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Old 07-14-2007, 12:48 AM   #1
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Smile Lets talk about fuel pressure

Here lately I've been seeing alot of fuel pressure threads so I decided to start a new post and share what little knowledge I have on the subject and for others to post any extra info they may have as well.

In general a stock NA Focus runs just a little over 39psi of fuel pressure so for arguements sake we'll just call it 40psi. When running ANY form of BOOST you need at least 1 "EXTRA" pound of fuel pressure per pound of boost you are running.

For example if you run 10psi of boost you "should" run 50psi of fuel.

The SVT.....

After playing around with Egbertnr's SVT and making SEVERAL datalogs I noticed something that was happening on his that happened on mine. When under boost on a WOT run once I hit 4500rpm the fuel pressure would start dropping off at an alarming rate and sometimes it would reach dangerously low levels... I'm talking about "blow the motor" levels.

After a few runs I decided to log the fuel pump's output and I found something that everyone should know. When running ONLY 10psi the fuel demand is so high that once you get around 4500rpms the pressure falls off fast... WHY??? The fuel pump is at 100% capacity ( when NOT running a Boost A Pump) here is a screen shot of what I saw....



Yellow is fuel pump output, Black is fuel pressure, and Red is RPM..... Interesting isnt it.

Once we figured out that the fuel pressure wouldn't hold, I decided to gowith a BAP, and it helped some... not alot but it was better than what I had.

A few months later I was playing around with some of the fuel settings in the ECU and "HOLY SHIT" I had fuel pressure and ALOT of it.....



As you can see with the BAP and the proper settings in the ECU you can get GREAT fuel pressure... at 19psi of boost I need 59psi of fuel, with the new settings you can see I was holding 62psi with ease.... NO aftermarket fuel pump, fuel rail, lines or anything else... all STOCK components... except the Boost a Pump.

And I want to add that at 360whp the fuel was starting to fall off... but I feel anything less than 350whp should be fine.

The Zetec..... Well there aren't as many good things to say about the Zetec. Even with the same pump as the SVT and with a BAP you are still limited to the amount of fuel pressure you can obtain on the stock stuff. For some reason Tom cant get the tuning that we figured out for the SVT to work on the regular Zetec and I dont have anyone close that has FI to experiment on.

There is still the possibility of using a return style fuel system and I'm sure that someone that has done it will chime in sooner or later.

I hope this has helped to shed a little light on the "Fuel Issues" that it seems like is plagueing alot of the FI guys out there.

John

EDIT: The correct datalog PID I used was the fuel pump duty cycle... I couldn't remember it last night.


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Old 07-14-2007, 09:46 AM   #2
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thanks john.

to bad i dont live closer. i would be glad to let tom experiment with my zetec. but since i dont i guess im going return.

john do you not have any problem with the fuel not getting to the pump under hard acceleration?
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Old 07-14-2007, 10:05 AM   #3
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No problems at all... at least thats what the datalog implies... and my independant FRP gauge.
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Old 07-14-2007, 10:11 AM   #4
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he can experiement with mine! i live in nc!!
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Old 07-14-2007, 10:13 AM   #5
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he can experiement with mine! i live in nc!!
But I'm in Ohio.....
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Old 07-14-2007, 11:03 AM   #6
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i think he means tom, lol.

do you know what keeps the svt from loosing fuel to the pump under hard acceleration. is the pump at a different part of the tank or something?
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Old 07-14-2007, 12:00 PM   #7
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would one of those fuel pressure regulators with the boost sensative things make a difference? i know that they're "supposed" to add 1 psi of pressure for every psi of bosst... so couldn't someone try to place one in the fuel line right before the rail where that ugly stock ufo disc looking thing is and hook up the vacuum line, add a gauge, and see if it works? granted you would need to change all the BAP and tune stuff to make sure the FPR was doing it and not all the other components set up. i could be completely wrong and not have the right idea about those things, because im trying to learn from you guys about this stuff
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Old 07-14-2007, 12:18 PM   #8
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a fuel pressure regulator is for a return style fuel system. the focus has a dead head system with a variable fuel pump (not return style). a fuel pressure regulator wont work unless you convert to a return style system. then yes that is what you want.
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Old 07-15-2007, 09:15 AM   #9
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The rising rate regulator is needed for the return style system, personally I dont think it would help on the stock system.. if it would even work right at all.
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Old 07-16-2007, 11:44 AM   #10
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In the older big thread around about fuel pressure Tom talked about the fuel pump. If you datalog fuel duty cycle you will notice that there are 2 ways to log it; one way goes to 100% and the other way goes to about 60%. Looks like the one that hits 100% is an ajusted value.

Tom said if you measure volts going through the ground it only is 6-7 volts when the input wire is 12V at max duty cycle at the fuel pump. Aparantly the ECU controls the duty cycle through the ground somehow. Tom said he "supergrounded" the wire and got 60 PSI on a stock fuel pump. That is all I can really remember does anyone have any ideas on how to proceed. It would be great if a BAP plus rewiring the fuel pump gives us the necessary pressure. I want to keep the stock fuel pump/tank therefor I dont want to do a return setup. Thoughts?


http://www.focusfanatics.com/forum/showthread.php?t=119193

"So i hooked the BAP up with the ground wire grounded and it fell to 54.6PSI and stayed there did 2 or 3 runs to back this up and she held perfect at 54,55PSI each run , this told me 2 things , 1) that the fuel pump , lines , filter , fuel rail were all large enough to do the job , 2) That it is a ECU controle meaning in the Diablo/SCT software there not giving us enough fuel pump controles or a design problem that cannot be controled with the ECU because the ground wire never goes above 7 volts at 100% pump dutycycle and fuel PSI at 23psi and it needs to go to 12 or 13 volts (full ground) ground "
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