Larger injectors or rising rate fuel pressure reg

Right, I was thinking the other day about the above, and pondered on whether smaller injectors with a rising rate fuel pressure reg would give better results than larger injectors? I would expect that idle would be much improved doingthe former, and still being able to deliver the larger quantities of fuel when required. Does that make mapping difficult though? Will a rising rate fuel pressure reg only give you a benefit when running standard ECU rather than aftermarket? Are standard fuel rail (when I say standard I mean 20vTurbo or G60 etc etc) fuel pressure regulators rising rate anyway? IIRC you can get higher pressure ones on things like porsches etc (3.5bar as opposed to 3bar)??

Discuss

Most modern fuel injected cars run a fuel pump in the tank and run much more fuel than is needed to the fuel rail. A regulator keeps a certain amount of fuel pressure in the rail and then sends the rest back to the tank.

Your car as well as most fuel injected cars have a one to one fuel regulator. It varies the fuel pressure by 'watching' manifold pressure. It uses a diaphragm to control pressure. Most Nissans at idle (vacuum) fuel pressure is 34psi and at full throttle (No vacuum in theory) 43psi. If you further put pressure (turbo or super charger) to the manifold, it further increased fuel pressure. In factory regulators, for every pound of boost, it adds a pound of fuel psi. Hence 1 to 1.

Most adjustable regulators are still one to one or close to that, however you can adjust the pressure at idle or full throttle for fine tuning. There is one other type of regulator that is used with aftermarket forced induction. These are rising rate regulators commonly called FMU's (fuel management unit). These regulators increase fuel pressure at a multiplication factor of boost. So instead of messing with complicated computers and injection duty cycles, these systems just increase fuel pressure to add fuel. They go inline down from the factory regulators and only start to add pressure under boost. So when you are off boost, you maintain factory tuning and drivability. Only as you get boost does the FMU begin to increase fuel pressure.

To check fuel pressure regulator:

Release the fuel pressure in the system by removing the fuel pump fuse and start the engine. After it stalls, crank it a few more times to release the fuel pressure.

Install the fuel pressure gauge on the inlet side of the pressure regulator, start the car and check for fuel leaks.

At idle, the pressure should be approximately:

235kPa / 2.4 kg/cm2 / 34 psi.

Disconnecting the vacuum hose, you should see:

294 kPa / 3.0 kg/cm2 / 43 psi.

Afterwards, connect a hand vacuum pump to the regulator and start the engine. As the vacuum INCREASES, the fuel pressure should DECREASE. If this is not the case, time for a new pressure regulator.

On a personal level
I dunno...

Almost a harking back to carbs and or Mechanical fuel injection?!!

However,yeah I guess it could work with a standalone.You could just modify your fueling map pulsewidths on a rolling/road in the boost areas to take into account the
higher fuel rail pressures.Not for me personally but could be interesting.I'll keep an open mind.

With my limited experience, I would have to say larger injectors would be better for the following reasons:

You would most likely require a remap with both.
With the rising rate regulator fuelling would be close but probably would benefit from a bit of tweeking.
I would say the tuning would be much more straight forward with a standard regulator as injector delivery is linear.
Even if fuelling is acceptable the timing would more than likely need tweeking anyway.

I don't like the idea that with the rising rate regulator the same fuel flow has to be maintained under higher pressure than with a standard regulator therefore hitting the upper limits of the fuel pump earlier.

With a good larger injector, the fuelling should still be very controllable at idling flow levels, you just need one's with a good spray pattern so as to not just blast the inlet manifold walls with fuel. (The one's Kev has look like the way forward)

My 2 p's worth, I'd love to hear the counter arguments though!

I remember chatting with Steve from Pitstop Developments about this a while ago - for G60's he would not map with aftermarket rising rate FPR's. Also when I talked with Vince about fuel pressure his advice was a normal but higher pressure fpr (such as the Porsche one) if I was having a problem with injectors getting close to max.

I would have thought adding in another variable of fuel pressure would make mapping more difficult - better to have either a higher than standard pressure or bigger injectors.

Yes, the G60 ones vary depending on manifold pressure - which makes people comments above seem quite strange. Perhaps it is to do with how the fuel pressure rises based on manifold pressure (is it linear or not for example). And yes there is a Porsche one (as mentioned above) which is higher than normal - 3.5 bar from memory. Nice simple little change really as off boost (as long as you are using a lambda probe) the map does not need changing as the injectors will automatically be dialled back based on afr.

Hmmm, I did think that the rising rate fuel pressure regs were non linear in their response. So, the reasoning behind non-linear rising rate regs is to allow standard management to be used when increasing charge density via super/turbo chargers then?

Does the Porsche 3.5bar FPR fit in a 1.8T manifold then?

I think there is a little confusion with this.

All FPRs "rise" the fuel pressure according to manifold pressure, so we need to distinguish between 1:1 FPRs and "Boost dependant" FPRs.

A 1:1 FPR will match the manifold pressure exactly. Throw in a bar of boost and your fuel pressure gauge will read 58psi if you have a 3 bar FPR and 72psi if you have a 4 bar FPR.

That's all nice and safe and should be setup to work within the Injector's linearity. I.e. between 10 and 85%. Remember, if injectors operate outside of their linearity, the fuelling can become unpredictable. Too small a pulsewidth and you get clumping and too high a pulsewidth and the spray pattern can change.

Don't forget the pump. If you fit a 4 bar FPR and the standard one is 3 bar, can your pump handle 5 bar at the required flow?

Pumps always deliver max flow and the pressure is governed by the FPR, but as pressure increases, flow reduces, so you need a good pump that can deliver the required Pressure AND flow!

The Bosch 044 is a classic pump. Huge pressure and flow and has a 12 Bar lockout (physically won't pump past that pressure), so it will sit at 5 bar all day long chucking out well over 220lph.

Anyway, back to FPRs... the "Boost dependant" ones behave like a stock 1:1 reg in vacuum, but when they see boost, they rise the pressure at a user selectable rate, anything from 2:1 to 10 or more :1. The BEGI FMU is the ONLY boost dependant reg to consider.

So if you set a rate of rise of 10:1, you can see why you need a Bosch 044 pump, or a pair of them but it depends on what boost you're running etc.

Then obviously you need some VERY good fuel lines. No cheap schitt from ebay, proper PTFE steel braided or Aeroquip rubber as a minimum requirement, and of course top quality fittings.

I ran a BEGI fpr on my first turbo install at 8psi boost and it was absolutely lush and a peice of p1ss to tune. All you do is set the rate of rise against what the AFR is telling you. As soon as the MAF and injectors max out, it's then up to the pump to shove the fuel in at a huge rate and imo it works very well.

People are afraid of that method for some reason, but if it's done properly there is no need fear it at all. I ran it like that for months with no issues.

As JMC says, don't mix methods though. Either go standard injectors, standard management + BEGI and poweful pump, or go big injectors + software and 1:1 fpr.

And VR6 Joni quite rightly points out that the fuel pressure method doesn't take care of ignition advance. The VR6 can take care of that to a certain extent due to the knock control, but you'd be surprised how little it takes off. Remember you are shoving loads of fuel into a lowered compression engine that is intercooled. As I say, do it properly and you have nothing to fear

The Brute force method is easiest and cheapest to setup, install and tune, but some people aren't happy with the fuel pressures involved or the lack of ignition advance control.

The big injectors + management method is more tunable and safer, but low duty cycles can cause serious idling and part throttle problems.

So neither method is 'right', it's user preference

Good info here everyone. Been thinking about all this a lot and ended up deciding to run a VR6 100L/hr@4 bar pump and a porsche 3.5 bar@ATM 1:1 reg that shouldn't make it much past 4 bar at the rev limit. As for injectors, I have some 350cc EV1 Saab Bosch injectors winging there way to me. They'll need cleaning and testing first but from what I can work out at 85% duty they should be enough for 250-260bhp. Was gonna try just going with just the FPR and pump but the more I thought about it and the closer I looked at the new head the less it made sense to not put some slightly larger injectors in there.

Good Read!

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I love that Max-Boost site

Your plans sound great and well thought out!