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How not to do a topfeed conversion

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Thought I'd break this out from the thread where I asked if anyone else had seen crazy fuel pressure fluctuations, so it stands as a reference.


Backstory is, had a leanout issue up top after YNANSB - targeting AFRs in the 10s, pulling into the low 12s near redline. My fueling setup is:


DW65c/AEM320 (tried both, currently on the AEM)

Brand new OE fuel filter

ID1000 top feed injectors

OEM 2006 WRX top feed rails, dampers and FPR


The hardline between the rails didn't fit the plastic LGT manifold, so I replaced it with two lengths of FI hose and secured them every 2 inches to the the coolant and crank breather pipes on top of the block after the manifold was on the motor, to keep them from whipping around.


After seeing the leanout, I got some low rate meter on my fuel pressure sensor, and did a pull. My measured fuel pressures were clustered very tightly around (43.5 psi + 1 psi/psi) of boost, right where they should be. After talking with Cryo, I decided to go the length of getting my high rate meter on it and cranking down the timebase such that I'd be able to see individual injector pulses. What I saw was crazy fluctuation over 30 psi peak to peak, at a frequency matching that of the injector pulses at a given RPM. The average value of the fuel pressure was exactly where it should have been, (43.5 psi + 1 psi/psi of boost).


After going down my troubleshooting fault tree, issue was narrowed down to the rails/FPR. Ended up getting some topfeed LGT rails and FPR on the car (still WRX dampers), and completely took care of the ringing in the rails. Now consistently nailing target AFRs. The moderate stumble/stutter I felt around 2500 RPM is gone as well.


It's possible that the rubber lines I used to jump the rails caused the ringing, but they were well secured and from what I've seen rubber lines tend to damp down fuel pressure oscillations. Not sure on this one. However, the 06 WRX rails have the FPR mounted directly on top of the #1 fuel injector, which seems to be a very likely candidate for driving oscillations as bad as what I saw in the rails.


I'll also note that during troubleshooting, I got a mechanical gauge on the car. The mechanical gauge definitely had the needle wiggling at the same RPMs as the scope, but way less amplitude (~5-10 psi). However, this is likely due to the mechanical response time limitations of the gauge.


Anyways, videos are here. Two 2nd gear and one 3rd gear pull due to traffic when taken, but I drove around with the scope for quite a while and saw the same fluctuations every time I got into any kind of load regardless of gear. Scope shows 12.5 psi/div vertically and 50 ms/div horizontally for both videos with the scope. Pardon the shakes, scope was temporarily secured to the steering wheel so I could grab a video.


Fuel pressure with 06 WRX rails/FPR/dampers




Fuel pressure with 09 LGT rails/FPR and 06 WRX dampers



Edited by awfulwaffle
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So kinda unrelated to the issue/resolution, but if you have 09 lgt rails now, how did you get them through the driver side of the manifold? I can't get mine to come off. The rail and all the hard lines up to the connection to and from the tank are one piece correct?



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Mmm OK. My hoses don't want to come off. It looks like the 05/06 you can't just slip it through without undoing the hoses. Maybe a heat gun will soften them up enough so I can get the manifold off. I've already removed the pesky bolt so that's not the issue.



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Yea I initially planned to do that but the fuel rail shields are right up against the a/c hard lines and the harness is there. I need to look at it again less frustrated and better light haha. I have the tgv unbolted and the wiring unbolted from the manifold. Couldn't figure out what it was hanging up on.



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