Is my fuel pressure math correct?

[SlickPyro]

I read topics relating to fuel pressure but I just want to make sure I am right. I just got my dsm and it is running real rich, I know this from all the black crud on my bumper above the tailpipe, I am getting ready to install an aeromotive afpr and the fuel pump is a walbro 255lph. I purchased this kit:
Import Evolution

To my knowledge the fuel injectors are stock. Lets see if my math is correct:

At 0 pounds of boost and no vacuum in the intake manifold fuel pressure should be the base fuel pressure for a 1g, 38psi with the vacuum line off (in fact all readings should be taken with the vacuum line off if I am correct). When adding boost you must add 1psi to the fuel pressure for every pound of boost. So when boosting at 10 psi my car should be running 48psi of fuel pressure. If I wanted to add more boost later I would add 1 more psi of fuel pressure for every pound of boost. So say later I wanted to bump up to 15psi on the bored out 14b in the laser the fuel pressure at 15lbs of boost would be 53psi. Also, when idling, there is 20 in. Hg in the intake manifold. At that time the fuel pressure should be 38-20 which is 18psi.

Thanks,
Brian

 

[toojung2die]

Set your base fuel pressure to 37 psi with the vacuum line to the AFPR disconnected and capped. You don't have to adjust anything else, The AFPR will increase fuel pressure in a 1:1 ratio with boost. The baseline fuel pressure never changes (unless you are using fuel pressure to tune and that's not always a good idea but it's an option).

 

[SlickPyro]

When the car is idling? set it to 37psi? I am not sure what "base" means so I don't know when to adjust it.

 

[snowborder714]

Your base fuel pressure (at idle, with vacuum line disconnected) should be set to:
1G auto - Spec 42.7psi (Standard value 41 - 46 psi)
1G manual - Spec 36.3psi (Standard value 36 - 38 psi)

When adding boost, the AFPR will add 1 psi of fuel pressure for every lb of boost. So if you're boosting to 20 psi, at peak boost, your fuel pressure will be 37 (base fuel pressure, assuming manual tranny) + 20 = 57 psi. You do not need to adjust anything for this to happen. The AFPR you purchased will do this for you. So the only thing you have to do is set your base fuel pressure and that's it. Double check it every once in a while and adjust as necessary.

As for fuel pressure at idle with the vacuum line connected, your fuel pressure will drop 1 psi for every 2 inHg of vacuum. For example, if your idle vacuum is 20 inHg (as you stated), your fuel pressure will drop 10 psi to 27 psi (again, assuming manual tranny) once you re-attach the vacuum line.

 

[toojung2die]

Yes, with the car idling and the vacuum line disconnected the AFPR your fuel pressure should be 37 psi. Where do you read 38 psi?. It will drop a few psi when you reconnect the vacuum line. You don't have to worry about changing anything else when adding more boost. As long as the fuel pump can keep up the AFPR will increase fuel pressure as boost increases. You won't have any worries about the 255 pump keeping up.

 

[SlickPyro]

Yup it is a manual trans. And I'm not sure where I got 38 from. Prob. from 36-38psi on another thread. I will set it to 37. Thanks for the help guys.

 

[Razor_nl]

I don't mean to hijack your post but it looks like your question is answered, so I think it's ok to post my related question here.
I have a '96 2G manual, JDM 6-bolt engine+ecu, big16g stock boost, stock 510's, walbro 190, Aeromotive A1000 fpr (bigger brother of the one above but basically the same with the 1:1 ratio)

Now I'm wondering... should I go with the 2G base pressure (which is about 40-42 if im right) or the 1G (36-38) ?
I think I have to go with the 1G since basically I have a 1G engine and ecu which controls the fuel flow. Or is there some reason where I can't think of that makes me needing to go with the 2G pressure ?

I will adjust it with a wideband and logger and such in a month or 2-3 but I don't have the option to do it before this time, so I would like to know what I should go with till that time.

 

[romeen]

You're correct, which base fuel pressure to use is determined by which ECU you are using, not by which generation of motor it is. If you will be using a 1G ECU for fuel control then you should set the base fuel pressure to the 1G level of 37psi.

I have a 6 bolt motor in my 2G using the stock 2G ECU. Therefore I am using the 2G base fuel pressure of 43.5psi.

 

[Razor_nl]

I'm using the ecu that came with the engine because it's an engine with a cyclone intake and the ecu needs to control that.
So then I'll go with 37 psi base pressure.
Thanks!