OKIE DOKIE. So, there's not a lot of SEDAN information for fueling. First you will say, they're the same, well, they aren't.
The difference is the pump assembly.
Here's the details for reference before I get into the issue:

2003.5 G35 Sedan 6MT
Walbro 255 In-Tank
Vortech V3 Si Trim, 3.12" pulley
De-Tuned to 395 WHP and 340 WTQ (Dynapack)
570cc Injectors (OEM from 2009 GT-R)

That's the important stuff.

Fuel PSI was steady at 57 PSI. No change from idle to load. (this can be a common issue for the coupe and 350z guys, requiring swirl valve/regulator modifications to allow more return flow at idle).
The PROBLEM is that at higher RPM (5800+) and under load the fuel pressure is dropping off all the way to 47 PSI. It's a linear drop depending on RPM and load.

Because I'm in boost at this time, the effect is compounded, making this a major issue, and causing the Injector Duty Cycle to be at 100% way before they should be.

The tuner put the rev limiter at 6000 RPM to leave some head room until I get this resolved.


The issue is I dont know why this is happening.

The Walbro 255 should flow plenty enough fuel for this power. The Injectors also. So the only thing I can think is perhaps there is something in the Fuel pump that is different on the sedans.

I'm sure I'll just end up going with a full-out return system here with a rising rate FPR, but I really want to know the reason before I waste money on something that MIGHT NOT even fix the problem.


Does anyone have any insight?

 

What you're experiencing is normal for a returnless system. You can try to overcome the drop in pressure with a boost a pump or bigger fuel pump but IMO a fuel return system is the way to go.

 

Really? I know that fuel pressure differential (at the injector) will change depending on boost/vacuum, but supply pressure shouldn't really drop unless there is a supply issue.

I know that traditional return style FPR's use vacuum and boost to change the fuel pressure respectively, but this is a single pressure regulator (the OEM one), so as long as there are no restrictions and the pump is capable, it should maintain one pressure all the time, period.
It's the differential pressure at the injector that will change.

Let me know if that above knowledge is incorrect.

I was also talking with Charles from CJ performance, great guy, he thinks the outlet of the sedan pump is quite a bit smaller than the 350z one and the G coupe one. This could be, in itself, the cause, right?

 

Sorry I don't know anything about the pump outlet in coupe vs sedan.