What is the function of a fuel pressure regulator?

[rroderiques77]

I own an 02 Maxima and have heard that Paxton makes a fuel pressure regulator for it. What exaclty is the function of a FPR? I am re-maping the ECM. Should I even bother installing an aftermarket FPR? Thanks for any help on this one.

[Q45tech]

Depends on the capacity of oem injetors.Each psi of boost adds 6.8% more air and thus 6.8% more fuel is required.

The flow of an injector is speced at 43.4 psi, so raising the fuel psi can increase the flow. FORMULA............take sqrt of new old ratio 55/43.4= 1.267 take sqrt =1.1257 or 12.57% so a 55 psi might be enough for 2 psi of boost.

Once you get above 60 psi of fuel pressure the springs in injector have a more wearsome difficult life.

Usually takes BOTH ecu reprogram and rising rate fuel pressure regulator to get serious fuel flow or increasing injector size and some other combinations.

The problem with larger injectors is that they may be too large and overfuel idle thus requiring an increase in idle rpm.

Juggle the choices to avoid uncivilized behavior.

[RBbugBITme]

I'm trying to understand where you're getting this 6.8% number. 1 psi doesn't equal a set amount of flow under all conditions.

[6SPD_FTW]

Very true. On my supercharged Corolla, installing a header and cat-back exhaust saw a 1psi reduction in boost, but an increase in performance due to less restriction.

Boost requires 2 things. 1) some method for pressurizing (turbo or s/c) and 2) resistance to flow. If the intake, head(s) or exhaust flow poorly (and thus increase resistance to flow), then boost will increase, but power and actual intake volume will decrease.

That 6.8% may be a very, very broad generalization.

Late,Trav

[Nismo_Freak]

Very true. On my supercharged Corolla, installing a header and cat-back exhaust saw a 1psi reduction in boost, but an increase in performance due to less restriction.

What you got was air/fuel mixture being blown down the exhaust on overlap, that reduces the intake pressure.

Boost requires 2 things. 1) some method for pressurizing (turbo or s/c) and 2) resistance to flow. If the intake, head(s) or exhaust flow poorly (and thus increase resistance to flow), then boost will increase, but power and actual intake volume will decrease.

Replace intake volume with mass flow and you have a better theory. Heat will also increase due to the increase in compressor pressurization reducing the density of the charged mass.

That 6.8% may be a very, very broad generalization.

1 PSI / 1.0 ATM = 6.8% Increase in Pressure / Flow Potential

There is not a 6.8% increase in fuel since the increased likelihood of knock shifts the mixture to the rich side. You would need to add something on the order of 7.2% - 7.5% fuel to the air to accommodate the heat of the ambient air and the increase in chamber heating on the compression stroke. The increase in fuel presence also gives a more favorable condition in the chamber.

[Nismo_Freak]

I'm trying to understand where you're getting this 6.8% number. 1 psi doesn't equal a set amount of flow under all conditions.

It's just a rough figure assuming 100% system efficiency. Obviously with boost comes variables in port flow due to increasing Mach value. Ultimately as the port migrates near 0.5 Mach it will begin to choke and you will need a large increase of pressure to result in ever dwindling increases of mass flow.

I'm sure he didn't care to type out 5 pages of fluid dynamics crap.

[crzycav86]

The function of the fuel pressure regulator is to maintain a constant pressure difference between the fuel rail and the intake manifold. So when you let off the throttle and the manifold pressure drops by 5 psi, your fuel rail pressure will also drop by 5 psi. The pressure difference must remain 43.5 psi, the base pressure.

The injector's fuel flow is a function of the injector's pintle cross-sectional area, and the fuel's exit velocity. The pintle area is based on the injector's design. and the exit velocity is a function of the pressure difference between the fuel rail and manifold.

the exit velocity can easily be derived from the bernoulli equation. Keeping a constant exit velocity gives a linear relationship between fuel flow and injector pulsewidth, which is what we want.

so to summarize, a fpr gives us a linear relationship between fuel flow and pulswidth. and no, you don't need a fpr for a mild NA tune.

[Q45tech]

Actually a FPR acts like an accelerator pump in a carb. When the throttle is suddenly opened [plenum psi increases and the rail pressure is suddenly increased [sure there is a lag] but not as much as the MAF to ecu lag complements the TPS signal so that the sudden accelation demand for fuel is partially or over compensated.

Being too rich is better than too lean as the cruise AF is transitoned from 14.7A/F > 12 or 10 A/F.

People forget that unsupercharged plenum pressure at idle or cruise is 9-10 psi [from earth's atmosphere and sudden rises to 13.7 psi at WOT.......................so 4 psi in plenum pressure increase causes a 9 psi increase in fuel rail this 27% psi increase causes a sqrt conversion or 13% sudden increase in fuel flow....................on an old VH45 engine Q45.

[crzycav86]

Your accelerator pump analogy is a little weak because the accelerator pump is a temporary enrichment. if you stab the throttle, the fuel pressure increases and remain at that pressure until you let off the throttle.

and efi systems already have an "accelerator pump" built in, which i talked about one of the other threads on this page. the fpr simply ensures a linear relationship between pulsewidth and fuel flow. you can draw other conclusions from that.

[Nismo_Freak]

Actually a FPR acts like an accelerator pump in a carb. When the throttle is suddenly opened [plenum psi increases and the rail pressure is suddenly increased [sure there is a lag] but not as much as the MAF to ecu lag complements the TPS signal so that the sudden accelation demand for fuel is partially or over compensated.

People forget that unsupercharged plenum pressure at idle or cruise is 9-10 psi [from earth's atmosphere and sudden rises to 13.7 psi at WOT.......................so 4 psi in plenum pressure increase causes a 9 psi increase in fuel rail this 27% psi increase causes a sqrt conversion or 13% sudden increase in fuel flow....................on an old VH45 engine Q45.

I agree with crzycav (partially because we have the same name).

The FPR is a linear response to manifold pressure as a means to provide stability in the fuel map. If you increase manifold pressure by 4 PSI the FPR will increase pressure at the rail by 4 PSI. The FPR is a linear device.

The tip in fuel enrichment on the ECU is what acts like an accelerator pump. It will increase the injector pulsewidth momentarily at large, sudden changes in TPS value. That is what provides the additional fuel (and timing change) on tip-in.

[Q45tech]

Measure the rail pressure on a Nissan/Infiniti with FPR you will see that at idle the plenum pressure is minus 18" Hg or 8.8 PSI and the rail fuel pressure is 34 psi.........................at WOT the plenum pressure is 13.7 psi and the fuel rail pressure is 43.4 psi.

9.4 psi fuel rail increase from a 5.0 psi plenum pressure increase is pretty far from LINEAR...............the oem is a 2.0:1 rising rate regulator.

As to my analog of a accelerator pump: look at the time delays of fuel pressure increase vs MAF and ecu lag to increase injector open time plus the actual flow rate of fuel mass into cylinder and you will see my analogy.

The MAF is too far [timewise] from injector and it takes 2-3-4 revolutions before the injector time catches up with the effect of the pressure ramp up.

On a Q45 the plenum internal volume has enough air for 2 revolutions at WOT we are speaking of what happens before the MAF wakes up and shows increased flow to match TPS increase in voltage.

Sure we are speaking of a time less than 1/20-1/10 of second but no stumble is no stumble.

[Q45tech]

Since a single PSI increase of fuel is is a square root function translating to mass ratio 44.4 /43.4= 1.0203 sqrt 1.011455 or 1.1455% more fuel [mass wise] and ......................................a 1.0 psi supercharger increase is a 6.8% air mass increase [assuming temperature of air doesn't increase] you would need the fuel mass to increase 6.8% this requires a 50 psi fuel rail pressure.

The usual FPR for a super/turbo is 6:1 for every single psi of boost the rail fuel pressure increases 6 psi..

With 4.5 psi boost you need a 75 psi fuel pressure: 75/43.4..........this gets hairy to try to use fuel pressure alone since such high psi will wear the fuel pump fast and may cause injector failures.

[Nismo_Freak]

My SR pulled way more vacuum than 18"

It was more like 24", and the fuel pressure rose 1:1 with boost pressure.

[RBbugBITme]

1:12:1499:1

They're all linear.

[crzycav86]

1:12:1499:1

They're all linear.

they're a linear pressure relationship, but the fuel mass relationship is quadratic.

Although I hold q45tech's posts in very high regard, I must disagree with his statement that the OEM fpr is 2:1.

On maf-based systems, it would be impossible to predict fuel flow without a MAP sensor or a fuel pressure sensor, because now the mass fuel flow becomes a function of the pressure difference, whereas the deltaP was constant with the 1:1.

Speed density systems would be able to compensate for the non-linearity because the desired fuel mass flow is already a function of the manifold pressure. However, I highly doubt that it is done because it is not necessary.

[StarPD]

Overlooked is the fact that the fuel/air ratio requirements vary so widely, and are so dependent on RPM, throttle opening, and load, that the complexities require vast computer capabilities to even map. Using simple formulas to calculate the requirements are so primitive in this regard as to be little more than trial and error, guesswork at best. There's a reason why aftermarket fuel and air intake systems along with ECU mapping are so often less than ideal for all situations, and frequently produce unintended consequences. Establishing the parameters for a racing, especially a drag racing motor that will run at WOT for a limited time is far easier than designing a system that is optimum for all everyday driving applications, wherein the variances in conditions is almost infinite. Well, not quite, but for all practical purposes, it's generally pretty applicable.

Perhaps the answer to the thread topic question is that under most conditions, a naturally aspirated Infiniti motor will not benefit from a higher than stock capacity fuel pump or pressure regulator. Add forced induction, and the equation changes. In that case, most forced induction systems are intended for racing use, and are far easier to map fuel/air ratios for than for ordinary passenger car use. Even so, I know of no supercharger/turbocharger manufacturer that produces a complete system for either M or Q 45s due to the low volume of those cars on the road compared with other makes.

Bottom line is that if all fuel/air system components are in as new condition, that's pretty much all you're going to get from a stock naturally aspirated M or Q45.

Now, you wanna improve ALL aspects of performance, incuding acceleration, fuel mileage, handling, and ride? The simplest and easiest way is to reduce weight (both sprung and unsprung), and alter weight distribution closer to 50/50. Moderate lowering of the vehicle can reduce Cd, which will further improve these areas, but including top speed.

How?Simple.Lighter wheels/tires, replacing battery with gel type and relocating it to trunk, removing spare tire, replacing exhaust sytem with a lighter one, removing some non-essential amenities, etc can remove up to 200 lbs, which is a considerable amount. You can get more, but such requires more drastic measure, which removes the car from "luxury" classification.

[Nismo_Freak]

StarPD, low load / moderate load tuning is done on the fly by the ECU. Do some reading into how the Long Term Fuel Trim (LTFT) operates and corrects the map through a conversion using the O2's. That's often why mid load tuning is futile to some degree in regards to fuel.

Besides, fuel is the easiest part to tuning. It's getting the timing map to be acceptable in relation to the fuel / dynamic cylinder pressure, and still make power, AND have a responsive motor that is annoying.

[crzycav86]

Throw in emissions and climate requirements and you've got one major headache!

[Q45tech]

Few have even mention the rail input pressure dampener tha derives it's control from the pressure hose just after the MAF where the differential is less than 12 " water column at 6,000 rpm vs 5" at 4,000 rpm.

The trimode fuel pump speed set by ecu, the dampener, the rail and the final pressure regulator work extremely well stock and with small ecu* mods but not with significant increased demand.................install a tee in the front rubber rail hose and observe and record the fuel rail pressures in a drag race or similar acceleration where the duty cycle exceeds 60%.

* Most ecu power mods actually reduce the fuel flow to lean out the factory rich safety.

It is one thing to command an injector open time [mass flow] but to trim it to perfection requires some knowledge of the ACTUAL MASS FLOW .......wideband O2 feedback since chassis dyno cannot emulate the load accurately in the real world.

[Nismo_Freak]

Throw in emissions and climate requirements and you've got one major headache!

Stay away from mountains, and emissions... hahahahhaa... hahahhahahahhahahha... good one.

[Nismo_Freak]

Few have even mention the rail input pressure dampener tha derives it's control from the pressure hose just after the MAF where the differential is less than 12 " water column at 6,000 rpm vs 5" at 4,000 rpm.

The trimode fuel pump speed set by ecu, the dampener, the rail and the final pressure regulator work extremely well stock and with small ecu* mods but not with significant increased demand.................install a tee in the front rubber rail hose and observe and record the fuel rail pressures in a drag race or similar acceleration where the duty cycle exceeds 60%.

* Most ecu power mods actually reduce the fuel flow to lean out the factory rich safety.

It is one thing to command an injector open time [mass flow] but to trim it to perfection requires some knowledge of the ACTUAL MASS FLOW .......wideband O2 feedback since chassis dyno cannot emulate the load accurately in the real world.

The hell are you rambling on about... fuel dampeners and ECU fuel pump controls are non-existent on 95% of the Nissan's built in the 90s. The Q45 has one, the 300ZX has one, and the other cars with the OEM Nissan big fuel pumps. Their controls are for noise, not fuel pressure. The 240SX, Sentra, Maxima, etc. all run full tilt all the time as there is no fuel pump voltage regulator assembly. The returnless cars obviously have them.

Oh and the fuel pressure regulator pulls manifold sourced pressure in every Nissan I've owned. That's a GA, SR, KA (E / DE), and I know the Maxima with the VE, VG, and VQ30 pull from the manifold as well.