OK you LT1 guys, I have a question

[JeepsAndGuns]

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.

[Six_Shooter]

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.