Fuel Rail Pressure

[Rich Buckner]

I am trying to determine a safe fuel rail pressure for my EFI system.

I am currently running a 3.2 Carrera fuel rail and am considering bumping the pressure up to 3.5 or 4.0 bar. I believe the stock fuel pressure runs at 3.0 bar.

Thank you.

[TurboKraft]

No harm in that, safe provided your hoses are in good shape.

Install the FPR from a 964, direct fitment, only difference is the M16x1.5mm threaded fitting off the bottom (same as threads on the sides of your existing FPR). So you change out that one line and you're done.

[sjf911]

Rich, the stock 3.2 fuel pressure is 2.5 bar.

Chris, are you retaining the fuel pressure damper on the right hand fuel rail when using the 964 FPR?

[TurboKraft]

Yes, we usually are.

[Tippy]

Why so high? I run the stock 2.5 bar at idle and 4.X bar under boost using MegaSquirt II.

Before with Protomotive rising rate FPR, I'd see well over 100psi under boost.

Point is, tuning idle with high base pressures may be difficult with big or bigger injectors.

[TurboKraft]

Tippy -- are you running full-sequential injection?

Most injectors are now designed to operate at higher fuel pressures (they atomize the fuel better) and are responsive, able to operate at minuscule pulsewidths without a problem. I cannot think of a late-model non-DFI car that doesn't run at least 4bar base fuel pressure.

Couple that with that fact that most newer ECUs have really good injector driver hardware, and you can control 2,000cc injectors perfectly at 4.5bar base pressure for an 850rpm idle and stoichiometric AFR.

[Tippy]

Open mouth, insert foot. . I'm running batch although I could run sequential.

I figured injectors that can be ran at high pressures/low duty cycles cost big $$$.

What kind of pricing and brands are you speaking of?

[sjf911]

Getting modern big injectors to work well at idle/low load is still a challenge because you need very good dead time, voltage, and small pulse width deviation correction curves/data and control.

[Rich Buckner]

Once again, Chris brings the facts of life to the table.

Thanks to all for your input.

3.5 Bosch unit it is.

[TurboKraft]

Tippy & sjf911:
We've had the most success with Siemens' injectors, very good spray pattern and great drivability at low speeds. Typically $510 - $600/set of 6, depending on size, and TurboKraft keeps these in stock.

For over 600HP applications we use and sell Injector Dynamics injectors
* ID1000 = $774/set complete with adapters
* ID1300 = $1236/set with adapters
* ID2000 = $1506/set with adapters
Injector Dynamics provides all those values for their injectors, so getting them to behave is effortless. (At least it is with AEM and Motec, minimal experience with MS's software.) The 800whp 993 ITB Twin Turbo we posted on YouTube -- that's got a super light rotating assembly and ultra-light flywheel, but it'll settle down to a perfect 850rpm idle if we tell it to, no weird behavior from the injectors.

A cool thing about the ID1300s: they are manufactured exclusively for ID by Bosch Motorsports and their internals are stainless steel, so impervious to all fuels. Using them in a 560+whp 3.4L Stroker right now, also perfect low speed drivability with them.

And (to get back on topic), that's using the plain 964 3.5bar FPR referencing manifold vacuum/boost. :-)

[Tippy]

I run Siemens Deka 55lb and have no problem adjusting AFR's at idle with MS II. But, don't know if I have to go with the pricier injectors to aid idle or not once going to bigger sizes. Supposedly I see granularity of thousandths of a ms with MS, but don't think it truly is that accurate?

BTW, sorry to hijack.

[alwaysflat6s]

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