What is a fuel system? - Boost Forum

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What is a fuel system?
Monday, August 29, 2005 7:11 AM
All you wanted to know about the J-Body fuel system... and more

Components
To understand the fuel system, you need to understand its parts. Here is the basic fuel system:

[Fuel Tank]--[Fuel Pump]--[Feed Line]--[Fuel Rail]--[FPR]--[Return Line]--[Fuel Tank]

The fuel injectors sit under the fuel rail and go into the head. So lets go through each piece:

Fuel Tank
The fuel tank obviously holds the fuel. In our cars (and many OBD-II cars) this tank is pressurized. This is why you hear a hiss when you open the gas cap. The computer monitors the pressure in the tank, so that when it drops below a certain point for a certain period of time, the tank or evaporative system has a leak. Usually this is caused by a loose or bad gas cap, but occasionally has other causes.

The evaporative (or evap) system is interesting in itself. If you've ever spilled gas you know that it "dries" up very fast. Its not actually drying, its evaporating. The evap system uses the evaporated gas and pipes it back into the intake manifold to improve gas mileage.

Fuel Pump
The fuel pump pulls the fuel out of the fuel tank and sends it down the rest of the system. The stock pump is not designed to handle much pressure over stock, no matter what other people have gotten with it. Do NOT trust the pump over 60 psi for any length of time. You may get away with it for a month or two, but one day the pump will burn out and fail when you really don't need it to.

You can pressure test a fuel pump to see what its maximum pressure is. First you need a gauge in the fuel system somewhere on the feed line or fuel rail. Next, start your car and let it idle. Clamp a vise-grip around the return line and wait until the pressure stabilizes, then unclamp it. That is your maximum fuel pressure. Just don't leave the line clamped very long, it will burn out your pump in a hurry.

Feed Line
This is a pressure line from the fuel pump to the fuel rail. There really is nothing special about it, other than its made for fuel injected cars (carburated fuel line is different... NEVER USE IT in an fuel injected system).

Fuel Rail
The fuel rail distributes an equal amount of fuel pressure to each of the injectors that are attached to it. The stock fuel rail holds enough fuel and fuel pressure to support well over 500hp. Aftermarket rails are really nothing more than dress up items. Fuel rails aren't really any technological wonder. Its a tube with a hole on each end, and 4 holes for the injectors. One end-hole attaches to the fuel feed line, and the other end has the fuel pressure regulator on it (FPR).

Fuel Pressure Regulator
This is a wonder of science right here. Basically, its a vacuum actuated valve. The more vacuum the FPR sees, the less it opens. If you are thinking to yourself... "well that doesn't sound right... at 0 vacuum (wide open throttle) you want more fuel, so you'd want it to open more", then you are just a bit backwards. The fuel pressure regulators job is given away by its name. It DOES NOT regulate how much fuel is in the fuel rail, but how much pressure is in it.

If you want to have a real world example of how you can understand this, get a drinking straw. Put one end in your mouth and put your fingers at the end so you can pinch the end closed. Now blow into the straw, but don't pinch the end, thats how the fuel pressure regulator is when there is full vacuum. You don't need a lot of fuel pressure going into the injectors because at full vacuum, the engine's throttle plate is fully closed (you are not accelerating).

Now, blow through the straw, but pinch the end shut (or varying degree's of shut), and blow through it. That is how the FPR is at WOT, where there is no vacuum. It increases the fuel pressure so each time the injectors open, more fuel is forced through them.

A common misconception about the fuel system is that the fuel pressure is regulated by the computer. It is not. The pump is running 100% (flow wise) all the time. The FPR chooses how much of this flow to hold in the rail (and increases fuel pressure by doing so), and how much to let back into the fuel tank (reason for the return line). The fuel pressure is completely vacuum controlled.

Fuel Return Line
This brings fuel back into the fuel tank. It is important to note that the fuel return line IS NOT a pressurized line. Don't use the return line to feed your extra injector's, or whatever else needs fuel. This line simply takes fuel from the fuel pressure regulator and drops it back into the fuel tank.

Fuel Injectors
I kept this one for last because it is a huge topic. Your fuel injectors sit under the fuel rail and inject fuel at the back of one of the valves, which, when open, suck fuel into the combustion chamber.

Fuel injectors are electronically controlled valves. They operate on a square wave signal from the ECU. Basically this means that the valve opens when the signal is high, and closes when the signal is low. The length of time that the signal is high (and the valve is open) is called the pulsewidth. The pulsewidth, as a definition, is the length (in time) of the pulse that opens the injectors.

Now injectors have a duty cycle. This is the amount of time they spend open. When an injector is open, it generates heat (electricity turns to kinetic energy in the form of heat, physics stuff). The longer the valve is open, the more heat it generates, and therefore the more prone it is to failure. When an injector fails, it will either stick open, or stick closed, both are very bad. When sizing injectors, you don't want to have the injectors working with more than an 80% duty cycle under the very worst conditions, and even then, not for very long.

Fuel injectors are sized at a certain pressure, usually 45 PSI. They are then tested at that pressure and rated for a certain flow (lbs/hr or cc/min). The more fuel that the injector can let through it at one time, the bigger the injector is. You can effectively change the size of the fuel injectors by changing the fuel pressure that is going through them. Obviously if the injector is rated at 45 PSI, and you push 60 PSI through it, then you will get a higher flow rate. Many times, this is desirable since you can get a lower duty cycle on smaller injectors and increase the fuel atomization.

There is a downside to just increasing the fuel pressure though. Actually, there are 2 downsides. The first is the fuel pump. Backing up the fuel in the rail causes the pump to try to push more fuel into the line (which increases pressure), but causes the pump to heat up and eventually fail. The second downside is called clipping. Clipping occurs when you try to push too much fuel pressure into the injector. The high fuel pressure can actually hold the injector closed, causing it to clip, or miss that pulse. When this happens often enough, it causes misfires and possibly engine damage.

Fuel system modifications
In the turbocharged kingdom, fuel is the king, queen, and town drunk. Most people will buy a fuel management unit (FMU), that goes in the fuel system and increases fuel pressure in a ratio with the amount of boost. Here is how an FMU goes in:

...--[Fuel Rail]--[FPR]--[FMU]--[Return Line]--...

Yes, the FMU goes after the FPR. (Ironically, in some fuel systems, the FMU completely replaces the FPR, this happens in the Hahn turbo kits for the Eco, but only because the FMU they picked also has a boost onset pressure, which regulates the pressure when you are not in boost as well). The FMU takes over after the FPR is fully open (in boost, you are making positive pressure, the FPR is maxed out at 0 PSI), and increases fuel beyond that, usually in a ratio with boost.

Now people who buy fixed rate FMU's are usually just buying the maximum ratio FMU they can buy, which really isn't all that smart. Buying a 12:1 FMU increases fuel pressure 12 PSI for every 1 PSI of boost, and it does this OVER stock pressure. So lets say your FPR at 0 PSI is putting out 55 PSI of fuel pressure, but you are boosting at 10 PSI with a 12:1 FMU... You are adding 120 PSI to your fuel system! This is WAY over the amount of pressure the pump is capable of supplying, and is WAY too much fuel for the injectors.

The best thing to do is to buy an adjustable FMU. These can adjust the rate, usually from 1:1 through 12:1 so you can pick and choose what the best ratio is for you.

Electronic Mods
These types of mods are done by many, ranging from the SAFC to the Megasquirt and many others. I will go over a bit how these things work, and what the different types are:

SAFC:
The SAFC is a popular choice due to how cheap it is, and how easy it is to hook up. The SAFC works by modifying the signal from the MAP sensor to make the computer think the air pressure is different than it actually is. The biggest problem with the SAFC is that when the MAP sensor is maxed out in voltage (5 volts), the SAFC can't really add any more fuel (computer doesn't know how to interpret any additional voltage). The SAFC should be used only to fine tune a fuel curve.

Megasquirt, E-Manage:
These are a bit less popular due to the complexity and cost, but they are the best choices for electronic fuel management. They work by altering the pulsewidth of the injectors that the computer sends out, adding or subtracting fuel based on many variables (called Maps). These maps are grid-like sheets that have 2 variables, and you enter a value in the graph. The variables are the X and Y axis of the sheet, and the values are in the middle.

Final notes
The biggest thing to remember about electronic mods, or the fuel system in general, is that it is not electronically controlled. The SAFC does not increase fuel pressure, and niether does the Megasquirt or the E-Manage.

There may be things I missed, but thats about it, feel free to add.

-Shifted





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Re: What is a fuel system?
Monday, August 29, 2005 7:12 AM
GREAT POST!


Re: What is a fuel system?
Monday, August 29, 2005 8:19 AM
thanks I hope I didnt push you over the edge to much. I was reading this stuff and thinking damn this is awnsering everything I needed and more. Great post copy and paste to faq


"Kick azz is my boost hero!!! "
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Re: What is a fuel system?
Monday, August 29, 2005 8:43 AM
Sticky!!!!!
Wish this was around 3 or 4 months ago



Re: What is a fuel system?
Monday, August 29, 2005 9:54 AM
Great info. STICKY!!!!



Re: What is a fuel system?
Monday, August 29, 2005 11:46 AM
Great post! How about some info regarding high and low impedance injectors, as I know this is a weak point for many who are learning about this kind of stuff? Actually if you can identify some direct replacements for '00-'02 2.4's in the 22-24lb/hr range while you're at it I'd greatly appreciate it!


15.3 @ 89.97mph, 14's on the way?

For Sale:
Getrag '00-'02 2.4 8000kms $800
Re: What is a fuel system?
Tuesday, August 30, 2005 12:32 PM
Ian Brydon wrote:

Great post! How about some info regarding high and low impedance injectors, as I know this is a weak point for many who are learning about this kind of stuff? Actually if you can identify some direct replacements for '00-'02 2.4's in the 22-24lb/hr range while you're at it I'd greatly appreciate it!


Aren't the 2.4 injectors already 24lbs/hr?
that's what i've read.


Re: What is a fuel system?
Tuesday, August 30, 2005 7:35 PM
Great info Ron. This thread should be !!!STICKY!!! and added to the FAQs.

Also I will add this info too.

2.2 and 2200 Injectors:
-99 --> 17Lb/Hr (178.5cc/min) Low impedance (2-6 Ohms)
00+--> 17Lb/Hr (178.5cc/min) High impedance (10-15 Ohms)

2.3 Injectors :
All are Low Impedance (2-6 Ohms)
2.3 LO --> 26Lb/Hr (273cc/min)
2.3 HO/W41 --> 32Lb/Hr (336cc/min)

2.4 Injectors:
-Mid 99 --> 24Lb/Hr (252cc/min) Low Impedance (2-6 Ohms)
Mid 99+--> 24Lb/Hr (252cc/min) High Impedance (10-15 Ohms)
SC Injectors --> 30Lb/Hr (315cc/min) High Impedance (10-15 Ohms)

Ecotec Injectors:
I don't have this info, then if anyone can help me? That would be highly appreciated.

You can use High Impedance injectors with a low impedance injector driver/ECU but not viceversa

Stock Fuel Pump:
Free Flows 80psi@28gph / 45psi@20gph

This is some info that I have collected, so if you see something wrong please check me.


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Re: What is a fuel system?
Wednesday, August 31, 2005 12:52 AM
95-97 2.2 injectors are high impedance, don't know about any of the others though.


Kevin Johnson
Jbody Club of Alberta
<a href ="http://www3.telus.net/public/styder" target="_blank"><img src="http://www.j-body.org/registry/poundinfire/j-body_sig.gif"></a>
Re: What is a fuel system?
Wednesday, August 31, 2005 1:25 AM
i just jerked off on this post and now its all sticky.


-Borsty
Re: What is a fuel system?
Wednesday, August 31, 2005 6:20 AM
"Fuel Pressure Regulator
This is a wonder of science right here. Basically, its a vacuum actuated valve. The more vacuum the FPR sees, the less it opens. If you are thinking to yourself... "well that doesn't sound right... at 0 vacuum (wide open throttle) you want more fuel, so you'd want it to open more", then you are just a bit backwards. The fuel pressure regulators job is given away by its name. It DOES NOT regulate how much fuel is in the fuel rail, but how much pressure is in it."

the more vacuum it sees, the MORE it opens to let the excess run off into the return line. no vacuum (wot) the regulator "valve" closes off the opening to the return line, thus building pressure in the rail. i have taken apart a fpr from a 2003 ecotec. looks pretty simple to me



Re: What is a fuel system?
Wednesday, August 31, 2005 6:50 AM
^Thats right, I mixed up the word less with more.





4cyltuner.com - Information Source For 4 Cylinder Tuners
Buy stuff from CarCustoms Ebay! Won't be disappointed!

Re: What is a fuel system?
Wednesday, August 31, 2005 3:25 PM
VTLoki wrote:

Ian Brydon wrote:

Great post! How about some info regarding high and low impedance injectors, as I know this is a weak point for many who are learning about this kind of stuff? Actually if you can identify some direct replacements for '00-'02 2.4's in the 22-24lb/hr range while you're at it I'd greatly appreciate it!


Aren't the 2.4 injectors already 24lbs/hr?
that's what i've read.


I've read that also, but I believe the '96-'99 2.4's w/ low impedance injectors were 24lb/hr, while the later 2.4's w/ high impedance injectors had 19lb/hr injectors but ran a higher base fuel pressure. It's a topic of debate I guess.


15.3 @ 89.97mph, 14's on the way?

For Sale:
Getrag '00-'02 2.4 8000kms $800
Re: What is a fuel system?
Tuesday, September 06, 2005 10:13 AM
no. the high impedance 2000+ 2.4L injectors were also 24 lb/hr. the ecotecs are 24 lb/hr and high impedance as well.


Re: What is a fuel system?
Tuesday, September 06, 2005 12:18 PM
JB Bika (slo eco) wrote:

no. the high impedance 2000+ 2.4L injectors were also 24 lb/hr. the ecotecs are 24 lb/hr and high impedance as well.


Yes, but the fuel pressure on the '00+ 2.4's is ~48psi, while the earlier 2.4's are ~ 43psi, and I've heard the eco's are as high as ~60psi. Even though they flow the same rate, the injectors on the newer engines are smaller. To compare injectors, you must flow test them at the same fuel pressure, which is not happening at this point.


15.3 @ 89.97mph, 14's on the way?

For Sale:
Getrag '00-'02 2.4 8000kms $800
Re: What is a fuel system?
Wednesday, September 07, 2005 7:00 AM
i've posted this link many times before. its part of RC engineering's site... calculate flow rates HERE


Re: What is a fuel system?
Wednesday, September 07, 2005 7:02 AM
also note that i wasnt saying the injectors were the same by any means. they are very different. just stating the flow rates on the two vehicles ended up being similar.


Re: What is a fuel system?
Wednesday, September 07, 2005 10:03 AM
JB Bika (slo eco) wrote:

also note that i wasnt saying the injectors were the same by any means. they are very different. just stating the flow rates on the two vehicles ended up being similar.


At least we're on the same page here. I have never argued that the end result is different, but in order to compare injectors (apples to apples), you must compare at the same fuel pressure. My issue is the capacity of my stock fuel pump, as it currently exceeds 60psi at WOT (no vacum) with my base set at 50-52psi, as tuned with my AFPR and WB O2 sensor at 12.9-13.2 A/F ratio at top of 3rd gear @ WOT. I'm under the assumption that the stock fuel pump should not be trusted to exceed 60psi so I want to upgrade my injectors with something the stock ecu will be able to handle, and be able to back off my base fuel pressure back into the safe zone, blah, blah, blah.

Bottom line: the fuel injector flow rates mean NOTHING if they are not dealing with the same fuel pressure, which we are not at this point. I'll have to take a closer look at the RCEngineering link you posted, I played with it a bit last time but didn't spend much time with it.


15.3 @ 89.97mph, 14's on the way?

For Sale:
Getrag '00-'02 2.4 8000kms $800
Re: What is a fuel system?
Wednesday, September 07, 2005 12:35 PM
no prob. its a good calculation tool. i wanted to post that link in here anyways since the topic is stickied.


Re: What is a fuel system?
Wednesday, October 26, 2005 10:43 AM
another good fuel managment system is the sds eic, its extra injectors added either to the charge pipe before the t/b or i've seen them welded into the intake itself. All thats needed depending on which kind you get is a wbo2 to tune it, it has a controller with a x and y knob that is used to control the adding of fuel. not as complex as megasquirt but people have made good power with these.


cavaliers been laid to rest
The saturn is being born
a new cavalier is in the mist
Re: What is a fuel system?
Friday, October 28, 2005 7:11 PM
The SDS EIC is the EIC kit that HAAS Performance used in their kits. And we all know what happened to HAAS Performance...I've heard bad things about the SDS EIC but I've heard good things about it from others. Some people complained that it would cause the car to lean out like crazy, but there is a certain level of tuning involved.




I was a retard, and now I'm permanently banned.

Re: What is a fuel system?
Friday, October 28, 2005 11:38 PM
with sds it sometimes leans out on the top end because the way fuel is added with sds, boost is in a curve, sds is in a straight line. kinda complicated i think thats the best way i could describe it sorry. there are 2 ways the system can be controlled with the 2 knob controller or a labtop program


cavaliers been laid to rest
The saturn is being born
a new cavalier is in the mist
Re: What is a fuel system?
Saturday, January 07, 2006 8:57 AM
Thanks Man, That is a very great write up for me to begin my quest for "Properly Boosting My Z24".







Live Free Or Die


Re: What is a fuel system?
Sunday, January 15, 2006 8:15 PM
Nice find
Re: What is a fuel system?
Tuesday, February 14, 2006 2:29 PM
Okay, here is my attempt at asking a question and not looking dumb.....(i dont think its gonna work) Okay....
So An FPR is used in line with the return line after the fuel rail, and it's purpose is to lean out the fuel from larger injectors while the car is NOT boosting.....Y/N maybe?

Going off of that ^ (and my confused understanding) The FMU takes over after the FPR is fully open, then taking over and adding more fuel by blocking off the return line...Y/N maybe?

SO the FPR (?)increases/dreases(?) the fuel pressure before boost and the FMU increases(?) fuel pressure during boost......ummm....yeah.

So: -No boost=FPR dealing with it
-Boost=FMU dealing wth it

Is there any way to only use one?

How wrong am I....Please be nice...



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