OK you LT1 guys, I have a question

What is the fuel pressure with the vaccume line unhooked from the fuel pressure regulator? I tried googleing it and I find everything from 39 up to 45, and then some saying anything between 40 and 47 is within factory specs. wtf? Are the gm FPR's really that bad from the factory? I know on my 93 jeep 4.0, the factory fuel pressure with the vaccume line unhooked is 39 psi, no range, just 39 psi, its either right or wrong. Vac line attached can vary based on engine vaccume though.

My edelbrock fuel rails that go with my intake, uses a bolt on FPR. From what I can find online, it appears the edlebrock one is set to 45 psi, witch is a bit more than I want. (I do not have one, the intake and rails did not come with it) I started trying to figure out a way to attach a hose barb to the fuel rail where the edelrock one went, but as thin as it is (cause the hole is big) it was looking like I might be stuck with theirs, or buy another left side rail so I would have threads on both ends I could attach lines to, and use a universial adjustable one. I measured the hole and looked at the pull a part seeing if I could find any factory fpr that might fit. I stumbled across a lt1 intake setting in a beat up and stripped out buick, I looked and low and behold, the fpr on it looked almost identical to the pictures of the edelbrock one I saw online, just the fuel return line is diffrent. I remove it and measure, and its the correct diamiter. I buy it for a few bucks and test fit it when I got home. TA DA, it fits perfectly. Looks like they are using a lt1 style fpr. But what pressure is this thing gonna run at? I dont plan on running the vaccume line to it. I would really like a adjustable one, but those are so dang expensive!
According to my calculations, with the injectors I plan on using, somewhere between 39 and 41 psi seems like it would be ideal.

Just about every older MPFI (up to about '96 depending on the application) GMs were 43.5 PSI for base fuel pressure. IIRC there were a couple early exceptions.

Since you will be tuning the ECM, this shouldn't really be an issue, especially with how little the flow rate will be effected by 3 to 4 PSIG.

in an MPFI set-up you really need to use a vacuum reference to ensure the delta pressure differential remains constant throughout the load range.

Fuel pressure on 1994, 1995 LT1 without the vacuum line should be 43.5psi. Factory specs are 41-47psi. Should drop about 6-8 PSI with vacuum. Depending on vacuum of engine and depending on fuel pump.

I can tell you for sure that with the vacuum disconnected on my 93 LT1 fuel pressure is 43.5psi.

Quote:

---

Originally Posted by Six_Shooter

in an MPFI set-up you really need to use a vacuum reference to ensure the delta pressure differential remains constant throughout the load range.

---

Umm....delta pressure differential? What is that?
The tbi doenst use a vaccume refrence, and the ecm varies the injector pulses to compensate for low load and high load. Does it not do the same when in mpfi mode? I am still learning here so please school me. I know theres that spot in the bin where you enter the injector flow rate. So if you enter the flow rate of the injector at the pressure its running with no vaccume, then hook up the vac refrence, its now not flowing the same ammount untill low/no vaccume. Is that not gonna throw off things with the pcm expecting one flow rate, and theres actually a diffrent one?

Well it looks like the fpr is no good, I hooked up a hand vaccume pump, and its not holding vaccume. It leaks off really fast. No biggie, I only paid a couple bucks for it, and looking at the parts stores, a new one is not too very expensive.

Delta pressure differential means the difference in pressure from the fuel rail to the intake manifold. In an MPFI set-up where the injectors are downstream from the throttle body, there is vacuum or low pressure at the injector nozzle, that will allow the injector to flow more than when the pressure is closer to atmospheric, such as when the engine is at WOT, removing any other variables. Just talking the pressure differential here. It is because of this that the vacuum referenced FPR is used in MPFI set-ups to keep that pressure differential across the injector the same at all loads, or intake pressures.

TBI doesn't use a vacuum referenced FPR, because the injector is in the open atmosphere therefore always a consistent pressure differential across the injector.

To look at it a different way, say you had a forced induction engine, where the intake pressure goes above atmospheric. In this example lets say that the rail fuel pressure is 40 PSI. Lets not use vacuum/pressure reference to illustrate what happens.

So at idle, you have 40 PSI fuel pressure and a vacuum in the intake. Lets assume this intake pressure is -10 PSI, so now there is a delta pressure differential of 50 PSI across the injector. You start driving the car, and start flogging on it. You get the intake pressure up to 0 PSI, so now you are at 40 PSI delta pressure. there is less flow potential now than at idle. You push harder on the pedal, and the intake pressure comes up to 10 PSI, so now you have a delta pressure of only 30 PSI, keep going, intake pressure rises to 20 PSI, so now there is only a 20 PSI delta pressure differential and reducing the ability of the fuel to flow through the injector. Really flog on it and get that intake pressure up to 40 PSI, now there is a 0 delta pressure and no fuel will flow through the injector, because both sides of the injector have equal pressure.

In reality, you could never get to that point where the intake pressure and fuel pressure are equal, because the amount of fuel flowing at lower intake pressures would be too little to actually keep the engine running (assuming someone isn't using huge injectors to compensate for a poorly designed fuel system, just thought I'd throw that in there for the nit-pickers. :P ).

Remember fluids like to flow from high pressure to low pressure, so the more consistent that pressure differential is, the easier it is to predict the amount of fuel that will flow through the injector.

Quote:

---

Originally Posted by cmaje72

I can tell you for sure that with the vacuum disconnected on my 93 LT1 fuel pressure is 43.5psi.

---

My 94 too!
But just like GM specs for TBI there is a varible, I don't know why they did this as it is not correct. TBI specs are 9 to 13 PSI? :yikes: Wrong! I don't care what GM service manuals say! It may run at 9PSI but the lower you go from 13PSI the worse things get. I have never seen one higher then 13.5PSI. These are all done on my same ol trusted gauge.

For instance, I have never worked on a stock engine in person that ran well at anything other than 13PSI. As a matter of fact if it is not 13PSI I usually find something wrong like a pump or clogged filter then when fixed it's 13PSI. If it's higher than 13PSI it's usually a ubstruction in the return line. Check return line pressure and should be 0PSI. If it's 2PSI you will usually find inlet line 2PSI higher.

They may have varibles to compensate for gauge differences. I have checked cars set to xxPSI on their gauge and it is yyPSI on my gauge. I have seen diffences as much as 4PSI on gauges.

Ok, I think I understand what your saying now. So basicly its like a siphon or suction effect, where the higher vaccume of the engine at low load is actually kinda helping to suck the fuel out of the injector, then at low/no vaccume, the fuel is simply being pushed out the injector by the fuel pressure alone. So at high vac, its sucking the fuel out, so less pressure is given so they dont flow as much, then low/no vac, pressure goes up for more flow.
Do I have that more or less right?

No, not really, don't think of it like a siphoning, that would require a different principal.

It's just simply pressure differential. The pressure difference between the fuel rail and the intake runner in this case. The vacuum referenced FPR is used to keep this pressure difference more or less the same at all engine loads.

If its not like the vaccume is basicly helping to suck the fuel from the injector than I just dont understand what your talking about.
But you know way more than I do and I value your advice. So I will run the vaccume line to it. So in the bin where you enter your injector flow, do you enter the flow when the vac line is unhooked?