Fuel pressure formula ???

[Muzzy]

is there a formula i can use to calculate the amount of fuel i will be pushing if i get an adjustable fuel pressure regulator. i plan to get a fuel pressure regulator to push my 370s on my SR a little more.

is that a dumb idea?? i dont plan to get too much more out of them. like 400cc+ if possible. im dont think im ready 4 550ccs.

$450 for a set of injectors or $125 for a regulator. if that helps.
....oooorr should i jus go to the yard and source up some RX-7(550s) injectors??

[Muzzy]

stop holdin out on me bitches

[Kid Zelda]

Ok, I will help


Check this.

If you had 370cc injector tested at 43 psi and now tested at 50 psi, it would go like this.


50
----- = 1.1627 * 50 is the new fuel pressre, 43 was the old fuel pressure
43

Now find the SQ root of 1.1627 = 1.0763

Now multiply the old flow rate by sq root

370cc X 1.0763 = 398CC is the new rate @ 50 psi

you're welcome

[Muzzy]

tank ye very much sir

[89s132b]

Math sucks, just use this. You will have to divide your 370cc/min by 10.5 to convert to lbs/hr but that is it...

http://www.csgnetwork.com/fiflowcalc.html

[Muzzy]

lol kidzelda's way is easier...to me...no conversions or anything....stright foward.
this should go FAQ i think or reference or somethin. we need a formula FAQ..sound good??

[fliprayzin240sx]

So whats the most PSI you can push out of the 370?

[NismoSilvia270R]

So whats the most PSI you can push out of the 370?

i wonder this too. i dont know too much bout injectors. want to know little more.(searching as i type in another window)

and how does duty cycle affect, or is affected by, changing fuel pressure with a fpr?

[Mikel]

i wonder this too. i dont know too much bout injectors. want to know little more.(searching as i type in another window)

and how does duty cycle affect, or is affected by, changing fuel pressure with a fpr?
Duty cycle won't be affected, the ECU doesn't know you've bumped the pressure, you just get more fuel for a given duty cycle.

Kinda a moot point anyway, if you're bumping the fuel pressure to push the injectors a little further, they're probably running static anyway.

[NismoSilvia270R]

i was thinkin that whole thing was sketchy..

[Muzzy]

come on come on!!! more info. im not the only one with questions about this idea.

my other question is...wont i run super rich at idle without an SAFC if u up the fuel pressure?????

[89s132b]

The SAFC cannot adjust fuel when your ECU is in closed loop (i.e. while idling)
Your O2 sensor feedback will maintain your a/f ratios regardless of your fuel pressure and SAFC setings. But still you don't want to pump it up too much

Rule of thumb is don't run more than 60 psi. It takes 4 times the fuel pressure to double fuel flow so it doesn't help as much as you would think anyways. Raising the pressure to extreme levels is very hard on the pump and can lead to leaks or failures in the plumbing and injectors themselves. Plus it changes your injector latency so they won't be firing at quite the right (optimum) time.

It is much better to buy the right sized injectors.

hope this helps

[Jeff240sx]

The injectors aren't affected nor limited by an increase in fuel pressure. The pump and regulator are. Running a lot of pressure on a pump is a sure way to kill it. Also, around 60psi, I wonder if the 255L/hr walbro can keep up. It's flow rate drops alot after 60psi. So much so that a slight hiccup and it may cost you a motor.
And the AFC doesn't raise fuel pressure. It changes the MAF voltage to trick the ECU into thinking more air is flowing into the motor than there really is.
-Jeff

[89s132b]

If your REALLY want to increase the fuel pressure anyway, I'd suggest the 255lph "high pressure" version. It will withstand the higher pressure better than the regular 255lph pump. The biggest strain will be on the wiring (see graph). The stock wiring sucks. It would be a really good idea to run a low gauge wire idrectly from your battery to the pump, then relay it with the stock wiring. I did this to my car and a lot of the high HP guys (not me - LOL) do it too.

Regular 255 pump
http://www.autoperformanceengineering.com/pics/255lph.jpg


High Pressure 255 pump
http://www.autoperformanceengineering.com/pics/hp255lph.jpg

[Muzzy]

i know it doesnt change the fuel pressure...but doesnt the safc control injectors also??? because it will be pushin more fuel then normal all the time even at idle so wouldnt it cause it to be rich at idle????

yea im runnin the walbro 255 but not the hi-pressure one.
i can skate on 50psi dont u think??

[89s132b]

i know it doesnt change the fuel pressure...but doesnt the safc control injectors also??? because it will be pushin more fuel then normal all the time even at idle so wouldnt it cause it to be rich at idle????


No, you won't run rich. Your O2 sensor will sense your afr's and adjust your fuel accordingly.


yea im runnin the walbro 255 but not the hi-pressure one.
i can skate on 50psi dont u think??

Yes, but if you have the 255 and stock fuel rail you are running about 50 psi already. I'd go 60psi if I were you.

[orion::S14]

...but doesnt the safc control injectors also???

No...SAFC modifies the MAF signal to trick the ECU into spraying less fuel...SACF does not connect to or directly control injectors.

FYI...

- Brian

[Mikel]

Two things:

The Walbro 255lph High Pressure will keep up with some pretty high fuel pressures. I was running 43psi static and 1:1 rising rate indexed to boost on my GN. This translates to about 65psi at full boost. I believe it's part number GSS340

If you get a good AFPR, you can adjust point of onset so that your idle fuel pressure is the same as stock, and fuel pressure doesn't begin to rise untill you see positive manifold pressure. A good example of this is Cartech's regulator.

[j6k]

here's a formula, i think it's from GRM or some engineering book i have...

New flow = Old flow x (New pressure/Old pressure)^1/2