED a re-tune after this but I did have my tuner make some adjustments because all my fuel lines were different and the aftermarket regulator I changed over to was a different one than in the past (the new one is built into the surge tank as opposed to be remote-mount standalone on the firewall).
On the OEM system the pump runs in a limited capacity at idle and part throttle, so the injectors are barely open. The excess pressure the pump is creating is minimal and is returned back to the tank via the regulator.
At 100% duty cycle (WOT) we're not only asking the regulator to increase the pressure directly with boost we're also opening the injectors and dumping some of that volume/pressure into the engine. And all of this is happening very quickly on the OEM system.
When you hardwire the pump and set it to 100% duty you're forcing the pump to make pressure as much as it can all the time. When the vehicle is idling and the injectors are barely open the OEM FPR simply can't dump enough pressure to drop down to 43.5 psi so you see the pressure increase accordingly. This leads to increased fuel consumption, poor running (injectors can leak if they have too much static pressure), as well as premature pump failure. You're essentially dead heading the pump since the injectors are 'closed' but it's making the same pressure it would normally be making with the injectors wide open.
On the OEM system the pump runs in a limited capacity at idle and part throttle, so the injectors are barely open. The excess pressure the pump is creating is minimal and is returned back to the tank via the regulator.
At 100% duty cycle (WOT) we're not only asking the regulator to increase the pressure directly with boost we're also opening the injectors and dumping some of that volume/pressure into the engine. And all of this is happening very quickly on the OEM system.
When you hardwire the pump and set it to 100% duty you're forcing the pump to make pressure as much as it can all the time. When the vehicle is idling and the injectors are barely open the OEM FPR simply can't dump enough pressure to drop down to 43.5 psi so you see the pressure increase accordingly. This leads to increased fuel consumption, poor running (injectors can leak if they have too much static pressure), as well as premature pump failure. You're essentially dead heading the pump since the injectors are 'closed' but it's making the same pressure it would normally be making with the injectors wide open.
That is what I was worried about with aeromotive 340 pump suppled with the Boyd tank- that the stock oem regulator could not return enough of the added volume of the Aeromotive
pump that fuel pressure would rise. If minimal it could be tuned out. I am using Microsquirt ecm.
However when I asked the question before , Tom Wallace replied it built 2 818's with Boyd tank and 2L mtrs and stock fpr w/o oem pump controller. ( no Subaru ecm) and he did not have a problem.
the OEM FPR simply can't dump enough pressure to drop down to 43.5 psi so you see the pressure increase accordingly.
Ok that is what I was looking for, thank you Keith. So bottom line is that the aftermarket regulators are able to cope with much higher flow rates than the factory regulators.
On the VWs I worked on in the past, it was never an issue. I did a Corrado G60 swap into a Rabbit. The Rabbit had CIS injection which was high pressure and the G60 had Digifant which was low pressure. I simply ran the factory pump from the rabbit which put out way more fuel than Digifant needed, but the factory fuel pressure regulator on the G60 engine took care of the pressure. It was just plug and play (as far as fueling went anyway). I guess the Subie regulators are just made to operate only within a narrow range.
You are close in understanding the "Why" but pressure regulators are designed for a relatively narrow flow range and the higher the flow the greater the "Pressure Droop" See [ http://www.beswick.com/basics-pressure-regulators ]. The short answer is that even with a pressure regulator the pressure to the injectors will vary. The Fuel Pump, FPR, injectors and ECM need to match up.
I thought I had a good understanding of the fuel system.
Since all fuel systems for our Subaru engines are designed to hold a constant 43.5psi of differential pressure. Why is a tune needed if a pump or regulator in the fuel system is changed?
Sounds to me like your covering up a fuel system problem with a tune.
Bob
PS: I have a OEM jet pump on my return line that has restriction. I could see this causing a problem if I would go to a higher volume pump.
Last edited by Bob_n_Cincy; 08-18-2017 at 03:55 PM.
Thanks Sixstar,
This helps explain why a tune is needed if you change regulators.
It also explains why Subaru put two "Fuel damper valve" in my fuel system. Their purpose is to get rid of pressure pulsation.
Bob
Last edited by Bob_n_Cincy; 08-18-2017 at 03:54 PM.
Thanks Sixstar,
This helps explain why a tune is needed if you change regulators.
It also explains why Subaru put two "Fuel damper valve" in my fuel system. Their purpose is to get rid of pressure pulsation.
Bob
It's possible to add dampers directly into your fuel rails on some aftermarket rails. Radium has a provision for them and sells them.
That's the damper in the middle of this Radium Subaru rail:
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Originally Posted by SixStar
