what is correct fuel pressure on an all stock 1989, LB9?

would i see any benefit from bumping the fuel pressure up a bit with an adjustable regulator? injectors are stock

 

not really any benefit on a stock engine. But an adjustable fpr is a good investment if you plan on doing any modifications to your engine in the future. If you plan on adding parts such as better flowing heads, aftermarket tpi parts, cam, etc, the fpr becomes a necessary tuning tool.

 

I have a slight hesitation when I hit the gas, rpms drop slightly before shooting up, I have a feeling it's bc my fpr is not holding pressure and the hesitation is pressure building up bc of a leak somewhere.

I have a pressure guage coming in the mail, is there a way to test the fpr?

If i have to replace it I'll go w an adjustable. I heard somewhere TPiS suggested bumping up the stock pressure a few psi will benefit even a stock motor

 

That "explanation" is not likely correct.

Could however be, that your fuel pressure is simply low; that being the most common cause of hesitation in an EFI setting.

Correct stock fuel pressure should be around 43 psi with the engine idling and the vacuum line to the FPR disconnected, and around 38 with the vac line plugged in. It should be able to hold that same 43 with the engine at max RPM and max power with your foot on the floor; i.e. tape the gauge to your windshield and watch it as you go up a steep uphill on-ramp or something and make sure it doesn't drop. If it does, most likely cause is, needs a new fuel pump.

"Stock" settings serve MANY masters, besides just "runs the best" or "perfect" or whatever. They are dictated by emissions, all-weather, CAFE, altitude, and above all emissions, among other things. The factory also has to give priority to emissions considerations. Oh and did I mention that emissions are important in developing "stock" specs as well? In case I forgot, let me throw emissions into the mix of things the factory puts ahead of "runs good" when designing a tune. So yeah... just opening The Book and setting everything to The Spec may or may not give you the "best running" combo possible; instead, FINDING that combo, is the process we call "tuning". Something EVERY would-be hot-rodder needs to master. And in an EFI car, the first and easiest tuning handle to grab and twist on, is the fuel pressure. Next easiest is ignition timing.

 

That's helpful, my gauge should be here by Monday. I'll check the pressure both ways. What does removing the vacuum line to the fpr do? And also is there a way to differentiate between the fuel pump and the fuel pressure regulator my fuel pump is new so I suspect te fpr

 

THIMK:

Things flow from a place of higher pressure, to a place of lower pressure.

So what is "fuel pressure", to begin with? That is, what is it measured, with respect to? And what is the result of it?

IOW, if you have a fuel pump that puts out, say, 40 psi, and you run a hose from wherever the gauge says 40 psi into a box that has let's say 50 psi in it, how much fuel comes out? Or, if you have some fuel sitting around somewhere and a gauge says "0 psi" (it's just sitting in a bucket or something), and you put the other end in a box where the gauge says "-14.7 psi" (vacuum), you'll still get flow, right? In fact, wouldn't that flow be the same as if you put fuel under 14.7 psi of pressure and let it flow out into "0 psi"?

That's the deal with the vac line. The pressure inside the manifold varies quite a bit; at idle, if you have 15" of vacuum, then since atmospheric pressure can be called either 14.7ish psi or 30ish" of vacuum, you would have about HALF of atmospheric pressure added to the TOTAL "fuel pressure" that's impressed across the fuel injectors. IOW at that instant the pressure drop across the injector, which is the amount that drives the fuel flow, is around 7.5 psi HIGHER than what your gauge reads.

THAT'S what the regulator is all about: it makes the pressure in the line change more or less in lock-step with the vacuum, such that the pressure drop across the injector is kept as close as possible to a constant at all times. The pressure is set at some value when it sees no vacuum at all (simulating wide-open throttle), and is reduced by the amount that the pressure in the manifold drops i.e. the vacuum. In order to do that, it has to be exposed to vacuum. Hence, the vacuum line.

In cars with forced induction, the FPR has to be able to go the OTHER way too: if for example you put 1 bar (1 atmosphere, or about 14.7 psi) of EXTRA pressure into the manifold by externally pumping it, then the FP has to be RAISED 14.7 psi.

 

Thanks that's helpful info. I got my fuel guage today here are my findings

For my 305tpi
At idle: 38psi
Vac line disconnected: 48 psi

When I rev the engine, the psi briefly rises to 42, this is when my hesitation happens, rpms drop slightly before shooting up, then psi drops back to 38.
So it seems like it's getting too much fuel for that brief moment, causing the hesitation Is it normal for the psi to rise for a quarter second when revving before leveling out again?