I just made a couple of posts in the Australian forum where I mentioned "Closed Loop" and "Open Loop". I thought it might be a worthwhile thread if I started an explanation of what this all means, and how it works in conjuntion with piggy-back ECU's.

I repost this info here for those that don't frequent the Aussy forum... Happy reading!

Closed Loop
This condition is normally encountered at part throttle openings, such as encountered in "normal" driving. A simple explanation of what is happening here is that the PCM (Powertrain Control Module) is continously monitoring the O2 sensors in the exhaust. This device tells the PCM if the engine is running Rich or Lean. The RX-8 also has what is called a wideband O2 sensor, and this is used to tell how rich or lean the engine is running - hence the PCM gets a very accurate reading.

Locked away inside the PCM is some reference tables where, for a particlar load/RPM/thottle condition, the target AFR is found. The PCM continously monitors the AFR and adjusts the fuel delivery constantly to try to obtain that AFR. Constant feedback/adjustment, hence the term "closed loop"

There are a few other things that come into play here, known as STFT and LTFT - and this is all related to "learning". I'll leave a discussion about that untill another time.

Open Loop
This condition is normally reserved for WOT. The feedback loop from the O2 sensor is no longer operational. Hence the term Open Loop. In this case, the computer determines the amount of fuel delivery (and spark timing) from some calibration tables that are again locked away inside the PCM.

One of the main sensors the PCM uses to determine the fuel requirements is the MAF (Mass Air Flow). The way piggy back ECU work is that they alter the signal the PCM is getting from the MAF in such a way that the PCM grabs a fuel "doseage" value from a diferent part of the fuel table. If you alter the MAF signal up or down by the right amount, you can pinpoint more or less fuel - hence adjust the mixture richer or leaner. Depending on the fuel maps in the PCM, this altered MAF signal probably needs different adjustmets at all RPM points.


So where does this leave us?

What is all means is that a piggy back fuel controller shouldn't make any difference to normal, part throttle (Closed Loop) driving. It should not make any difference to economy either, or to emmissions.

Why is that? Because in Closed Loop, the computer is continually adjusting the fuel delivery to obtain a target AFR. The target AFR is locked away in the PCM. To my knowlede the fuel controllers don't fool the PCM with a butchered O2 reading. Over time, the different amounts of fuel required at different load points will be learned by the PCM (this is where those STFT and LTFT come in), and negate the changes to the MAF signal.

That is also one reason not to play with the MAF too much - in closed loop there will be no differences, and if you fiddle with screens etc you will bugger up the accuracy - they rely on a nice laminar flow of air.

But with a fuel controller that has been calibrated properly to fool the computer to think a different amount of air is incoming at WOT, then since the computer is in closed loop, you can alter the performance of the engine. A "Piggy Back ECU" is really fooling the computer. Done properly by an experienced tuner, one can achieve measured gains - if the state of tune is conservative in the first place.

The other answer is to get inside the computer and find all the look-up tables, understand their structure, and change them. That is a lot harder to do.

I hope this has been an interesting lesson in de-mystifying the working of your cars "computer" to some extent. I love understanding what is going on inside there.

Cheers,
Hymee.

HYMEE

I am truly fortunate to have availability to your LOOPY knowledge for the testing of maurices stage 1 kit for the Australian RX8 S .

Go fed ex go 2 days to count down .

cheers

michael

I am really amazed that no one has really posted more on this. :)

Cheers,
Hymee.

probably, Hymee, because very few of us fully understand it.
I, for example, am a pre-IT baby boomer. If it is more complex than tuning a SU carbie, then I'm too old.
jack.

Doesn't the open loop also apply to the engine at start-up?

Yes, I believe that is the case. I have witnessed that via my OBDII scan tool.

I would say the reason is at startup, there is a fuel/spark table that is looked up against intake temperature as one of the main variables. The O2 sensors don't work untill they get to a certain temperature either, so that would also rule out closed loop at cold start.

Cheers,
Hymee.

This is great info Hymee, thanks!

This all comes into play greatly with turbo charged vehicles, i believe the code is po103 or something like that, if you trigger this cel and use a pocketlogger (this is a great tool to have for obdII cars. using an apexi s-afc can help tune for this you will neeed to tune your high and low throttle settings, but most importantly to rid yourself of this code using the s-afc you will need to change the %'s for high and low throttle something like 30% low 70% high. stft short term fuel trim ltft long term fuel trim, these are used when tuning a car under low throttle and high throttle you want to keep them as close to 0 as possible. This means your 02's are getting proper readings and the fuel mixture is correct in each rpm level. one thing to look at to help you tune your car is youe 02 volts a good range for tune is between 85v and 94v.

The target AFR is locked away in the PCM. To my knowlede the fuel controllers don't fool the PCM with a butchered O2 reading.


Can we get canzoomer to respond to that?

One thing I would ask would be, are the reference tables completely locked away? Is it known that you a) cannot change the values, say, target AFR, and b) cannot redirect the PCM to look at a separate device such as the piggy-back that has its own programmable reference tables/target AFRs?


I need to go back and read some of canzoomer's and syntrix's threads during testing to match it up in my brain that they were in fact just tuning open loop/@ WOT. I have it in my brain for some reason that 1) things run pig rich even at partial throttle/idle, more or less in some ranges of the powerband but still pretty rich overall, and 2) that piggy-back units or programmable fuel mapping devices like the greddy were addressing that, and not just open loop/WOT.

Maybe you guys can help me out in the meantime until I can go search and read up.

Originally posted by jeffscardinal
This all comes into play greatly with turbo charged vehicles, i believe the code is po103 or something like that, if you trigger this cel and use a pocketlogger (this is a great tool to have for obdII cars. using an apexi s-afc can help tune for this you will neeed to tune your high and low throttle settings, but most importantly to rid yourself of this code using the s-afc you will need to change the %'s for high and low throttle something like 30% low 70% high. stft short term fuel trim ltft long term fuel trim, these are used when tuning a car under low throttle and high throttle you want to keep them as close to 0 as possible. This means your 02's are getting proper readings and the fuel mixture is correct in each rpm level. one thing to look at to help you tune your car is youe 02 volts a good range for tune is between 85v and 94v.

Can you elaborate on what you have written? It is hard to read, and I can't see where you are going with it.

The RX-8 has a wideband O2 sensor. Wideband O2's use current, not voltage to report the lambda. "Normal" voltage based O2 sensors are not linear, and are really only good to indicate rich or lean or stoich. But not the degree of richness or lean-ness. It would be advisable to tune with the wideband, not the normal one (which the RX-8 has as well).

If you reset the fuel trims, the PCM will re-learn them after a while anyway. That is what closed loop is all about.

Cheers,
Hymee.

Wow awsome info thanks Hymee I am learning so much.

Two observations:

First, the RX-8 does not appear to delineate between "open" and "closed" loop.
Since the front O2 sensor is used to set lambda values for the PCM under wide open throttle, there isn't a real "open" loop anywhere in the programming.

Second, the PCM in the RX-8 seems to assume a certain response curve for the MAF and trusts it will remain that way.
That means that adjustments to the "closed" loop parts of the map DO have the same effect as those to the "open" part.

When I do tuning runs for the E-Manage, I do 5 seperate logs:
One each for 20%, 50%, 70% and 100% throttle and one for a standing 1/4 mile.
I then tune each of those ranges seperately. The PCM does respond in kind and does not attempt to "un-learn" what I have dialed in.
I actually found that I could make a significant change for the better in the drivability at part throttle.
At 70%, the lumps and sags in the power curve that happen at 4200 and 6200 RPM are more pronounced in the OEM tuning. I dialed those almost completely out with the E-Manage.

hmm.. I don't frequent these discussions much so please excuse me if my post is not 100% relevant to the programming of the ECU and other computer functions.

So after reading a couple of the posts.. Hymee you have a pretty good example but it may be a little to complicated for those who don't know about some of the functions of the ECU (I will admit I only have a rough grasp)
But , I really haven't seen a good definition of Open loop and closed loop. systems.

So open loop:
getting away form a complicated thing as your ECU we can liken this to a simple washing machine. You turn it on and it goes through its cycle. How do you control the cleanliness of your cloths? well you can't unless you modify some of the inputs (soap wash-time etc). This is similar to some of the ECU function. It takes an input and then produces a set output. Nothing complicated here

Closed loop:

This is where things get fun! A simple example would be your toilet. You flush but instead of just filling the tank for a certain amount of time (like you washing machine) your toilet has a sensor (the floating thingy)! This produces a "signal" (just a lever) back to the valve which will start to proportionally close the valve until the toilet tank has reached the proper water height.

This simple example can then be extrapolated to a much much more complicated functions of the ECU.

Now I am sure you can probably see a lot of closed loop systems now.. ie cruise control.. etc

A little bit more technical as I don't have much experience with (but would like to learn): the electronics. So Hymee you are saying that the piggy back system is directly connected right before the PCM. So essentially the piggybacked-unit is just spoofing a different value to the PCM?

This seems like a really hack job(sorry if this sounds rude) as I am not used to seeing things done like that. Ideally you would want to really put in a new look up table in the original ECU but I guess that is kinda out of the question right?

Also the piggy backing can be tough because one of the interesting things about closed loop control systems is that they can go unstable! So that Piggybacked ECU has really got to be tested well for the values it outputs(and the time lag it induces) hehe.

Interesting stuff!

if you guys have anything I could read more about this I would love to get my hands on it! I would really like to be able to see a flow diagram to really understand how all the systems work together!

A closed loop system will only become unstable if the feedback time exceeds the response time of the original system.
That wont happen here.
Think of a piggy-back air/fuel controller as a pair of sunglasses and you have a better analogy.
Your irises will react at a rate determined by the amount of light hitting your retina.
Now, put on a pair of sunglasses.
Your retina simply gets less light, so your irises don't react as much.

Originally posted by Maniac
Two observations:

First, the RX-8 does not appear to delineate between "open" and "closed" loop.
Since the front O2 sensor is used to set lambda values for the PCM under wide open throttle, there isn't a real "open" loop anywhere in the programming.

Second, the PCM in the RX-8 seems to assume a certain response curve for the MAF and trusts it will remain that way.
That means that adjustments to the "closed" loop parts of the map DO have the same effect as those to the "open" part.

When I do tuning runs for the E-Manage, I do 5 seperate logs:
One each for 20%, 50%, 70% and 100% throttle and one for a standing 1/4 mile.
I then tune each of those ranges seperately. The PCM does respond in kind and does not attempt to "un-learn" what I have dialed in.
I actually found that I could make a significant change for the better in the drivability at part throttle.
At 70%, the lumps and sags in the power curve that happen at 4200 and 6200 RPM are more pronounced in the OEM tuning. I dialed those almost completely out with the E-Manage.

Jeff,

Firstly I want to make it clear from the outset that I am not trying to discredit your results. I am quite impressed with what you have done, and give you lots of kudos for being a pioneer with the eManage on the RX-8. I have seriously considered going down that path.

With all due respect though, the ECU does have Open Loop and Closed loop modes. I know because the computer tells me so.

I would like to know the "longevity" of the closed loop gains you see. In theory (and I admit none of us know the exact inner workings of the PCM yet), over time the LTFTs and the STFTs should adjust to achieve the same AFR's as it has been "told" to obtain.

What I would like to do is go for a drive with some logging happening, and produce a graph of "Fuel System Status" (open or closed loop), Speed, RPM, TPS, Wideband O2, Commanded (target) O2 etc.

I am waiting for a hardware update that will allow me to log more paramters at once - hence more frequent updates. Then I need to do some work on extracting the raw values from the logs, and plot them on a graph of some sort.

I have been looking at some OCX's for doing that. If anyone has some info on OCX's that can be used for graphing, please let me know. So far I have been looking at "Dragonfly" and "Flipper Graph Control"

Cheers,
Hymee.

Originally posted by Kaliken

So Hymee you are saying that the piggy back system is directly connected right before the PCM. So essentially the piggybacked-unit is just spoofing a different value to the PCM?

This seems like a really hack job(sorry if this sounds rude) as I am not used to seeing things done like that. Ideally you would want to really put in a new look up table in the original ECU but I guess that is kinda out of the question right?



Yes - Essentially a Piggy back is a "hack job", but that sounds derogatory to the kind of effort Maniac and Canzoomer have put into their unit. It is spoofing the values, and the trick is to spoof the correct amount at all the RPM / Load points.

Yes - Essentially think of most Piggy Backs plugging in "before" the PCM - to alter incoming signals.

One exception I know of is Ric Shaws tunable unit. It does not alter the signals coming in, but takes control of the duty cycle of the fuel injectors as well as the ignition firing "on the way out". I am quite impressed with this concept. One advantage is has is that you are not messing with any thing else the computer might be controlling. As an example, think about this - Lets just pretend that the PCM controls the amount of oil being injected to protect the seals via a decision it makes upon the signal it is getting from the Mass Air Flow sensor? What happens to the doseage then if you spoof your MAF? You can see why vehicle manufacturers void powertrain warranties due to piggy back PCM's.

"Ideally..." - I believe you are correct. It is "kinda" out of the question, the ECU software is not something you can go get out of Open Source.

If someone can swing me a copy I would more than happy to step up to the plate ;)

Cheers,
Hymee

Very interesting point concerning maipulating outgoing rather than incoming signals.

Great point Hymee....about how the MAF signals being altered.

Without knowing the entire script from the OEM PCM, its hard to tell what else the computer may or may not base "piggyback warped values" to make other decisions. Interesting thought.

About the point brought up about how a Piggyback FC is hack as compared to new lookup tables and values placed on the PCM itself...I agree to a point.

I'm a purist however...and would rather a reprogrammed ECU over a unit to fool the ECU that I currently have. What Ric Shaw is going to sell is somewhat unknow to most at this point as far as the specifics....and how it interacts with the ECU compared to Canzoomers.

What is certain is that Racing Beat plans to modify the ECU with new software in the form of a software update. You send then your ECU...they fash it with the new software designed by them....done. What the power gains are...we dont know yet, but I like the idea of a refashed ECU rather than whats offered at this time. RB says they are very close to completion........it may be worth the wait.

Not to mention the fact that Racing Beat has been tuning rotaries for about as long as Mazda has been selling cars in America.

Are you logging through the DLC on OBD-II?
That is why you get the "open" and "closed" loop indications.
The car is required to indicate those points in the tuning to conform to OBD standards.
However, the computer continues to trim fuel based on the WBO2S, even at WOT where the car would, theoretically, be in "open" loop.
I have a separate WBO2S in front of the cat that is always installed, always on, and I log just about everywhere I drive.
I've been doing this for over a month.
My A/F ratios have stayed where I put them and I can adjust them at any throttle position.
If it of any use to you, I can send you a TON of Excel pages of data dumps from all kinds of driving scenarios.

thanks guys.. this is fun stuff. and all the props to canzoomer and maniac figuring all the stuff out to create a piggyback unit! I can fully understand the amount of work that went into it.

very cool!

And maniac your analogy is pretty good considering we are taking into consideration things that move at the speed of light. namely electrons in a circuit. So time delay would hardly be a factor here. But there are a lot of mechanical control systems that can go unstable because of time delay (or in the proper control terms: phase lag).

This is realy interesting stuff as it pertains to a lot of theings I studied in college. I will have to keep posted on this topic!

Originally posted by Maniac
Are you logging through the DLC on OBD-II?
That is why you get the "open" and "closed" loop indications.
The car is required to indicate those points in the tuning to conform to OBD standards.
However, the computer continues to trim fuel based on the WBO2S, even at WOT where the car would, theoretically, be in "open" loop.
I have a separate WBO2S in front of the cat that is always installed, always on, and I log just about everywhere I drive.
I've been doing this for over a month.
My A/F ratios have stayed where I put them and I can adjust them at any throttle position.
If it of any use to you, I can send you a TON of Excel pages of data dumps from all kinds of driving scenarios.

Jeff,

If you would be happy to send them to me, I would be more than happy to have a look over them. As long as you realise I am no trying to have an argument with you, just trying to share experiences. I know you have put a lot of time, effort, and money into this.

It is indeed interesting that the AFR's are staying where you put them. It is like the computer is not even adjusting anything as it would in closed loop. That is incredible.

I hope the computer is not lying through it's OBDII DLC (yes, that is what I am reading). If it was it would surely be in breach of the whole CARB Emissions thing that is basically enshrined in law.

And if it was lying through that, what hope would the dealer have via the WDS system? It communicates with the vehicle via the same connector.

Just a quick question - You said "The car continues to trim fuel" and you also said your "AFRs stay where you put them". To me that is a bit of a contradiction. If the car continues to trim fuel, that is because it is trying to get target AFR's, however your AFR's are maintained. From what I understand, you are not altering the target AFR's per se, but essentially the amount of air flow detected. Do you see what I am getting at. Anyway - my theory is not worth much compared to your practical results!

Keep up the good work!

Cheers,
Hymee.

Trim was the wrong choice of words (since that already has a definition via OBD).

So "alter", or "change" then?

OK, I think I now know a little bit more that helps me understand the experiences Jeff has posted.

In Closed loop, the "target AFRs" are locked away in the PCM, and the PCM looks for a target AFR based on RPM and MAF (and maybe something else). So the trick in tuning is then to alter the MAF signal just the right amount so the computer goes looking in another part of the table and gets a target AFR that is the one you are looking for. The computer then adjusts the fuel delivery until that AFR is obtained via the wideband O2 (Lambda) sensor.

This is where the fuel trims come into play: the computer records the adjustment it last made for that condition, and it is looked up first - sort of like a level 1 cache - and hopefully gets the AFR right the first go. If it doesn't get the AFR right the first go, it "Trims" it (via the closed feedback loop), and stores that adjustment in the Fuel Trims, to be used next time that condition is reached.

All of this is happening many, many times per second. It is really a marvel of engineering what that little computer is doing! Most people simply put the key in, start it and drive it.

Cheers,
Hymee.

PS - Thanks to Canzoomer for explaining to me how the MAF signal can be used to our advantage in Closed Loop!

This is a great discussion that should be sticky. I can't remember, is WOT really 70% throttle or greater? I know its not just 100% pedal to the floor.

Given what we know about closed and open loop, how does the emanage control a turbo setup the under partial conditions?

ahh that last question of yours is excellent considering the timing of jeff just having put his now TCd car back on the road:)

how is the emanage handling things MazdaManiac?

From what I learned, closed loop control means your outputs are dependent on your inputs, whereas open loop is independent of input. Is that what you're getting at Hymee?

open loop still uses inputs and gives outputs......it just doesn't use o2 derived A/F adjustments........ie feedback loop

Actually, I think you mean your input is dependent on your output (in closed loop) not the other way 'round.

jds

From what I learned, closed loop control means your outputs are dependent on your inputs, whereas open loop is independent of input. Is that what you're getting at Hymee?

What do you want from me, I failed my design class. :rolleyes:

I have an interesting set-up thanks to Acosta Motorsports. They have installed a Super AFCII air fuel controller. This is similar to the VTECH controller for those former Honda people as myself to adjust the AFR for multiple RPM points. To tune properly, they just installed an AEM Wide Band O2 air fuel gauge. This gauge is very accurate, in fact real time AFR readings, which is what is suggested for AFR tuning. I have been playing around with unit and "feel" some improvements. I have not had a chance to dyno, but I have made adjustments to low throttle as well a high throttle to add some fuel where the AFR was too lean. The air fuel gauge is mounted on the dash next to the A pillar. I am having fun monitoring since the gauge is digital but also has multi LED's, green (rich), yellow (stoich.) and red (lean). I am on a never ending quest to keep improving a good thing !

I'm really shocked at what I typed in my last post on this thread (it was a long while ago!!!). I'm not afraid to admit it though.

I remember typing that post after a long conversation with CZ about how Closed Loop can be tuned. I'm a bit shocked and embarrased that I let him convince me of some of that.

Closed Loop
By definition, Closed Loop mode means the computer is constantly monitoring the Air:Fuel Ratio (AFR), via the 02 sensor, and making fuel delivery adjustments to maintain the "ideal" air/fuel mixture (Stoichiometric) for complete combustion. This is an important function for reasons of efficiency and the environment. It is a feedback loop - if something happens to make the AFR richer or leaner, the computer puts in less or more fuel respectively. The only way to trick the computer would be to fudge the O2 sensor - but there is no point to that at all. It is a highly accurate device, and what it is detecting is very valuable info.

The engine is running in "Closed Loop" mode most of the time, except in "heavy" acceleration and also decellerating on a closed throttle. More on this later.

Open Loop
This is when the driver is commanding "power". The best AFR for power is richer than "stoich". And things are changing so rapidly (i.e. RPM rising, Air flow rising, Throttle position changing rapidly etc), that the computer does not really have time to react in a "feedback loop" manner. Hence the loop is no longer closed, but rather "open". In this state, the computer is looking at all of it's inputs (RPM, MAF, TPS, Temps, BARO etc...) and working out how much fuel to dump in for each injector squirt by looking at a fuel delivery schedule table locked away in the PCM somewhere. It doesn't take note of the AFR and make any adjustments - as there is no feedback.

Just where does the computer go from Closed Loop to Open Loop? Simply put, light throttle will be Closed Loop, and heavy throttle will Open Loop. A scan tool (such as the sCANalyser) can tell you directly if the engine is running in Closed Loop or Open Loop, as there is a mandatory ODBII parameter for it. But you can also tell by looking at the AFR. In the RX8 we already have a wide-band 02 sensor, and if we monitor that, or even any other o2 sensor shoved up the tail pipe or stuck in the eahaust manifold, we can easily see when it drops out of closed loop into open loop, and vice versa. How? Well when it is in closed loop, you see a nice AFR of 14.7:1 (or Lambda = 1.0) or very close to that. As soon as the AFR reading deviates much from that, you know you are in open loop. Simple!

I hope this clarifies my position. I am a firm believer in not tuning the car in closed loop. It is totally pointless, and without merit. By definition, the car is tuning itself in this condition, and all of my experience with logging RX-8 real-time data shows it does a very good job of it.

Cheers,
Hymee.

NOTE - I have not logged an "exact" Throttle Position where the transition from Open to Closed loop operation happens. Nor do I believe it will always be at a certain throttle opening. It probably depends on a number of inputs, such as TPS, MAF and RPM. It is probably more of a % Calculated Load type of thing, rather than a single physical sensor input.

I have an interesting set-up thanks to Acosta Motorsports. They have installed a Super AFCII air fuel controller. This is similar to the VTECH controller for those former Honda people as myself to adjust the AFR for multiple RPM points. To tune properly, they just installed an AEM Wide Band O2 air fuel gauge. This gauge is very accurate, in fact real time AFR readings, which is what is suggested for AFR tuning. I have been playing around with unit and "feel" some improvements. I have not had a chance to dyno, but I have made adjustments to low throttle as well a high throttle to add some fuel where the AFR was too lean. The air fuel gauge is mounted on the dash next to the A pillar. I am having fun monitoring since the gauge is digital but also has multi LED's, green (rich), yellow (stoich.) and red (lean). I am on a never ending quest to keep improving a good thing !


When i talk to marcus at acosta he says you should see about 9 hp the most.i was going to have him install it and dynoing it but told me right off the bat.you will not get much.so i decided to stay away.

9 HP is the same ballpark to what we have seen on the AU cars and a piggy-back. So a few of us agree that the best use for a piggy-back would be in FI.

Cheers,
Hymee.

Sorry to resurrect this thread but I was going through it and am still unclear as to the sub-thread between Hymee and MazdaManiac on the PCM still reading to the WBO2 sensor in open loop. Doesn't this mean that the PCM always operates in closed loop? If not, then for what purpose is PCM still reading the sensor?

The PCM operates in both closed loop and open loop. This can be verfied with an ODBII scan tool such as the sCANalyser. Even watching the O2 sensor readings you can see when it is drops out of closed loop into open loop, as the AFR alters dramatically from stoich.

Cheers,
Hymee.

This is where the fuel trims come into play: the computer records the adjustment it last made for that condition, and it is looked up first - sort of like a level 1 cache - and hopefully gets the AFR right the first go. If it doesn't get the AFR right the first go, it "Trims" it (via the closed feedback loop), and stores that adjustment in the Fuel Trims, to be used next time that condition is reached.


This cache-lookup behaviour seems to explain something my wife first noticed on our '8. Shifting upgear on a certain rpm, you can actually notice a tiny anomaly in the sound on the exact rpm you shifted up on, after downshifting and reaccellarating past that rpm (shortly after the upgear shift that is). Probably the gearshift event causes a suboptimal entry in the cached table for that particular rpm/load. It gets passed that suboptimal entry too fast to actually retrim is guess, but the effect is noticeable.
This "memory effect" only occurs in a short timespan, so the cache expiry time is probably short.

This cache-lookup behaviour seems to explain something my wife first noticed on our '8. Shifting upgear on a certain rpm, you can actually notice a tiny anomaly in the sound on the exact rpm you shifted up on, after downshifting and reaccellarating past that rpm (shortly after the upgear shift that is). Probably the gearshift event causes a suboptimal entry in the cached table for that particular rpm/load. It gets passed that suboptimal entry too fast to actually retrim is guess, but the effect is noticeable.
This "memory effect" only occurs in a short timespan, so the cache expiry time is probably short.
Sorry to go off-topic but :

Who the hell are you Barf??? Are you Leo???

Fabrice

Sorry to go off-topic but :

Who the hell are you Barf??? Are you Leo???

Fabrice

That's me allright! I never manage to stay anonymous. ;-)

For those of us with a turbo, it would be great if we could tune closed loop so we could get rid of that crappy dongle. How difficult would it be to spoof the O2 sensor.

That's me allright! I never manage to stay anonymous. ;-)
Hi Leo,

The <<Dutch RX-8 club webmaster>> gave it away a bit...

F

So in closed loop, if we are wanting to try to get better gas mileage, we'd have to have some way of changing the target A/F ratios the PCM is trying to achieve. I don't think it's 14.7 to 1 since if that was the target, I think we'd be seeing better gas mileage. Has anyone logged just their normal driving, steady state cruising and since it's closed loop what A/F the car was hitting? If it's 14.7 to 1 or clsoe to it and people are still getting 14 to 18 mpg, then, well, that sucks balls.

So changing exhaust and intakes should actually make your mpg go down since if your in closed loop mode for 85% of your driving it's looking at your MAF and O2's and then trying to hit its A/F targets in the maps for it's throttle position and rpm...correct. IE, if you swap intakes and it does breath a bit better or change exhausts and the overall breathing efficency of the engine is better, the PCM can tell more air is being sucked into the engine by the MAF. It uses that new MAF voltage for the incoming amount of air and adjusts the fuel to hit it's target. That was the drawback with the FD, using the speed/density system and not MAF, it didn't know when you changed intakes or exhaust since it wasn't actually measuring the amount of incoming air, just a table of values based on the speed the car was going and the density of the air. Seems the MAF sensor in a NA car would be better suited to show minute changes depending on the calibration of the MAF to changes in air and the voltage it take to see that change.

Am I off here?

Tim

So in closed loop, if we are wanting to try to get better gas mileage, we'd have to have some way of changing the target A/F ratios the PCM is trying to achieve. I don't think it's 14.7 to 1 since if that was the target, I think we'd be seeing better gas mileage. Has anyone logged just their normal driving, steady state cruising and since it's closed loop what A/F the car was hitting? If it's 14.7 to 1 or clsoe to it and people are still getting 14 to 18 mpg, then, well, that sucks balls. The PCM works in term of Lambda when it comes to AFR control. But you're correct, if you change the target values to run leaner, you'll get a better FE. And the target IS Stoich lambda (=1) or 14.x AFR.


So changing exhaust and intakes should actually make your mpg go down since if your in closed loop mode for 85% of your driving it's looking at your MAF and O2's and then trying to hit its A/F targets in the maps for it's throttle position and rpm...correct. IE, if you swap intakes and it does breath a bit better or change exhausts and the overall breathing efficency of the engine is better, the PCM can tell more air is being sucked into the engine by the MAF. It uses that new MAF voltage for the incoming amount of air and adjusts the fuel to hit it's target. That was the drawback with the FD, using the speed/density system and not MAF, it didn't know when you changed intakes or exhaust since it wasn't actually measuring the amount of incoming air, just a table of values based on the speed the car was going and the density of the air. Seems the MAF sensor in a NA car would be better suited to show minute changes depending on the calibration of the MAF to changes in air and the voltage it take to see that change.
Am I off here?
Tim No, your MPG should not go down in the area where it runs in closed loop. Although what you describe above is roughly correct, you need to think in term reduced intake and exhaust losses. For the same BMEP (actual power), you actually ran at a slightly lower load and therefore you bruned less fuel.

Fabrice

So we know for sure the PCM is set to try to get a target A/F of stoich (14.x to 1) under closed loop? Is that something that had been logged by more than a few individuals for closed loop driving?

So if the PCM does have it's target A/F ratio of 14.x to 1 during closed loop, what's the deal with such crappy gas mileage. Does that debunk the myth that it the car was set up to run pig ass rich to save the catalytic converter and also the fact that some individuals were claiming they lost their engine to carbon build up? How could there be that much build up if it's running 85 to 90% (if not more) in closed loop mode.

Tim

I've seen close to stoich for closed loop in more than one '8. It doesn't debunk the myth about pig rich... catalyst protection strategy typically will only kick in open loop where the PCM is typically not targetting stoich.

I haven't read the latest on carbon buildup.. can someone else summarise the latest theory?

Tim,

Lots of us have monitored the AFR and in closed loop they run very close to 1.0 Lambda (Stoich). Especially those of use with a sCANalyser ;)

They do run too rich when under WOT or heavy load - i.e. open loop.

In closed loop on the highway at a steady 100 or 110 km/h, MAF v's Lambda v's VehicleSpeed logging shows about 9'ish litres per 100km fuel consumption in my experience.

Cheers,
Hymee.

14 tp 18 is in town miles and is based on how much driving vs sitting you are doing, how quickly your car warms up(cold it is in open loop)how you accelerate and how you shift.

on the highway under light load the car is in closed loop hunting stoich and easily returns 24 mpg or better when travelling at 55-70 mph in 6th gear

the problems with engine seizing are in automatics that have 1 oil cooler that are in extreme heat areas(las vegas phoenix lubbock etc) and sit in traffic alot or never get revved above about 35k rpm. its an oil issue not a fueling issue. dont be suprised if all 2006 ATs come with 2 oil coolers in the US

That is great to hear. I think I'm going to order the scanalyzer from Mazdaparts.com for the shits and giggles and log some long drives and also city driving, etc just to see what our car is doing. So like mentioned above, doing the intake and exhaust, maybe possibly the high flow cat, would theoretically help us get better gas mileage and save a few pounds as well? Since it's breathing more efficiently and not need as much fuel for the same HP output?

Tim

That is great to hear. I think I'm going to order the scanalyzer from Mazdaparts.com for the shits and giggles and log some long drives and also city driving, etc just to see what our car is doing. So like mentioned above, doing the intake and exhaust, maybe possibly the high flow cat, would theoretically help us get better gas mileage and save a few pounds as well? Since it's breathing more efficiently and not need as much fuel for the same HP output?

Tim
The difference in FE will only be marginal and far from offsetting the cost of your intake and exhaust modifications.

Most modern petrol engines require heavy fuel enrichment at high engine speed, high load for catalyst protection. Changing catalyst for a better one won't change anything unless you re-map the original PCM with a higher catalyst temperature limt (presently around 960°C).

However, using scANalyser will show you where the engine runs in closed loop, where it goes really rich so you can learn how to drive economically when you wish to do so.

Cheers,

Fabrice

I don't know if I'm disrupting the forum by reviving an old thread, but I'm having the "darndest" time finding a convincing answer to the question: Why do car manufacturers these days program cars to go open loop at WOT? Why not stay in closed-loop mode at WOT?

1. I've read a post saying that at WOT, the computer can't keep up with all the monitoring and reacting. This is hard to believe since stuff happening inside an engine is actually pretty slow (kHz) when compared to computer calculation cycles (mHz). Also, good O2 sensors are very fast. So I don't buy this argument unless I'm missing something.

2. I've read a post saying that it's to keep exhaust temps down to make the catalytic converter last a long time. Is this true?

Any other reasons?

both are correct. the systems cant monitor and react fast enough at higher rpms and you cant run stoich at higher rpms or you'd burn the cat. it is more of a hardware issue than a computing issue. but the PCM does use the fuel trims it learns during closed loop to trim the open loop fuel requests so it should be fairly accurate to what its asking for. in open loop the front 02 sensor acts mostly as a cat temp sensor to watch for over temp situs.

thank you for bringing up the older thread instead of starting a new one.:) anyone else that has question scan review this thread and hopefully find the answers they are looking for:)

No need to appologise for reviding an old thread!

Zoom44 is correct. The main problem (I believe) is that by the time the "last" does of fuel is injected / compressed / burned and then heading down to the cat, and then the O2 sensor does it's thing and taken it's measurement, the engine will most likely be at a different load point - after all, in WOT the RPM's would normally be rising pretty fast, especially in the lower gears.

I also believe the MoTeC can do fast lambda, and you can have a target AFR/Lambda table where you can enter "non-stoich" values such as required at WOT. I haven't seen this in operation, and I believe you also need to get the base mappings close to start with, otherwise to many iterations of the feedback loop would be requied, and then the problems as mentioned about with latency become an issue. If you had a computer controlled dyno/retarder, you could spend more time at each load point and get it accurate - either "manually" or automatically via this closed loop style of approach.

Cheers,
Hymee.

I don't know if I'm disrupting the forum by reviving an old thread, but I'm having the "darndest" time finding a convincing answer to the question: Why do car manufacturers these days program cars to go open loop at WOT? Why not stay in closed-loop mode at WOT?

1. I've read a post saying that at WOT, the computer can't keep up with all the monitoring and reacting. This is hard to believe since stuff happening inside an engine is actually pretty slow (kHz) when compared to computer calculation cycles (mHz). Also, good O2 sensors are very fast. So I don't buy this argument unless I'm missing something.

2. I've read a post saying that it's to keep exhaust temps down to make the catalytic converter last a long time. Is this true?

Any other reasons?Most manufatcurers still use switching HEGOs (0-1 V narrow band) sensors for closed loop fuel control. It is only suitable to target stoich AFRs. Anything rich or lean is impossible to target with it. And, as Zoom said, at high speeds and loads conditions, the fuelling MUST be rich, over-rich even, to prevent the cat temp to go above about 970°C as it would result in a reduction of cat longevity.

Now, the RX8 is fitted with a wide band lambda sensor. Why they haven't used it to run in closed loop in all run conditions is a mystery to me.

Fabrice

Thanks for all the great replies. It seems like latency is the issue. :)

I'm not sure I'm totally clear on all the functions of our stock wideband.

As I understand it, the(front O2) narrow band serves the typical closed loop feedback function to keep the car near 14.7. The front O2 sensor does not serve as a cat temp sensor.

The wideband rear 02 sensor serves as the cat temp sensor under pretty much all conditions and as an idiot check for the narrow band sensor. What I'm not clear on is whether or not the wideband is working in both open and closed loop or just one of them. I would think it wouldbe helping under the open loop scenario to better dial in the fuel under WOT. Does the wideband also report back under closed loop? I would think it would, just as a check.

Most cars get away without a wideband currently, so Im guessing the open loop monitoring serves an emmissions function (as it helps dial in afr and helps protect the cat).

I'm revisiting some of these issues in the context of some interesting LTFT observations I've been reading about, I'm thinking that most folks would see greater hwy fuel mileage by reseting the ecu before each trip.