Fuel pressure

Stan Weiss · Jun 26, 2009

bergs said:
yes, they are the ford racing green top injectors. so i guess that means that the 39psi that i see at wot is correct?

Stan Weiss said:
NO!

If you have a 42 lbs/hr injector that is rated @ 39 psi and at WOT you have 10 psi boost with 39 psi fuel pressure you have a 29 psi pressure differential and your injectors are flowing a max of 36.22 lbs/hr. The question becomes how much fuel does your engine need?

I calculate that a 4.6l at 6000 RPM with 10 psi boost needs a mass air flow of around 3460 lbs/hr. With an AF of 12.5:1 each injector need to flow 34.6 lbs/hr

(3460 / 12.5 ) / 8 = 34.6

Is the "NO" in my post easier to read now?

Code:

 A/F              lbs/hr needed
 10.5000     41.19
 10.6000     40.802
 10.7000     40.421
 10.8000     40.046
 10.9000     39.679
 11.0000     39.318
 11.1000     38.964
 11.2000     38.616
 11.3000     38.274
 11.4000     37.939
 11.5000     37.609
 11.6000     37.284
 11.7000     36.966
 11.8000     36.653
 11.9000     36.345
 12.0000     36.042
 12.1000     35.744
 12.2000     35.451
 12.3000     35.163
 12.4000     34.879

Let me clarify that the injector lbs/hr is what is needed for that air fuel ratio. Based on what duty cycle you want as your max you than need to calculate injector size. If I want a 11.0:1 AF @ 90% duty cycle 2 WOT then 39.318 / .9 = 43.6867. You would need a 44 lbs/hr injector

blkhorse · Jun 26, 2009

seen the no, but with all your flow data, you did not say what he should see, couldn't tell if you were saying too low or high, he needed an answer for what pressure he should see, not the science of flow

several posted a much better way to get a good idea of what he should see(see my previous post and trinity's) your input seemed to confuse

bergs · Jun 26, 2009

thanks for all of your posts. you guys all rule. i love that you can get on the internet and fellow car guys help eachother. that being said, i talked to my tuner and i need to make an appointment to verify the a/f ratio and such. he said that the fuel pressure should go up over 39 psi in a 1:1 ratio to boost at WOT. so at 10 psi of boost i should see 49 psi of fuel pressure. so we'll see whats up. the a/f was all good when it was tuned, but i had no FP gauge at the time.

GDawg · Jun 26, 2009

Good thread! (subscribed)

bergs · Jul 1, 2009

ok so i took it to the dyno today. it ran pretty much the same numbers as last year. fuel pressure was up over 50 psi the whole time. look on the graph to see the fuel pressure throughout the run. my gauge still only reads 40 psi for some reason, but i know the pressure is more than that and that my air/fuel ratio is safe at 12.0 or less, it was at about 11.8 most of the time. does anyone have the gt supercar pump and 42 lb injectors and an auto meter fuel presure gauge? if so what does it read for you? heres the chart:

Fuel pressure

Stan Weiss

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Fuel pressure

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