Switching from 24LB's to 30LB's

Thomas302 · Oct 27, 2004

Should I bring my fuel pressure down, or leave it alone. Its at 39psi w/vac, and about 44 w/o vac. Maybe this is a dumb question, I guess I should just put them in and see how it runs, and adjust from there, but I just wanted some advice. Also I have a motorsport extender, and I have the A/F set at 12.75, does that sound good. The car runs good with my current A/F ratio and fuel pressure, but I just wanted to know where I needed to start adjusting with the new 30's. Thanks.

Edit: Sorry I should have noted that is a 331ci 10.5cr with AFR 185's, TFS-R intake, 75mm tb and 76mm C&L mass air.

Daggar · Oct 27, 2004

12.75 AFR is good. Adjust accordingly to keep it there.

Thomas302 · Oct 27, 2004

N8Miller · Oct 27, 2004

If you did nothing else but switch to 30s, then you mgiht wanna turn it down a tad.

Grn92LX · Oct 28, 2004

39 psi with the vacuum off is fine.

Michael Yount · Oct 28, 2004

Leave the pressure stock 38-40 psi measured warm with the engine idling, and the vacuum line off the regulator and plugged. It's what the ecu and the recalibrated maf expect.

The computer's adaptive strategies will generally un-do small fuel pressure changes anyway. Within a relatively large range, the computer will adjust pulse width on the injectors in response to what the O2 sensors are telling them. It will do so to keep the a/f within the range of the programming in the ecu. So if you turn up the pressure, it will see a richer mixture, and it will simply reduce the pulse width to bring the mixture back to where IT wants it to be. If you turn it down to lean it out, it will decrease the pulse width. Over time, it will also make adjustments that affect the wide open throttle tables that are in effect even when it's in open loop and ignoring the O2 sensors.

So, a pressure change to affect a/f ratio will only work for a very short period of time before the computer adjusts for it. If you need to make mixture changes that will last - you'll need a chip, twEECer, pms, EEC tuner - etc.

Set it stock.

90Notch · Oct 28, 2004

"So, a pressure change to affect a/f ratio will only work for a very short period of time before the computer adjusts for it."

Not true under WOT, under WOT the EEC simply reads off preset values. This and startup (first few seconds or so) are the only times you'll see a difference changing FP. 12.8:1 is real good place to be with those heads so I say leave it be.

Michael Yount · Oct 28, 2004

90Notch - over time, the computer will adjust those 'preset tables' as well. The only way to benefit from a pressure change is to reset it, and clear the computer by killing the battery. Even then the change is only for as long as it takes the computer to adjust.

autoXr1 · Oct 28, 2004

Michael Yount said:
90Notch - over time, the computer will adjust those 'preset tables' as well. The only way to benefit from a pressure change is to reset it, and clear the computer by killing the battery. Even then the change is only for as long as it takes the computer to adjust.

Agree. I think I heard it takes about 150-200 miles for the computer to pretty much abolish the effects your fuel pressure adjustments. So, unless you plan on having a regular battery disconnect schedule, it's best to actually tune the proper way...EEC recalibration. The more I have learned about these adjustable fuel pressure regulators, the more I realize I may have been "suckered in" and wasted my money. These may be useful at the track for someone who is truly having AFR problems at the drag strip on an untuned engine, but not for much more than that, for all practical purposes. Ford injectors are calibrated for 39.5 psi, so it's best to just leave it there.

Your MAF and EEC calibration should match your injectors and combo. If not, the engine will not run anywhere near it's potential.

5spd GT · Oct 29, 2004

Michael Yount said:
Leave the pressure stock 38-40 psi measured warm with the engine idling, and the vacuum line off the regulator and plugged. It's what the ecu and the recalibrated maf expect.

I've set my fuel pressure by unplugging the vacuum line while it is running/warm and didn't plug the vaccum line while I turned the FP down or up...so I guess I can put my finger over the vaccum line (red) while I turn the FP up or down...how did my previous method effect the readings?

Michael Yount · Oct 29, 2004

It probably didn't affect things very much - yes, just put your finger or a piece of tape over the hose while you're setting the pressure.

5spd GT · Oct 31, 2004

Michael Yount said:
It probably didn't affect things very much - yes, just put your finger or a piece of tape over the hose while you're setting the pressure.

Just for FYI: I went out and "redid" the FP and when I plugged the line with my finger the FP went up 1lb so I backed the pressure back down to 39psi which is what I originally thought it was...

So in short when you plug it with your finger it ups the FP one lb...

Michael Yount · Oct 31, 2004

"So in short when you plug it with your finger it ups the FP one lb... "

The rest of the sentence is "...on my car." Different combos/cams make different amounts of vacuum at idle - the more radical ones need vacuum pumps to run the power brakes -- so it won't have that effect on everyones.

5spd GT · Oct 31, 2004

Michael Yount said:
"So in short when you plug it with your finger it ups the FP one lb... "

The rest of the sentence is "...on my car." Different combos/cams make different amounts of vacuum at idle - the more radical ones need vacuum pumps to run the power brakes -- so it won't have that effect on everyones.

I was almost going to put that down (on my car)...but I figured since I typed it and said I went out and changed it "they" would assume it was on my car

Highbredcloud · Nov 1, 2004

should fuel presure be measured with vacuum on or off? Currently I have 40psi with vacuum on and about 46-48 with vacuum off...is this right? I always mixed this up...lol...

Michael Yount · Nov 1, 2004

No - not right; set the fuel pressure with the engine warm and idling. Pull the vacuum hose off and plug the end of it. Set the pressure to 38-40 psi, then put the hose back on. It will usually drop down into the 32-36 range depending on how much vacuum you have at idle.

5spd GT · Nov 1, 2004

Michael Yount said:
No - not right; set the fuel pressure with the engine warm and idling. Pull the vacuum hose off and plug the end of it. Set the pressure to 38-40 psi, then put the hose back on. It will usually drop down into the 32-36 range depending on how much vacuum you have at idle.

Yep...I would usually mess with it after I pull into the driveway after I got home from college or got done running an errand so it is warm already...and get out and pull the vaccum line and set the pressure and then reconnect the vaccum hose...

Mine reads at 35psi (vaccum connected) and 39psi (w/ vaccum line disconnected)...

Switching from 24LB's to 30LB's

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