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Old 11-19-2001, 01:13 PM   #1
RigdenZW
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Question Explanation of Fuel Delivery - Long

I'm not sure if I should post this here or in the newbie section or what.. but I was reading the posts on getting an NA RS to 200hp etc, and Midwayman mentioned the link -

http://www.rceng.com/technical.htm#WORKSHEET

Now this peaked my interest and I'd like to learn more. If anyone could explain.. or point me in the right direction to find my own answers.. that would be awesome.

Here's what I've gotten so far... stock injectors are 280cc. Fuel Pressure is 43psi. BSFC is .45 - .50 for an NA motor. I can do all the formulas presented on the link above. The part I need help with is the theory I guess.

Duty Cycle? Is this the percentage of effectiveness from the injectors? Is 80% a safe range that the manufacturer decides upon? Why do most max out at 80% and what does it mean when they say max out?

BSFC? Someone mentioned that .45 was ideal (or best case) for NA.. the link says .45 -.50... what changes this variable?

How would you change your Fuel Pressure from 43psi? I'm assuming a better fuel pump? If that's the case.. then what does a fuel pressure riser do?

Like I said.. don't necessarily need the answers.. just a point in the right direction. This link seems to be helpfull too.. I've just glanced over it and will read it in more depth when I get the time.

http://www.autospeed.com/A_0102/P_1/article.html

Thanks in advance..
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Old 11-19-2001, 03:45 PM   #2
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I think I can answer your questions, but if someone out there thinks I'm spreading lies, please correct me.

Duty cycle: a 280cc(/min?) fuel injector at 80% duty cycle is pumping out 224cc(/min?) of fuel. Manufacturers probably decide on 80% because 100% is cutting it too close, and 50% is overkill for the application (of course, if 500cc injectors were cheaper and worked just as well in the 0-220cc range as 280cc injectors, 500cc injectors would probably be used)

BSFC: I'm fairly certain that this is a constant(ish) for an engine that depends on all sorts of strange thermodynamic-chemical-mumbo-jumbo.

Fuel Pump, Fuel pressure, and FPRs: Well, first of all, a fuel pressure riser does just that, it raises the fuel pressure between the pump and the injector. As for a "better fuel pump" that won't necesarily change your fuel pressure. From what I can tell, raising the fuel pressure is a crude hack for getting more fuel that has some undesireable side affects.

Other than that, I don't know where you can find authoritative answers to those questions, but those are my best guesses.
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Old 11-19-2001, 06:06 PM   #3
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Iowannaski is right, but for BSFC, I would say that you can change this somewhat... this is really just how efficient your engine is at making power... so if you have better cams, intake, exhaust, etc, your BSFC would probably be better... not a HUGE difference... 1 or 2 pounts
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Old 11-19-2001, 06:17 PM   #4
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Past 80-85%, most injectors go static, meaning they essentially stay open continuously, dumping fuel. Injectors, like everything else, have an efficiency range within which they perform quite well. Remember, you can have injectors which are too large, as well as too small. If you ran 750cc injectors, you'd likely be quite unhappy with your idle, even with proper tuning.


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Old 11-19-2001, 06:30 PM   #5
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You're mostly right
the 80% duty spec is because injector go static and cant close above 85% or so. Manufactures leave a little extra room by specing the 80% Using too big of an injector like one that only uses 40-50% decreases the "resolution" of fuel control. Basically It cant control the relatively small amount of fuel it need. (for example say a injector has a error of +/- 1% (just made that number up) , the error will be alot larger in absolute terms with the larger injectors compared to a properly sized injector.
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Old 11-19-2001, 09:46 PM   #6
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sorry, this only partly applies to the thread. i was just wondering why it's said that the injectors go static, or effectively wide open, at 80% if there's still 20% left. if the injectors are fully open at 80%, why is it not 100%? what happens in that extra 20% that makes it flow any more than at 80%?
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Old 11-19-2001, 11:08 PM   #7
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it's 80% of what it CAN flow... so 280cc/min injectors can pump 280cc's of gas an hour... but, to accurately control it, its gotta stay under 80% or it doesn't have time to close before it's gotta start spraying again, therefore it's staticly open...

In case it wasn't clear, I just reread that and want to add that it's not wide open at 80%, but it's open enough that it can't close before it has to open again for the next spray
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Old 11-19-2001, 11:39 PM   #8
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thanks for the explanation, seems strange to me that they cant just make quicker acting injectors that open and close with enough speed so that they dont keep shooting gas when they're not supposed to.

also, what confused me, shiv of vishnu performance once said on the board that the spray from a static injector is not even enough, that is, not spread out enough that it will burn as consistently as a mixture which came from an injector that has not gone static. and the explanation about not being able to close doesnt seem to address that part of it. anyone know why this would be a result of a static injector? or maybe i just thought this up, sometimes my mind gets jumbled and i link one thing to something completely different, but im asking anyways.

un otra cosa, i dont see how a percent (effectively a flow rate) makes much difference on how fast an injector closes. sure, the cross-sectional area of the cylinder or whatever it is inside of the injector is bigger when the flow rate is bigger, but is the speed of the closing of the injector relative to that flow rate? it doesnt seem like it should be. maybe i'm just taking the rule of thumb too far and it's merely being used as what it is, a rule of thumb. but i just thought i'd add to my post because it seemed somewhat relevant, and if not relevant, certainly interesting.

Last edited by elemental; 11-19-2001 at 11:45 PM.
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Old 11-20-2001, 01:03 AM   #9
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They can make injectors that work that way, they just draw more power, generate more heat, and don't last as long. Good for a race car, bad for a street car.


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Old 11-20-2001, 01:07 AM   #10
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Quote:
i dont see how a percent (effectively a flow rate) makes much difference on how fast an injector closes. sure, the cross-sectional area of the cylinder or whatever it is inside of the injector is bigger when the flow rate is bigger, but is the speed of the closing of the injector relative to that flow rate?
Assume the maximum rate at which the injector can cycle is constant, no matter what the rated flow (which is determined by the internal bore, as you suspected). If the cycle rate is constant, that means the injector is open for the same amount of time no matter the flow rate, and since flow rate is fuel volume over time, a higher flow injector will push more fuel in the same amount of time as a lower flow injector. This is why it is viewed as a percentage.


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Old 11-20-2001, 02:26 AM   #11
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To understand this, go get out your garden hose with a triggered spray nozzle. Squeeze the handle on and off rapidly and watch the pattern of the water. Notice how it comes out in puffs and swirls? (mist) Now squeeze it full open and watch the water. Not as much mist is there? Basically the same idea as your injector.
-B

Quote:
Originally posted by elemental

also, what confused me, shiv of vishnu performance once said on the board that the spray from a static injector is not even enough, that is, not spread out enough that it will burn as consistently as a mixture which came from an injector that has not gone static. and the explanation about not being able to close doesnt seem to address that part of it. anyone know why this would be a result of a static injector? or maybe i just thought this up, sometimes my mind gets jumbled and i link one thing to something completely different, but im asking anyways.
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Old 11-20-2001, 02:33 AM   #12
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yea, but the question was more along the lines of... let me make an example

for ease of use, 200 cc injector and 100 cc injector:

200 cc injector is at 80% duty cycle, 160cc, 40 cc "left" before 100%
100 cc injector is at 80% duty cycle, 80cc, 20 cc "left" before 100%

if the injector goes static at 80%, regardless of the rated duty cycle, (higher duty cycle injector, 20% more flow is increasingly bigger) you're saying that the speed at which the injector closes is related to the flow rate, to the percent (which varies with injector flow rate). what i am wondering is why this should be the case... the speed of the closing(stopping of flow) of a big injector should be about the same as the speed of the close of a smaller one(the valve should open close at same speed, even if it has to traverse a very tiny bit bigger diameter to cover the whole cylinder of a larger flow rated injector), correct? or is this an incorrect assumption, because if it is correct, should 80% really be a good way of measuring when an injector goes static?


thanks for listening to the ramblings

Last edited by elemental; 11-20-2001 at 02:38 AM.
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Old 11-20-2001, 02:43 AM   #13
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I'm having trouble even understanding your question.

It takes the same amount of time for a big injector to transition from closed to open and back from open to closed whether a small or large injector. In between, the injector is open a certain amount of time, called the duration. Larger injectors have higher flow rates, but do not open, close, or maintain a given duration any more effectively then smaller injectors. Why is it not clear from this that injector performance limits are based on a percentage of maximum flow?


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Old 11-20-2001, 03:15 AM   #14
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I think I got what you are asking... actual volume has nothing to do with duty cycles.

So no matter if it would flow 10cc or 1,000,000cc If the design goes static at 80% The injector cant close before it has to open again.

Lets put it this way.... 80% isn't really about 80% of FLOW, its about 80% of TIME

injectors are rated by flow at 300kpa (43.5psi about) at 100% duty (open all the time)
So if duty cycle is 80%, then it is open 80% of the time.
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Old 11-20-2001, 11:42 AM   #15
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You guys are making my day.. thank you.

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Old 11-20-2001, 01:03 PM   #16
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One of the reasons I asked these questions and am so interested is because I've seriously been thinking about getting Cobbs Tuned Cams and I want to make sure that I will be completely safe in the setup.

If the stock injectors are 280cc and the desired BSFC is .45-.50 then the range of hp I can expect to fulfill (according to the formula) is 170.6-189.6 before the injectors go static.

(I know that hp isn't additive when it comes to breathing mods.. but for simplicity....)
If stock max hp is 165 + 14 (cobb cams) + 5 (k&n + ganz intake + subaru performance pipe & muffler) = 184 hp

That's within the 170.6 - 189.6.. but it's still greater than .50 BSFC with 280cc injectors at 43psi can handle (again according to the formula). And if I decided to get a better exhaust setup later it will hopefully go over.

I know that there are probably a bunch of people running around with similar setups and no alternative fuel options. Should I be concerned? If not please explain..

Thanks guys for the awesome information so far..
-Dean

p.s. Also, I'm assuming that the ECU will handle the fuel management? I think there are two modes that it runs in.. open and closed.. one is for WOT and the other is normal driving? Gah.. so much to learn.. so little time.. hehe

Last edited by RigdenZW; 11-20-2001 at 01:09 PM.
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Old 11-20-2001, 01:41 PM   #17
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I run the cobb street cams +catback +UDP +Intake. Im not having fueling issues. *however* I dont have an A/F meter to see if Im getting rich up top if the injectors are going static. However, you can probably count on 85% duty without worrying too much. And in any case if you do make an injector go static, you'll just run very very rich. Not a dangerous condition. For some minor gains a FPR (fuel pressure regular- fixed rate @ 50psi) and a SAFC should give you the ability to tune how you need. The ecu should take care of the additional fuel pressure in closed loop mode (low rpm, low throttle) and the safc can take care of the open loop mode (high load, high rpm) Since the safc should "lose" its settings for the open loop mode, this should work well for a NA setup
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Old 11-20-2001, 01:48 PM   #18
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for your example 184hp is probably about right, btw...
also, the car was designed to run on 87 octane, so if you do experience pinging because you're running lean, 92 will probably help a bit in the regard(don't push it tho!).
Regarding the ECU... Yes, there is an open loop mode and a closed loop mode. Open loop doesn't rely on the O2 sensor for feedback to adjust the fuel levels. The fuel is dispensed exactly how the maps are set up. This means you won't lose power due to what the O2 sensor would read as rich. Closed loop is for cruising, gentle accelleration, and possibly idle. It trys to keep the AFR at a "clean" level. This is to help with emissions and economy.
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Old 11-20-2001, 02:47 PM   #19
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When this thread is done.. I think I'm going to print and file it.

How does the safc lose it's settings in open loop mode? I thought that was when you needed it?

Other than that, I think I'm getting it. I'm sure there will be more questions down the line... especially if I have to start managing the engine myself as opposed to plug an pray mode with the ECU.. but I'll save them for another thread..

Now to look into FPR's and SAFC's.. I'll search for type and cost..but if you guys have any off the top of your head and wouldn't mind posting..

-Dean "the learning machine"
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Old 11-20-2001, 04:01 PM   #20
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On MAP-equipped cars (MY00+), the ECU learns around the changes from the SAFC in closed-loop mode. In open-loop mode (20-100% throttle, depending on who you ask) the ECU doesn't try to learn at all, so the SAFC can function correctly.
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