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Radium Fuel Pressure Regulator Kit for Subaru

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Hey all,

Im just starting out to get my car set up and ready for some tuning and one thing I've been recommended to get on the car first is a better fuel pressure regulator since the OEM FPR setup has a weird loop in it that I guess causes strange fuel stumble issues when tuning. This is the kit I was looking at getting:

https://www.radiumauto.com/FPR-Kit-08-21-Subaru-STi-P955.aspx

Which seems nice since I wouldn't have to buy a bunch of tools to properly cut my own lines and add AN fittings.

One thing Im a bit confused on though is that the OEM fuel setup does have fuel pressure dampers inline but this kit that is supposed to fix fueling issues actually deletes the fuel pressure dampers and doesn't replace them? This seems to be the case with most/all of the FPR kits for this car but deleting the dampers seems counter productive if the goal is stable fuel pressure no?

Am I missing something? Should I look at a different kit?

Any input appreciated.

From what I can see and read in that description, it looks like the OEM fuel rails are demand feed so there would be pressure fluctuations occurring each time and injector opens and drops the pressure, before the demand feed system supplies fuel back into the rail. With the replacement system, all of the supplied fuel flows through the rails, and then into the FPR, doing this greatly minimises the pulsations in the fuel pressure.

G,

I did the R&D on this while I was at COBB. For those not familiar, this is specific to the 08-21 Subaru STI vehicles (not WRX, not older STI models) and the primary issue is one of very extreme oscillation of fuel pressure which occurs at a high frequency. When logging fuel pressure at low frequency i.e. typical OBD port logging at 5-20 Hz, fuel pressure usually appears reasonably stable, but high frequency logging with external loggers and sensors shows the actual issue. The engine will suffer from poor combustion, misfire, etc. mostly in a certain RPM and fuel flow range, but it has less extreme broad effects as well.

It's a pretty complex issue, and in my experience the solution involves rails, lines, FPR so it's not a matter of swapping out one part. Materials, hose lengths, routing, vacuum reference source and more all turned out to be key components in creating a complete system to stabilize pressure. I tested several FPRs, various dampers sold aftermarket, some of which were OEM parts, some not. One of the most surprising bits of data I found in testing was the dampers a few companies were offering at the time, all made the issue worse than stock. Dampers

I use this COBB kit up to 400 whp on E85, 525 on gas:

https://www.cobbtuning.com/products/fuel-system/subaru-fuel-system-package-sti-2008-2021

It's plug and play which avoids varied lengths etc. which change the fuel pressure behavior.

Or that kit but with ID1300 injectors and a Walbro 274 pump instead for up to 500 on e85, 650 on gas.

If you're going to run more power than that, then I use a different FPR, lines, etc.

The new FPR in the Radium part you linked came out about a week ago and I have one in hand, but haven't had a chance to try it yet. They took some of my feedback on the performance of the prior version, did their own re-development, added some brilliant touches like the swivel fitting and no tool adjuster, so I'm excited to try it as a universal FPR when I have time. In terms of a solution for the 08-21 STI, I'd want to test the whole kit on that platform.

Great info guys! Thanks for the insights.

You're welcome!

@MikeMcGinnis Actually had another question around this since it sounds like you've done some pretty thorough testing on these setups...

Im curious if you can comment specifically on how things ran with just the OEM rails/lines but aftermarket pump, FPR, and injectors (eg:1050x)? I understand this probably wouldnt be the ideal or correct solution, but its come to my attention that running out of injector will happen pretty quickly during the upgrade path (with just an intake and downpipe) so im wondering whether doing just injectors would cause more harm (fuel pressure instability) than good (not running lean)?

The short answer is poorly, not significantly better than stock. The extra rail volume helps act as a damper and the lines themselves are the most critical part. The stock system being small volume hard lines really exacerbates the issue.

Installing injectors which flow about double stock causes the fuel delivery error to effectively double when there's fuel pressure oscillation, so it gets worse. With the right combo of lines, rails, regulator though, 1050x work fantastic, passed emissions lab testing on my calibration with flying colors and that really shows the level of precision regained by getting hold of fuel pressure.

Unrelated, upgrading the intake and downpipe in an area where it gets cold will cause boost creep on that particular vehicle with stock turbo. I'd just do the intake until you're ready for a turbo upgrade which will incorporate a higher flow wastegate and you won't have creep.

Ok yeah that makes sense. Good to know.

Re: Downpipe & Boost creep

Hahah... yeah I just realized this will also be an issue Im unable to resolve with tuning changes. Guess this downpipe is going to be sitting around in a box a while longer 😂 .

Sounds good. Best to wait til you have a package of parts that can work well together.

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