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Correct Fuel pressure under WOT

18K views 49 replies 7 participants last post by  ScrapYurImport 
#1 ·
Im running a S3 with Siemens 1025's, return line, BNF, rewire, Walbro 255, Aeromotive FPR. I cant get the car over 70PSI at WOT, it falls to around 65PSI at redline. My tuner is adamant that I'm 8-10PSI to low but the injectors are rated at 58PSI. Ive had my tank down and been over the entire fuel system trying to improve on this but it won't go over 70PSI even if I turn up the FPR. Im curious what people are running for WOT fuel pressure. Ive searched a lot and not turning up much. Any help/feedback is greatly appreciated!
 
#3 ·
Source is full plenum vacuum, set to 58PSI it reads around 48-50 at idle when connected to the source.
 
#5 ·
Thats why I'm posting here, I'm running 20lbs of boost and 70 PSI fuel pressure... fuel pump is a walbro 255.

I removed the gauge from my FPR to plumb 5 ft of fuel line to a gauge in cabin. Its just a temporary solution but it reads accurate. To spare any pissing matches I prefer to leave him unnamed but lets just say that he's a reputable and well known tuner on the forums.

I have not tried turning the boost up on my own to see if the pressure follows (its tuned on Diablo and boost by gear so I didn't want to mess).. Adding fuel does change as it should though.

I guess my question is should the 1:1 increase be from the base pressure that is set before Vac is connected or from the resting pressure after Vac is connected.

If its after I'm pretty much dead nuts where I should be, if its the other way around then I guess I should be seeing around 78PSI? If this is so where could I be loosing that 8PSI?

The other part to this is, why is it necessary to have that high of pressure if the injectors max flow rate is rated at 58PSI? I maybe I'm confused about something, I'm going nuts trying to figure this out and its stalled finishing my tune for weeks.
 
#6 · (Edited)
I guess my question is should the 1:1 increase be from the base pressure that is set before Vac is connected or from the resting pressure after Vac is connected.
Technically it will add 1PSI of boost pressure on top of the "static" fuel pressure. "Static"=No vacuum line connected

If its after I'm pretty much dead nuts where I should be, if its the other way around then I guess I should be seeing around 78PSI? If this is so where could I be loosing that 8PSI? Not always an exact 1:1, it will be close but not always exact.

The other part to this is, why is it necessary to have that high of pressure if the injectors max flow rate is rated at 58PSI? I maybe I'm confused about something, I'm going nuts trying to figure this out and its stalled finishing my tune for weeks.You really don't but at idle etc. you would not be able to tune the injector cycle small enough on an injector that has to be large enough to provide fuel at 20 PSI of boost using a fuel pressure of 58 PSI all the time. So a fuel regulator that increases the pressure of the fuel is what is used when in boost. Think of it like opening and closing a valve to let water out of a tank for 1 second durations at a time at a certain pressure, increasing that pressure will push more water out the tank for that same time of a second.
 
#7 ·
The reason you want a 1:1 FPR is because you're effectively keeping the pressure at the injector tip constant. Injectors flow more at higher pressures because as Hybrid said they are effectively small valves at a very simple level. Lower pressure = lower flow, higher pressure = higher flow. Your injectors weren't spec'ed at a max flow of 58 PSI, that's just one of the data points.

Let's say you have a fuel pressure of 58 PSI with 0 boost and 0 vacuum in the intake. That's the amount of pressure your injectors have at the tip, where they're spraying. At 20 pounds of boost with no boost referenced FPR you have effectively 38 pounds of pressure at the tip of your injector. Adding the 20 pounds of fuel pressure back by for your 1:1 FPR and you're now back to the 58 PSI at the tip.
 
#8 ·
it won't go over 70PSI even if I turn up the FPR.
That's the key detail right there^^

If you can't manually force the FRP to hold over 70psi EVEN AT IDLE, then either:

1) The the regulator is crap(leaking seat)
2) Your pump is crap
3) You actually have a low pressure Walbro 255(or cheap junk replica) that maxes out at ~68psi instead of a genuine High pressure Walbro that maxes out ~100psi.
 
#9 ·
Thanks a lot for that information guys, it helps me understand a bit more about what factors could be playing into this. I really want to learn along the way of getting this fixed.

To be safe I had only been turning the pressure up at idle 5 or so PSI and then taking a ride to see what the WOT pressure was and if it would go up in a 1:1 fashion. That doesn't seem to be resulting in anything more than 70PSI under WOT, however I didn't turn it up anything drastic.

So after reading what Duster said I wanted to confirm that I couldn't get it over 70PSI. I turned the FPR up at idle and its able to hold over 80PSI at idle. I didn't want to go any farther than that to see what the limit was because I figure 80PSI is sufficient. Does this rule anything out?
 
#12 ·
I turned it up to 60PSI at idle and took it for a ride and it won't go over 68-70 PSI. Is there anyway that this could be vacuum or boost related? I find it interesting that Duster says there is a 255 that only flows 68PSI.. makes me think this could be the case (pump was done by previous owner). Is there any way to test if an FPR is bad besides swapping in a new one?
 
#15 ·
I find it interesting that Duster says there is a 255 that only flows 68PSI.. makes me think this could be the case (pump was done by previous owner). Is there any way to test if an FPR is bad besides swapping in a new one?

No you already proven your pump can sustain 80psi at idle, remember? This proves that at least low flow rates, it "works" and is NOT a low pressure pump.

This isn't rocket science. Just eliminate the possible causes. Force the regulator to hold 80psi by manually adjusting it and go run it.

If it still drops, you know the Regulator isn't the issue but either a weak pump or other restriction BEFORE the regulator, like an undersized and/or clogged filter.

What filter are you using?
 
#13 ·
#14 ·
Summit has it listed wrong. That's the correct fuel pump for the SRT-4 and they can support much more pressure than that. If you want the 255 high pressure pump look at Modern Performance or Modern Automotive Performance. Buy from a reputable shop like either of those two or you have no idea if you're getting a fake. I'd stay away from ebay and Amazon.

The 450 you linked to is not a direct fit pump, you'll have to modify the canister to get it to work. If your signature is correct there is no reason for you to go through the trouble, the 255 and a bottleneck fix will support your setup.

Where are you located in Wisconsin?
 
#16 ·
If your signature is correct there is no reason for you to go through the trouble, the 255 and a bottleneck fix will support your setup.

Where are you located in Wisconsin?
My signature is from my car in high school lol, I just left it because it reminds me of the good ole days. This is my second SRT. Im just curious if the 255 will be sufficient for E85. I live just south of Milwaukee in Franksville.

Force the regulator to hold 80psi by manually adjusting it and go run it.

If it still drops, you know the Regulator isn't the issue but either a weak pump or other restriction BEFORE the regulator, like an undersized and/or clogged filter.

What filter are you using?
I forced it to 82Psi @ idle and it drops all the same at redline. The previous owner had a shitty aftermarket filter in there (Ive had it apart to make sure its not plugged). I wanted to eliminate the FPR before making a decision. I ordered a K&N 81-1000 to replace it. If that doesn't work I guess ill be buying a new fuel pump to boot.
 
#21 ·
I was able to get K&N to cancel my order before it shipped (surprisingly good customer service via live chat). I ended up ordering the Fore with the SS filter, its only $10 more than the fuel lab and looks to be a well engineered design. I like the fact that they sell so many adapters and accessories for it, should I ever need to change the fuel system I know I can get what I need for it. Thanks for the input everyone, hoping this does the trick.
 
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#22 ·
Wow im having the exact same problem you are i just have 750 injectors and big turbo instead of s3. Just redid my fuel system with the new 255 pump kit from modern performance and agp return line kit, boomba fuel rail and all. Did you put the fpr before or after you fuel rail? I read that people thought it was beneficial to put it after the rail so thats what i did but im getting the same results you are at 15 lbs. Of boost. Very interested in how yours turns out please keep us update!
 
#23 · (Edited)
Well I just installed the new Fore filter, Im really happy with it. Its extremely well built, you can tell there is care in the manufacturing process.

Unfortunately the issue is the exact same.:tongue:

Im going to go over the lines that are after the fuel filter again and see if there is something thats not right. The previous owner had the aftermarket stainless lines mated to the factory fuel line after the fuel filter and even though it doesn't leak or look kinked I'm beginning to think that may be the issue. Im going to try and rig something up so that I can test the fuel pressure under WOT before the fuel filter to eliminate the pump being the cause.

Did you put the fpr before or after you fuel rail?
The system looks like this:

In tank: Re-wired Walbro 255 (Canister has bottle neck fix and Gates submersible fuel line for return)

Outside the tank: The fuel line is 6AN stainless braid up to the Fore fuel filter, then the stainless lines attach to a rubber fuel line with a fitting, that connects to the factory fuel lines. The factory lines run into the fuel rail, which is an 03 rail. Off the schrader valve on the end of the fuel rail, there is about 1-2ft of stainless fuel line which runs to the FPR. Then off the FPR the stainless braided return runs back to the tank.
 
#25 ·
Yeah we pretty much have the same fuel system i just did the bnf today with a fst filter and im still getting the same results. I might do a -6na line from the filter to the rail like the guy above me just said. A buddy from work thats been trying to help me is suggesting maybe a inferior/bad fpr. With the price of AN line i dont know which is cheaper to replace to see if anything changes lol.
 
#26 ·
This is what I'm looking at ordering. I have a -6AN elbow fitting for the other end of the fuel rail adapter already, so you may need one of those in addition. I don't know how much fuel line you will need either I think 6ft will be sufficient for me.

Pro-Flo 200 Series Stainless Steel Braided Hose
-06AN
Length: 6'
SKU # 555-100911
$26.99

SAE Quick-Connect EFI Adapter Fitting
Push-On Style
5/16'' (Hard Tube) SAE Quick-Disconnect Female
-06 AN Male
SKU # 799-640860
$16.47

Straight Max Flow Hose End
-6AN
SKU # 555-100001
$4.99
 
#27 ·
Im gunna pick some line up tomorrow and put it in if you want to wait and see ill probably have the results posted before lunch time. Im probably gunna need like 8ft. My filter is mounted to the outside of the spare tire well, just behind the tank and i plan on bringing it up to where the stock line ends now and the 2 foot piece i have now connecting to the 5/16 line will replace the stock plastic line from the 5/16 to rail.
 
#28 ·
Definitely let me know your results. Im leaning more towards it being my fuel pump at this point, but if it works for you it may be the same for me.
 
#30 ·
After talking to the guys at my local speed shot im not 100% sure thats the problem. I just bought a fuel lab and the -6an lines so im going to install them now so ill let you know the outcome. But my reg. Is definitely my problem, as it was the guys had a hard time finding a reg in stock that went over 70 psi because aparently thats the average
 
#32 ·
Let me know how it works out for you.
 
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