This question's 4 all of you running an adjustable FPR!
 

Mine's an Aeromotive (first one was an OBX...seal blew...piece of crap, IMHO). Also, my car's a '97 with an '02 3.5 swap. Anyway, I've tried 3 separate gauges (all screwed directly into the FPR gauge port): AutoMeter, Ashcroft (from a local flow systems company) and the cheapy OBX gauge. First, I don't trust the OBX gauge as the needle sticks occasionally and the psi reading fluctuates by as much as 5. Here's the problem: When I first start the car (with the FPR vacuum line disconnected), I can adjust the pressure to 51 (or whatever I choose). When I reattach the vacuum line, the psi falls by 4 or so (which, I'm assuming, is completely normal). After the car has idled for 20-30 seconds, it falls again to 42 (on the AutoMeter) and 46 (on the Ashcroft). At that point, turning the adjustment screw has absolutely no effect on the gauge display. It's not until I turn the car off and on again that I can adjust it. Question: Why do the gauges always fall to either 42 or 46? Does it have something to do with the 4th gen fuel pump not having enough flow capacity or ??? The car's running perfectly...I'm just kind of frustrated by this. Thx.

One other thing I didn't tell you to try in the pm...put the gauge directly in front of the fuel rail on a tee. See if it reads differently. The pressure you want to measure is the pressure going into the fuel rail anyways. Do that before you try anything else.

OK...I'll try that as well. Now I need to find out where to locate a tee with barbed fittings at two ends and a female 1/8 NPT fitting at the third :slant: . Ideas?

Get a tee that has 3 threaded 1/8 NPT sides, get 2 barbs that fit the tee, and put the gauge in the top of it, like I did with mine.

http://i3.photobucket.com/albums/y87.../FuelSetup.jpg

Thanks GoalieKeg...that's exactly what I've decided to do.

You aren't supposed to hook up the vacuum line, the 3.5 has a constant output fuel pump unlike the 3.0. Set it to 51-52 psi and forget it.

You can do it either. The ECU will adjust its short term fuel trims to get a stoich mixture in closed loop no matter what the fuel pressure is. I will say though that 51 PSI full time is the best option due to the superior atomization at higher fuel pressures.

The A32 has a constant output fuel pump too but uses a vacuum referenced fuel pressure regulator to adjust FP. The A33B has a fuel pressure regulator too, but it's fixed.

The A32 and A33(VQ30) ECUs are programmed for rising pressure FPR. You can in theory tune for constant rate, but you would need higher injector duty/duration to compensate for the rising manifold pressure. The only real reason the VQ35 has constant FP is because the FPR is in the tank and it was easier for Nissan to just tune it for constant FP than to run a vacuum hose back to the tank enclosed FPR.

What do you mean? 96sleeper has proven that the A32 ECU will adjust its short term fuel trims to bring the closed loop mixture back to stoich despite the higher than stock fuel pressure (full time 51 PSI) and larger injectors. His closed loop a/f ratio was stoich even AFTER he threw in a 255 Walbro pump. The ECU doesn't care what the fuel pressure is. It was adjusts accordingly. If the level of adjustment required is excessive a DTC will be set off but no one has managed to push the ECU that far.
:scratch:
Higher fuel pressure full time is better since it allows for better fuel atomization at even small throttle angles, so it was worth keeping it full time. I don't really see the point of rising rate fuel pressure in an n/a vehicle. IMO of course...

The whole point of the rising rate fuel pressure is to have a constant fuel pressure differential across the injectors at all times which a constant pressure output does not do. As the fuel pressure at the injector is reduced when the throttle is opened on the variable pressure systems so is the pressure in the intake manifold where the injectors are located thereby keeping the differential constant throughout the complete range.

According to the experts injectors are designed to operate under the same pressure differential at all times which probably makes the 3.0 a better design. Having the fpr etc in the tank and having a non return system is a cost cutting measure certainly not done for economy or efficiency of the engine.

I see what you're saying but i don't quite understand the significance of that phenomenon.
Returnless > return style. The fuel stays much cooler with the returnless setup which is undeniably better for power and efficiency.

so what would it take to convert to returnless?

As far as I see all i would need is to cap off the return line and put an fpr in line with the feed line.

Any thoughts?

But, isn't Jime saying that the 3.0 fuel setup is more efficient as far as pressure is concerned. I understand having a returnless line will keep things cool, but is there a way to have the best of both worlds. Lets say a returnless line with variable pressure as done on the 3.0 platform.

Returnless keeps the fuel cooler and has been proven to be better. There is plenty of other threads about this. I would recommend converting to it since it is really not that hard.

Rising Rate means that there is a constant difference between the pressure of the manifold and the pressure of the fuel. It is always (normally) 3bar or 44psi above what the manifold pressure is. Since the manifold pressure changes the FP must also change. This is how the VQ30 fuel system is done.

Constant means that the fuel pressure is the same no matter what. This is how the VQ35 stock fuel system is. The ECU takes into account for the fact that the fuel pressure is not going to change depending on the manifold pressure and thus has different fuel tuning.

The ECU will not adjust for open loop, which is just as important if not more so. The ECU can only compensate so much, and it will completely put open loop out of tune.

It is a trade off. I promise peformance wasn't the reason Nissan or anyone else did it. When I redo my fuel system which probably won't come until right before boost, I will do a returnless rising rate.

Jime I don't understand what your take on this is. On the first post you say to use constant and then you say that rising rate is better.

In the first post I was just saying KISS (keep it simple stupid) and run it like the 3.5 was designed to run ie fixed fuel pressure.

Second post I was trying to explain why the 3.0 system was really a better design.

A rising rate system is designed to keep the pressure differential constant across the injectors because of the varying intake manifold pressure. The fixed pressure system has a 10-11 psi differential across the injectors. Thats why a dry nitrous system is inferior to a wet. In a dry system the fuel pressure is increased to allow for the increase in oxygen from the nitrous and the injectors were not designed to operate at that pressure. Same with boost, it works but its operating way outside the parameters the engineeers designed it for and injector lockup, premature injecture failure etc is a likely possibilty.

I've done some research and looked into this thread as well as the 3.5 FAQ thread.

http://forums.maxima.org/showthread....eturnless+fuel

To make a returnless fuel setup...it is said to put a tee in the fuel rail line going from the fuel filter to the fuel rail. Then one leg of the tee goes the fuel rail, the other leg of the tee goes to the FPR, and from there back to the tank.

Now. If your looking at the fuel rail there is an ingress and egress for the fuel to travel. From the description above and the link I have that one leg of the tee goes to the fuel rail clossest to the radiator. The the other leg of the tee goes to the FPR and out to the return line. Now common sense would say that the egress part clossest to the firewall is capped off? But this is not clearly said.

Lastly this has been done on the 3.5L swaps, but I'm more interested in doing this for an N/A application on a 3.0 setup with DEK ruel rail & injectors.

Understood.......:laugh:
I understand but why's this important? It's easy as pie to tune the open loop a/f ratio.

Which is fine. That's what piggybacks are for.
Higher fuel pressure = better atomization. Based on this, i will be keeping it at 51 PSI full time.
Agreed. He seems to be a proponent of rising rate systems and had the oppotunity to keep it as such on his 3.5 swap but didn't. :confused:

I understand the theory of what you're saying. I still haven't seen an explanation as to why it's important though, or how much of a difference it actually makes.

I doubt it makes much of a difference either way if any at all. I just think its probably best to use the type of system the engineers designed for the engine which is why I am using the constant pressure. However, I am still using the return system because its much easier than installing the fpr in the tank. Even though I am using a fuel cell I still have the complete pump, float etc mounted in it so I can have a gauge etc.

Currently I am also using a MAP vs MAF and just getting A/F leveled out so so much for using what the engineers designed. I am doing it mainly to get a more consistent 1/4 time for brackets. Seems to be working much better so far but I will put it to the test this weekend at a 3 day event. I want it to work like a carburated engine that doesn't control anything but the fuel and its always the same. I can adjust my 1/4 times according to the weather as I have a weather station and et predictor and don't want the MAF making any adjustments for me.

These motors aren't as picky as you think they are. JClaw and a couple others ran a return-style fuel rail with an A32/A33 OEM fuel pressure regulator @ 34/43 PSI with no problem. The setup wasn't ideal, but it operated perfectly fine and his car was very quick. Nissan didn't design the system specifically for the 3.5. They just wanted to head in a new direction with fuel delivery.

Where are those pictures of your setup? :squint:

Sorry...apparently, I can't post photos (or am I sorely mistaken?). You probably couldn't make mine out unless I removed the entire intake assembly. Anyway, my setup looks very much like GoalieKeg's. I took his advice and installed the gauge just before the rail (psi still drops from 51 to approximately 46 or 47 after a half-a-minute or so. I've kinda' decided not to worry about it (as I think it's close enough and the car's running strong).