question on setting vs. measuring fuel pressure

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Jun 6, 2005 | 06:02 PM

Red Tornado

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From: OBAMA IS HITLER

question on setting vs. measuring fuel pressure

To measure FP, we leave the vacuum line attached from the regulator to the intake plenum.

To adjust/set FP, we disconnect the vacuum line. And normally, we should see a 7-10 psi or so jump in psi when disconnecting.

I dont understand how the two conditions reconcile. For example, my FP reading w/line disconnected is 52/53 psi. With line connected its 44/45. Which is my REAL fuel pressure reading?

Jun 6, 2005 | 06:40 PM

CFI-EFI

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From: The Top of Utah

The pressure IS what the gauge says it is. It is that basic. As the load on the engine increasings the more fuel it will need. The manifold pressure increases as the load increases. The fuel pressure regulator changes the fuel pressure according to the manifold pressure (demand). The "REAL fuel pressure reading" is what the gauge says it it is.

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Jun 6, 2005 | 07:48 PM

Red Tornado

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From: OBAMA IS HITLER

Jim, thanks, yes of course I understand that, but which fuel pressure reading is the real one - with, or without the vacuum line attached to the regulator - when hooking up the gage at idle?

Last edited by Red Tornado; Jun 6, 2005 at 07:51 PM.

Jun 6, 2005 | 07:56 PM

FD2BLK

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From: Charleston SC

If you are taliking about the point of refrene it is the number with the vacume line off. That will be the FP at WOT and will be what the injector is using at full capacity.

Jun 6, 2005 | 08:06 PM

Red Tornado

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From: OBAMA IS HITLER

oh i see. crystal clear. then 52/53 is ugly high fuel pressure, definitely NOT what i want. this must be reduced to 43-45 assuming the line is unattached. am i correct? for a '90 it should read 41-47 psi.

but wait......normal and even spirited driving means i would only flirt with WOT for very brief moments, typically just that period of time while approaching shiftpoint then shifting.

so......now the cloud comes back.....nominally speaking about what range should i be aiming for with the FP setpoint? again, the difference between the 2 readings in MY car is 7-8 psi.

i'm probably making this harder to understand than i should.

thanks to both of you for your help

Last edited by Red Tornado; Jun 6, 2005 at 08:13 PM.

Jun 6, 2005 | 08:34 PM

CFI-EFI

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From: The Top of Utah

Quote:

Originally Posted by bradvette

i'm probably making this harder to understand than i should.

It appears that you are. The REAL fuel pressure is what the fuel pressure gauge, REALLY says it is. Because there are different fuel requirements for different conditions, the "load adjustable variable fuel pressure regulator" allows the system to adapt to the needs of the conditions. Properly adjusted, the fuel pressure should change to meet the needs of the engine. I would expect your Factory Service Manual would list the correct pressure under specified conditions. If the fuel delivery and/or demand have changed through modifications, your chip burner may have to tell you what pressure will make your system provide the correct A/F ratios for your set up. Otherwise, tuning with a wide band O2 meter may be necessary.

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Jun 6, 2005 | 09:14 PM

65Z01

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From: SE NY

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Indeed it is quite simple, just set fuel pressure at idle with the vacuum line off to factory spec and foreget it.

If you want the optimum fuel trim put it on a dyno and adjust fuel pressure at WOT for best power.

Jun 6, 2005 | 10:22 PM

Red Tornado

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From: OBAMA IS HITLER

ok got it. thanks guys. disconnect the vacuum line then set it @ 43 psi as a starting point......which may or may not be adjusted further on the dyno.

thank again for clearing my cloud