Increasing Fuel Pressure

[Nicks89]

Im a new member to the form so i might as well tell a little about my bronco before i go on. I have an 89 bronco ii 5 speed that just turned over 148,000. I swapped out the original fm146 for a m5od after 5th gear exploded. While i had the transmission out i decided to pull the motor and replace all the gaskets, new crank and rod bearings, bought a stock cam for it, ported the heads, ported the upper and lower intake manifold, put a 58mm throttle body on, 19lb fuel injectors,headers, and converted it to mass air. I can post pictures of how i mounted the mass air meter, it looks like it rolled out of the factory with this set up.

Back to my question. Does anybody know of any way to bump up the fuel pressure a little bit? the mass air works great, but Id like to get it just a little more fuel. I haven't seen any aftermarket adjustable fuel pressure regulators, and id rather not send out my mass air meter to get recalibrated.

 

[RonD]

Welcome to TRS

Reads like a nice build, good work

The computer controls the pulse width of the fuel injectors, by using the Air Flow data and feed back from the O2 sensor(s).

So it can compensate for fuel pressure changes over time, most systems lose pressure as they get older, lol.
Computer just adds a few milliseconds to open time to get O2 sensor back to sweet spot.

If you bump up fuel pressure(or use larger injectors) computer will just shave off a few milliseconds of open time.
And you can bump up fuel pressure just by pulling off the Fuel Pressure Regulators vacuum hose, and plug up the hose of course.
That should add a few more pounds of pressure.


The MAF sensor is what the computer bases 0 fuel trim(open time/dwell/pulse width) on.
Computer calculates 14.7:1 air:fuel ratio and that becomes the floating 0 of short term fuel trim(STFT) that changes with RPM/air flow
If O2 sensor shows too much oxygen in the exhaust(lean burn) then computer will go to +1 STFT, add milliseconds to open time
Too little oxygen(rich burn) then computer will go -1 STFT, subtract milliseconds from open time.

If STFT gets to +20 or -20 computer will set a Lean or Rich code and turn on the CEL(check engine light)

If you disconnect O2 sensor computer will run engine slightly Richer since that is safer than Lean

There were EFIE devices made to highjack O2 sensor signals so you could run engine Leaner or Richer.
Never tried one so................


Basically the computer runs the show so higher fuel pressure won't mean more fuel flowing into the engine, just less open time for the fuel injectors.

 

[Nicks89]

Thanks for the info! I have a wideband o2 sensor hooked up, and it reads anywhere from 14-15.7, even at part throttle. I know 14.7 is the "perfect" mixture, but that seems a bit too lean at part throttle. Im worried about it pinging if its under a load, and i think it would run better if i could give it a little more fuel. Full throttle it drops to anywhere from 12-12.5 which is good for me. I'll hook up a pressure gauge and see what it is with the vacuum line hooked up vs unhooked and see if it makes a difference. Thanks again for the info

 

[adsm08]

No, 14-15.7 sounds just about right for part throttle or even idle. That puts 14.7 almost exactly in the middle of the range, and because the O2 is trying to adjust after the combustion even already happened things will never be perfect.

It drops to 12ish at WOT because the computer starts to ignore everything and just dump fuel. Running a gas engine at 12 all the time will probably foul plugs eventually. At least it is going to trash your HC emissions.

Taking the vac line off the FPR is going to max the pressure and make it run rich all the time.

 

[Blown]

I tune

The computer does trim fuel in Closed Loop for the best mileage and emissions to stoiciometric 14.7:1 It is relying on the O2's to do so, so you can't fool the computer. The Stoic 14.7:1 is for the cleanest burn, not the most power. The sooner you mash the gas pedal down the sooner it goes into Open Loop Fuel to meet the required fuel for the load on the engine.

However In open loop which includes WOT there is no trim. The computer will send what it takes to get a specific fuel ratio, say 13:1 at 2/3rds throttle. You can fool the computer in Open Loop with higher fuel pressure because the EEC is sending a preset amount of fuel. Increasing fule pressure will richen the mix across the whole fuel curve in Open Loop, not the best if you need more fuel at lower rpm in a boosted application becasue if you get it right at 2000rpm then it will likely be way rich at 5000rpm.

Now the best way to tune is not through increasing pressure but in actually adjusting fuel mix yourself for a given Throttle position and RPM. Increasing Fuel pressure increases the mix in a linear fashion not a curve which is better. Here is your Open Loop Fuel Table: (You can change the value in the cells with a tuner and fine tuning can be done in the MAF Transfer Function)

Function for when It goes into Open Loop, above the line is Open Loop Below it is Closed Loop Fuel:

Check-out Tweecer.com for the tuner I use.

 

[RonD]

Good info Blown

I had a question for Open Loop
Will the computer bias Open Loop tables using Long Term Fuel trims(LTFT)
My understanding was the LTFT was used to offset small air leaks and pressure drops which occur as engine system gets older?

 

[Blown]

Good info Blown

I had a question for Open Loop
Will the computer bias Open Loop tables using Long Term Fuel trims(LTFT)
My understanding was the LTFT was used to offset small air leaks and pressure drops which occur as engine system gets older?

Fuel trim in Closed Loop, No Fuel trim in open loop, it is running mostly off of what is set in the Open Loop Fuel table and MAF Transfer Function. This is why it is very important to utilize and monitor a fuel pressure gauge in a boosted application. If fuel pressure drops, it goes lean and goes boom! There is fine tuning done by the EEC utilizing Barometric Pressure, Engine Coolant Temperature, and Air intake Temp in Open Loop and Closed Loop.

I tried an FMU in my first build without a tune and it was lean at low throttle with high load, it predetonated and took out a head gasket. The MAF Transfer Function is a curve, not linear like boosting fuel pressure.
MAF Transfer:

And for shits an giggles, stock Timing:

 

[RonD]

So LTFT is strictly a diagnostic tool, not part of the adaptive software used in Open or Closed Loop, intesting.

 

[Blown]

So LTFT is strictly a diagnostic tool, not part of the adaptive software used in Open or Closed Loop, intesting.

LTFT is used in Closed Loop, you can monitor or datalog them with the Tweecer software. It may also be used in newer vehicles with Open Loop, I don't know.

 

[RonD]

OK, makes more sense.
Relearning an engines air leaks and fuel pressure issues on each startup of the computer didn't make sense to me.
Especially if LTFT is available