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Forum » Off Topic Discussion » Fuel pressure adjustment to change AFR at WOT

Fuel pressure adjustment to change AFR at WOT

Off Topic Discussion

If it's not really about tuning or wiring. Then it belongs in here.


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I have a 4AGZE 97ci supercharged engine with stock 34.8lb/hr injectors with a stock fuel pressure of 36psi. I have installed an AEM UEGO wideband AFR gauge and adjustable fuel pressure regulator. My goal is to find out what pressure to set my fuel pressure regulator at to achieve 12.5:1 AFR (theoretically ideal AFR for forced induction power) @ 6400RPM on a stock ECU.
If I punch the numbers in: 97ci x (6400/2) /1728 = 180cfm, 180x.076 = 13.68/4 = 3.42lb/min mass flow per cyl
3.42lb/min / 12.5 AFR = .274lb/min or 16.44lb/hr mass fuel per injector to reach 12.5:1 AFR at 6400rpm
So I would have to decrease the 36psi fuel pressure to drop the 34.8lb/hr to 16.44lb/hr?
A fuel pressure of 8psi if punched into a fuel flow after pressure change formula: (square root of (8psi/36psi))x34.8lb/hr equals out to 16.32lb/hr which just seems ridiculously low pressure so I'm obviously doing something wrong or I'm using these equations all wrong... If anyone has any advice, would be appreciated. I am not good at math and no longer confident in my understanding of EFI.

Hey Gerken, From what I can gather you are trying to calculate Mass air flow at 6400rpm in the first equations, But the rest doesnt make sense me... I dont know the answer for you But I can tell you that your well off the mark....

You would need to know what AFR the car is running at now to be able to work out how much extra pressure you need to reach your desired fuel pressure.

But just raising your fuel pressure is certainly not the ideal way to go about it... If you do not tune to suit it will make the whole tune rich/lean (depending what way you need to go from the stock AFR at full noise) If your adding boost to a normally un-boosted setup you can sometimes get away with a rising rate fuel pressure reg but its still a bandaid fix and timing would not suit either.

My car does have adjustable ignition timing via an adjustable cam angle sensor and timing light. I understand that manually adjusting this way will adjust for the whole tune, I'm just trying to see what I can do with what I have before I choose my ECU and start tuning on the computer.
The equations I was using was to find the mass of fuel per cylinder required to hit 12.5:1 air to fuel. so I divided mass air per cylinder over 4 injectors to get 3.42lb/min and divided that by 12.5 to get .274lb/min which i converted to 16.44lb/hr fuel flow to reach 12.5:1 AFR at 6400rpm.
In a month I will try and do a few "ramp runs" and see what my current AFR is at 6400rpm and 36psi and try and figure out the proper way to figure out the fuel pressure adjustment.