Welcome to the North American Subaru Impreza Owners Club Tuesday March 3, 2015
Home Forums WikiNASIOC Products Store Modifications Upgrade Garage
NASIOC
Here you can view your subscribed threads, work with private messages and edit your profile and preferences Home Registration is free! Visit the NASIOC Store NASIOC Rules Search Find other members Frequently Asked Questions Calendar Archive NASIOC Upgrade Garage Logout
Go Back   NASIOC > NASIOC Technical > Factory 2.0L Turbo Powertrain (EJ Series Factory 2.0L Turbo)

Welcome to NASIOC - The world's largest online community for Subaru enthusiasts!
Welcome to the NASIOC.com Subaru forum.

You are currently viewing our forum as a guest, which gives you limited access to view most discussions and access our other features. By joining our community, free of charge, you will have access to post topics, communicate privately with other members (PM), respond to polls, upload content and access many other special features. Registration is free, fast and simple, so please join our community today!

If you have any problems with the registration process or your account login, please contact us.
* Registered users of the site do not see these ads.
Reply
 
Thread Tools Display Modes
Old 01-20-2003, 04:48 AM   #1
M3sum
Scooby Specialist
 
Member#: 18226
Join Date: Apr 2002
Location: Bellevue, WA
Vehicle:
2001 M3
Laguna Seca Blue

Default How does our fuel system work?

I got some various questions here. I tried searching but didn't get any specific answers

How do you calculate injector duty cycle in relation to the wrx system. the injectors are ~420cc/min, pump is 130lph, but how do you calculate the cycle? I assume we gotta factor in the boost too.

Also, how much psi of fuel pressure does our system push PER psi of boost? A friend of mine is using a vortech 8:1 on his miata so he's getting 8psi of fuel per psi of boost so i am wondering what ours is on the stock FPR. Thanks.
* Registered users of the site do not see these ads.
M3sum is offline   Reply With Quote
Old 01-20-2003, 05:42 AM   #2
Austin
Scooby Guru
 
Member#: 7089
Join Date: May 2001
Chapter/Region: NWIC
Location: Western Wa
Default Re: How does our fuel system work?

Quote:
Originally posted by M3sum
How do you calculate injector duty cycle in relation to the wrx system.
Injector duty cycle is the percentage of time the injector is actually open compared to the maximum time available to be open.

If you know the time the injector is open during a given cycle (injector on time), you simply divide that number by the maximum injector on time.

Injector on time is based on load determination and determined by the computer's fuel delivery equation.

Maximum injector on time is based on motor RPM.
Austin is offline   Reply With Quote
Old 01-20-2003, 05:59 AM   #3
Austin
Scooby Guru
 
Member#: 7089
Join Date: May 2001
Chapter/Region: NWIC
Location: Western Wa
Default Re: How does our fuel system work?

Quote:
Originally posted by M3sum
Also, how much psi of fuel pressure does our system push PER psi of boost? A friend of mine is using a vortech 8:1 on his miata so he's getting 8psi of fuel per psi of boost so i am wondering what ours is on the stock FPR. Thanks.
Your fuel pressure regulator sets a static fuel pressure of 43.5psi. 43.5psi is 3 bar.

Static means referenced to atmosphere. The FPR is referenced to manifold pressure/vacuum. At idle, when there is 15-18-ish inches of vacuum in the intake manifold, fuel pressure is lowered by that much.

The stock FPR is a 1:1 rate.

29.92 inches = 14.7psi = 1 atm. 1 bar = 14.5psi, so it's almost a 1:1 conversion.

At idle, your fuel pressure should be 35-38psi.

Under boost, fuel pressure is more than 3 bar. Boost pressure + 3 bar = fuel pressure.

If you're at 14.5psi boost (which is 1 bar boost), then your fuel pressure should be 4 bar.

Injectors are rated at 3 bar. The stock 420cc injectors flow that much at 3 bar fuel pressure.

With a 1:1 regulator, the differential pressure across the injector (fuel rail pressure - manifold pressure) remains at a constant 3 bar. Because of that, the only variable to determine fuel delivery is injector on time.
Austin is offline   Reply With Quote
Old 01-20-2003, 06:43 AM   #4
hotrod
Scooby Specialist
 
Member#: 14141
Join Date: Jan 2002
Location: 13.239@102.85 @ 5800 ft on 13T
Vehicle:
2002 Impreza WRX

Default Another bit of info

Austin has already done an excellent job of covering most of your questions, but here is a thread that you might want to look at for some additional information on the subject.


http://forums.nasioc.com/forums/show...&highlight=IDC

By the way the injector dead time (IDT) for our High impedence injectors is 1.3 - 1.4 ms according to the folks at RC engineering.


Larry
hotrod is offline   Reply With Quote
Old 01-20-2003, 07:08 AM   #5
M3sum
Scooby Specialist
 
Member#: 18226
Join Date: Apr 2002
Location: Bellevue, WA
Vehicle:
2001 M3
Laguna Seca Blue

Default Re: Re: How does our fuel system work?

Quote:
Originally posted by Austin

If you're at 14.5psi boost (which is 1 bar boost), then your fuel pressure should be 4 bar.

Injectors are rated at 3 bar. The stock 420cc injectors flow that much at 3 bar fuel pressure.

Cool thanks for all that info Austin!
I guess what I am still confused is what you said here. Do you mean that at 3 bar fuel pressure, the injectors are flowing 420cc?? What happens at boost and 4 bar fuel pressure? How can it flow more than its rated? Or maybe im just not getting what you are saying......
M3sum is offline   Reply With Quote
Old 01-20-2003, 07:17 AM   #6
Austin
Scooby Guru
 
Member#: 7089
Join Date: May 2001
Chapter/Region: NWIC
Location: Western Wa
Default Re: Re: Re: How does our fuel system work?

Quote:
Originally posted by M3sum


Cool thanks for all that info Austin!
I guess what I am still confused is what you said here. Do you mean that at 3 bar fuel pressure, the injectors are flowing 420cc?? What happens at boost and 4 bar fuel pressure? How can it flow more than its rated? Or maybe im just not getting what you are saying......
At 3 bar differential pressure, a 420cc injector will flow 420 cc's of fuel per minute. Injector on times are measured in milliseconds. A 420cc injector will flow 7 cc's per second, or .007cc's per millisecond.

Your boost guage shows you pressure inside the intake manifold. The spray end of the injector is subjected to intake manifold pressure. If intake manifold pressure is 14.5psi (1 bar) and fuel pressure is 58psi (4 bar), then there's 3 bar differential pressure across the injectors. If intake manifold pressure is 0psi, then fuel pressure is 3 bar, and d/p across the injectors is 3 bar. If intake manifold pressure is -8psi (about 17 inches of vacuum), then fuel pressure is 35.5psi, so there's still a 3 bar d/p across the injector.

That 3 bar differential pressure remains constant so that, when the computer wants to deliver a certain amount of fuel, all it needs to do is vary the injector on time.
Austin is offline   Reply With Quote
Old 01-21-2003, 01:33 AM   #7
M3sum
Scooby Specialist
 
Member#: 18226
Join Date: Apr 2002
Location: Bellevue, WA
Vehicle:
2001 M3
Laguna Seca Blue

Default

Damn Austin, you know too much, but thanks for your info!

Anyways I *think* I get it now. Previously I had the misconception that the 420cc/min just means the max the injectors will flow but say at idle it will be doing 100cc/min. I guess the injector flow stays constant but the duty cycle varies.

So to rehash, basically what you are saying is that it will flow 420cc/min at any point that its on, the only difference being that under light load, it will have less injector on time, and at heavier loads, it will have more injector on time which than equates to higher duty cycle.
So with the STi 565cc/min injectors, since its squirting more when "on", it doesn't have to be "on" as often....

Correct me if I am wrong.
M3sum is offline   Reply With Quote
Old 01-21-2003, 02:29 AM   #8
Austin
Scooby Guru
 
Member#: 7089
Join Date: May 2001
Chapter/Region: NWIC
Location: Western Wa
Default

Quote:
Originally posted by M3sum
it will flow 420cc/min at any point that its on, the only difference being that under light load, it will have less injector on time, and at heavier loads, it will have more injector on time which than equates to higher duty cycle.
You're exactly correct.

With my TEC I see/log my duty cycles all the time. At idle, I'm usually at 1 or 2% duty cycle. During cruise, I'm usually <15% duty cycle. Only under high boost (about 20psi) at high rpms do I approach 90 or 100% duty cycle with my 650cc injectors.

Keep in mind that, for the same injector on time, duty cycle at 3500 rpm will be half what it is at 7000 rpm. That's because at slower engine speeds, there is more time available for injectors to be open.

Here's a screen capture of my current map's duty cycle table. Keep in mind that 101kpa = 14.7psi, so about 205kpa is 15psi boost. Only at 21 psi boost at 7000 rpm do I hit 100% duty cycle.
Austin is offline   Reply With Quote
Old 01-21-2003, 04:53 AM   #9
SuicidalLabRat
Scooby Specialist
 
Member#: 4096
Join Date: Feb 2001
Location: Constantinople
Vehicle:
noun [ ve·hi·cle ]
a carrier,as of infection

Default

Or you can run a really high base pressure and push your injectors, though you may become prone to pattern failures.



SLR-
SuicidalLabRat is offline   Reply With Quote
Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
How does a Cobb Accessport work and will it work with my parts? Twin_Cities_WRX General Community 5 03-08-2009 03:39 AM
How does a Cobb Accessport work and will it work with my parts? Twin_Cities_WRX Newbies & FAQs 2 03-07-2009 08:22 PM
Lost Oil Pressure ?? how does the oil system work 500hp sti StiChip Normally Aspirated with bolt-on Forced Induction Powertrain 5 11-26-2007 01:13 PM
Fuel system work CrowbarKen Mid Atlantic Impreza Club -- MAIC 14 06-11-2004 10:21 AM
How Does WRX Fuel Injection Work? 03GLHWRX Factory 2.0L Turbo Powertrain (EJ Series Factory 2.0L Turbo) 4 05-09-2003 06:59 PM


All times are GMT -4. The time now is 08:59 PM.


Powered by vBulletin® Version 3.7.0
Copyright ©2000 - 2015, Jelsoft Enterprises Ltd.
Powered by Searchlight © 2015 Axivo Inc.
Copyright ©1999 - 2014, North American Subaru Impreza Owners Club, Inc.