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Old 01-17-2005, 03:11 AM   #1
Back Road Runner
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Default Curious - Open Loop/Closed Loop

Ok, I've come across this in the past. The Subaru engines lock the Air-Fuel mixture under a certain rpm to save fuel economy, correct? I've also come across people mentioning throttle position as the determining factor for switching between open and closed loop(WRX 2.0L?). (If I'm mistaken, correct me please)

*my car, '02 Forster 2.5L N/A, stock (other engines may work differently)


The engine has always behaved...wierd. I'm not exactly certain what it is, but I'm assuming it's related to this in some aspect, maybe even how the ECU handles things too.

The engine feels like it has more power than it outputs. In fact, it feels oddly restrained up until above 4k where the air/fuel ratio gets unlocked. The jump in power output is noticable, and the engine sort of comes alive and feels like it should throughout the whole powerband.

Now, I'm curious is it possible, even with the stock ECU, to actually unlock the air/fuel ratio all the time so that the engine can adjust it as needed at any rpm? Would an aftermarket ECU or a piggy-back system be needed?

Why do this? Well, I personally seek 2 things. First, I kind of hope to unlock a little power that seems to be trapped by this default setting. Second, the engine doesn't behave in a way that I'd call "smooth." I'm also hopping to decrease the noticable (abrupt) changes that take place with the engine through the rpm range AND when gettting on and letting off the throttle(slowly or quickly-more of an instant response than current).

I'm not worried about fuel economy either. I'd think if one drove the car light, it would naturally run the mixture relatively light. If the throttle is light, and there isn't much load on the engine, it shouldn't need to suck gas anyways. I don't know how the Subaru ECU handles things, so I can't say what it would do.


So...
any of you guys actually ever worked on this aspect of the ECU before? Is it unlockable? Is an aftermarket ECU needed? Price involved in such a thing?
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Old 01-17-2005, 12:53 PM   #2
bofh
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Search for a post by Vaus about his OLM Gadget cause it sounds like what you need.
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Old 01-18-2005, 03:13 AM   #3
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Nice little post about the mapping. However, I'm kind of leaning towards a different approach. I'm really curious if it's possible to keep the car in open loop where the air/fuel mixture is dynamic according to how you drive. That is open loop right?

I do have another curiousity. Now, what I seem to experience has nothing to do with throttle position. However, as stated in Vaus' post and what I've seen elsewhere(pertaining to WRX'es only), the open and closed loop changes are throttle position dependent. This may be a design difference between the 2.0L WRX'es and the 2.5L NA, but my issue isn't throttle related. It seems purely rpm related with very specific changes are specific rpms. I've inquired before and have been hinted to the open/closed loop issue causing the changes, hense my current inquiry.

Now, you may want to suggest other "problems" like bad sensors and hesitation, but that's not it. It's specific, repeatable, and always present aspect of the engine's behavior. It's basically when the engine/ECU does something.

I don't want to narrow down what I want to hear for a response, but I am mainly focused on attempting to eliminate these changes and hopefully lock the engine in the open loop always, if possible. If what I'm looking for is odd or vague, it's only because of my lack of knowledge on the subject. Feel free to clarify aspects as well as state limitations.
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Old 01-18-2005, 07:57 PM   #4
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I have no clue, and I am not afraid to admit it! However, you may want to talk with the folks at openecu.org where the entire purpose is tearing apart the Subaru ECU. I know there are a couple of NA guys there as well.
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Old 01-18-2005, 08:24 PM   #5
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ok let me just clear this up because i'm a bit confused and not sure if you know what you're talking about (no offence at all please)

closed loop = the ecu using the o2 sensor to shoot for 14.7:1 AFR for idle, and light load conditions.
open loop = the ecu using its preset maps and applying A amount of fuel for B amount of air at C rpm and D load. usually richer than closed loop fueling.

i am not sure where the N/A ecus cross over from closed to open. some sort of obd-II logger should be able to tell you. watch for when the AFR shoots down from the 14s. http://www.rs25.com/ may be a good starting point for finding out the cross over area.

good luck
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Old 01-18-2005, 08:37 PM   #6
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Is this an automatic?

You sure you're not just hitting peak power or shifting?
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Old 01-18-2005, 09:01 PM   #7
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A couple of things-

Closed loop fueling means that the AFR's are being measured constantly, and they are programmed to hit a specified target. For example, you could run a car in closed loop fueling, however, the target might be 12.0:1.

Also, more fuel doesn't equate to more power. Just because the car goes into open loop fueling doesn't mean there has to be a surge of power. A car makes peak torque at stoich AFRs, so when the car is running rich, the excess fuel is being used for cooling.

I don't know much about the Forrester engine management, or even what a dyno plot looks like. Is it supposed to pull harder above 4k? Maybe it's got a VTEC motor


Mike
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Old 01-19-2005, 01:41 AM   #8
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Thanks MikeWRX-NJ for clearing up some understanding. Yes, I pretty much don't know what I'm talking about, but I'm trying to learn. Oddly, I like the sound of closed loop better, continuously monitored and adjusted to the optimum 14:1 ratio we were taught in school. That's what I like to hear. The premapped thing seems odd to me. Sure, with a good amount of testing, you can set it to work in a specific manner to achieve specific results, but I don't see the point of ignoring the sensors and running a predefined ratio that may or may not be right for the conditions(envirionment, car load).

As well, crazymikie, I do understand that there are different power outputs through the rpm range. These types of characteristics exist for all cars and are dependent on the motor. I am aware of the torque curve and its resulting power curve. I've torn down a rebuilt an engine and have taken several years of physics. The mechanical and power aspects I understand well. It's just the electronic aspect I don't(not electronics or computers in general, lol, I know plenty). The ECU and how it controls the engine is one aspects of car engines I am just starting to learn. There are a lot of things I don't know about that, open/closed loop for one.
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Old 01-19-2005, 08:34 AM   #9
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well the thing is that the o2 sensor is only partially a wideband. meaning it does read other than rich/lean but many people say its not very accurate. Now this is just on the WRX, i'm not sure what the case is on the N/A. the reason you have to go open loop is because the o2 sensor isn't accurate and can't read richer than 11.25:1. on a WRX 11:1 is considered a safe tune that makes decent power. that is a boosted motor though.

in reality 14.6:1, or stoich, is the ideal for a complete and chemically ideal burn. It is NOT the ideal for power though, the ideal for power lies between rich best torque of 11.5:1 and lean best torque at 13.3:1. where a particular car can run depends on octane and other things. I suggest you pick up "how to tune and modify engine managment systems" by jeff hartman. it's a $19.99 book that is worth it's weight in gold when it comes to tuning theory, and it's written so that a newbie can understand it, but it goes advanced enough to cover things well.
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Old 01-19-2005, 09:07 AM   #10
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Back Road Runner- I meant no offense, so I apologize if I stated the obvious for you. I guess what I meant, overall is that there is no reason why you should have to feel CL/OL switchover if tuned correctly. I believe there is a standalone EM called 'Hydra' that runs CL fueling 100% of the time. It is an idea that intrigues me as well. Why adjust fuel trims and whatnot when you can just tell the ECU what you want and let it figure out how to do it.

MikeWRX-NJ- That sounds like a good read. Thanks for the pointer!


Mike
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Old 01-20-2005, 03:36 AM   #11
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Thanks MikeWRX for the heads up on the book. I may look it up.

No offense taken crazymikie. Perhaps it would have been better initially stating my knowledge on the subject + related areas. So polite; you're mom must be proud


I'm not really looking for open or closed loop only. The actual biggest problem is that the Subaru engine is...well...the worst behaving engine I've used. Now, I should clarify that some as that statement is overly general. Basically, in smoothness of operation(not vibration), it doesn't do well to hide changes or to provide the driver with smooth, instant power. It almost seems to lack refinement. I'm used to old cars, both carburated, throttle bodied, and sequencial fuel injection in a slew of makes and models from different companies, all cars +10 years old. They ALL felt smoother and more in tune with the driver than this engine. Behavior through the rpm range and throttle response were better. I know it's not the Subaru engine itself but rather how it's controlled. It's kind of my quest, so to speak, to find and solve the problem associated with its poor behavior. The closed/open loop stuff is currently my focus.
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Old 01-20-2005, 09:48 PM   #12
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On a turbo engine max power does not equal stoich. It is in a much richer area basically what Mike WRX said, and that is a great book BTW!

I learn stuff every time I pick it up. I wish there were more people like hartman willing to share his recipe for power...
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Old 01-21-2005, 03:39 PM   #13
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I'm sold. So where can I buy the book for $20 bucks? Have a linky?



I looked on ebay, and found two - one with a bmw on the cover (looked older) and one with an focus on the front. Is the focus cover the latest?
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Old 01-21-2005, 04:22 PM   #14
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