Fueling & Injection - base fuel pressure?

i have a 2000 firebird built ls3, 225 afr heads, magnafuel 4303 pump,twin
70mm turbo's what should my base fuel pressure be???

4bar...~58.8 PSI

i have a 3bar

I'm running 3 bar fuel pressure in my TT c5. I'm running an aftermarket ECU though

your base fuel pressure can be whatever you want it to be....

the standard for injectors I believe is 43.5psi or 3 bar
some have/list ratings at 58psi, 4 bar.
Its all relative.

Being you are boosted, do you have a Fuel pressure regulator? What size injectors do you have? have you ever logged duty cycle or has the car not run yet?

It all depends on your setup and what you are doing with it. Keep in mind however, that as you increase boost, this pressure is pushing back on the injector delivered fuel pressure, that is why people go Fuel pressure regulator.

your base fuel pressure can be whatever you want it to be....

the standard for injectors I believe is 43.5psi or 3 bar
some have/list ratings at 58psi, 4 bar.
Its all relative.

Being you are boosted, do you have a Fuel pressure regulator? What size injectors do you have? have you ever logged duty cycle or has the car not run yet?

It all depends on your setup and what you are doing with it. Keep in mind however, that as you increase boost, this pressure is pushing back on the injector delivered fuel pressure, that is why people go Fuel pressure regulator.

i have a fpr, 120lbs, e85, duty cycle was 85% on 14lbs of boost,my base fuel pressure is 41

what is the problem with your current 41psi. is your regulator boost referenced?

i was told to do this with my set up.

set base to 43.5 vac line off, then at 14 psi boost you will have 58.5 psi,

i think if you change your base psi you will have to re tune your car.

what is the problem with your current 41psi. is your regulator boost referenced?
i was told to do this with my set up.
set base to 43.5 vac line off, then at 14 psi boost you will have 58.5 psi,
i think if you change your base psi you will have to re tune your car.

I think you have misunderstood how & why people do the FPR.
If base is set to 43.5psi, that means with no boost, he will be pushing fuel into the engine at 43.5psi. When you go to 1 PSI of boost WITHOUT a FPR, that becomes 42.5 psi that he is pushing fuel into the engine with.




you will have to re-tune, as you will be changing the Injector flow rate essentially.

you can squeeze some more out of those injectors if you bump up the pressure, you should be able to scale your IFR according to whatever pressure you adjust to. and then likely lower your offset vs volts vs vac table in hptuners or the similar table in efi live.

Also, likely your MAX FPR pressure can go to is 75psi...
so 41 + 14psi boost = 55psi, so you have plenty left over.


If you up your base pressure to either 43.5psi (57.5psi at FPR at full boost)

or 58psi (72psi at FPR at full boost) as long as your injectors don't give you funny issues and you can change the tables accordingly, which should not be a problem at all, scale your IFR and your offset vs volts vs Vac table and you should be fine. Did you have injector data from the manufacturer for either of these tables? With your Injector flow rate table, it will stay constant, as you are holding fuel pressure constant across the board.

if you adjust your 41psi to 43.5psi BASE, then your Duty cycle will drop to about 82%. ( Sqrt (41/43.5)) = 0.97 * 85 = 82
Your 120LB/hr injectors are likely rated at 43.5psi so depending on your display in hptuners, on one setting you will have 120 across the board.

It simply scales from there if you change your base pressure
for instance if you go to 58psi on these 120 LB injectors
= 120 LB/hr * ( sqrt ( 58psi/43.5psi) ) = 138.56 LB/hr injectors at 58psi

if you need some help with understanding all that, PM me.

I think you have misunderstood how & why people do the FPR.
If base is set to 43.5psi, that means with no boost, he will be pushing fuel into the engine at 43.5psi. When you go to 1 PSI of boost WITHOUT a FPR, that becomes 42.5 psi that he is pushing fuel into the engine with.



Actually I told him to do it..... Madmax is running dual GSL392's. They do not like big pressure.

You not only have to boost reference for pressure compensation, but you also have to do so within the limits of the pump. If you try 58# base pressure & boost reference an additional 20#, you need 78psi. Most pumps will not be happy at this pressure or the flow will drop off significantly when you need it most (at high boost). The best way is to choose an injector that will support your power level while maintaining a max boost referenced pressure of under 65-70psi.

yep... forgot about that. good catch.

what is the problem with your current 41psi. is your regulator boost referenced?

i was told to do this with my set up.

set base to 43.5 vac line off, then at 14 psi boost you will have 58.5 psi,

i think if you change your base psi you will have to re tune your car.

i guess i have no issue just wondering if i bump my fuel pressure up from 41 to 48 would my inj duty cycle down it's at 86% at 14lbs of boost

Actually I told him to do it..... Madmax is running dual GSL392's. They do not like big pressure.

You not only have to boost reference for pressure compensation, but you also have to do so within the limits of the pump. If you try 58# base pressure & boost reference an additional 20#, you need 78psi. Most pumps will not be happy at this pressure or the flow will drop off significantly when you need it most (at high boost). The best way is to choose an injector that will support your power level while maintaining a max boost referenced pressure of under 65-70psi.

the injectors I had were rated at 4bar, but im getting the impression thats an oddball number? probably to inflate the injector size or something....

is it odd to see them rated at 4 bar?

i guess i have no issue just wondering if i bump my fuel pressure up from 41 to 48 would my inj duty cycle down it's at 86% at 14lbs of boost

Yes, your IDCs should go down if you up the pressure as the injector will be flowing more fuel with the same open time.

If you up the pressure and the IDCs don't come down, you might be reaching the edge of what your fuel pump can support.

As fuel pressure increases, injectors flow MORE and fuel pumps flow LESS.

Yes, it is more of a standard practice to rate them at 3 bar. 4 bar pressure makes them flow approx 15% more. Sort of a confusing way to make you think you have bigger injectors, when compared to others.

Other than using tuning constants, I refer to the 3 bar pressures.

Set/measure rail pressure with FPR reference hose temporarily removed (i.e. FPR exposed to BARO)... this measurement is the base pressure;

then use this base pressure to calculate the IFR table using the squareroot formula (and populate the same value in all cells horizontal/flat across).