I see several people tuning to run open loop. I notice that my car runs so much better in OL than CL. Are there any downsides to running OL all the time?

 

I think most of us, after a few years fo tuning begin to realize a statement made by Grumpy many many years ago.

"14.7 AFR is for the converter, not the engine".

It's true. Some motors like to be leaner, some like to be richer.


The only obvious problem with open loop, is you won't get any o2 correction. So if something is drasticly different from one day to the next, the car simply wont fix it self.

What would be ideal is, to modify the ECM to use a wb-02, then change the routines so the target AFR is true AFR.

But then again, some engines might like different AFR's at different rpm/loads.

-- Joe

 

Well put.

If i had a driver today i would be working out an open loop tune instead of typing about it :-)

Gonna give it a whirl and see what happens

later
Jeremy

 

Which table are you modifying to keep the car in open loop?

 

For your '90 running the $8D mask, set this parameter to $ff , 255 decimal:

RBob.

 

I'm just getting into this so bare with me.......but I don't understand the parameter. I see the min coolant temp for CL but I don't understand changing the parameter

 

If you change it to the highest value that you can ($ff, or 255 dec) it will not go into CL. It is a really high temperature that your engine should never reach, therefore staying in OL.

 

>I notice that my car runs so much better in OL than CL<
Perhaps something is wrong, such as:
too much erg,
one lame fuel injector,
too much spark advance,
AE incorrect
etc.
14.7, should run fine.
.
contactpatch

 

14.7 should run fine for whom?

-- Joe

 

I am one of those who is running open loop. Personaly I would not do it unless you have a wide band O2 sensor, otherwise you have no idea of where you are in your ATF ratio.

That being said, my car runs great in open loop! The major reason that I stayed in OL is my NB O2 was not correctly adjusting my BLM's and even if I changed the "window" in which the NB O2 saw as stoich one part of the MAP vs RPM would beither real rich or real lean.

In open loop it is at about 14.7 in all cruse areas (took about 20-30 hours and 65 changes to get there) and 12.5~12.7 in PE mode. Much much happier .

 

The car used to run great with the super ram and 74219 cam but I recently switched over to a single plane efi and the fuel is now way off. I haven't changed the timing tables any, but when I tune to 128 the car runs terrible.....backfiring through the intake....slugish ect

 

Sounds like the AE is way off. Might want to pump it up at the lower %tps and work from there.

-Brian P

 

Maybe I should specify some more. The back firing through the intake doesn't begin until right before PE. Would this be effected by AE....if so which table do you begin in? There are so many AE tables
Thanks for the help

 

In closed loop, the ecm forces richer and leaner then 14.7 so that it AVERAGES 14.7. The amount richer and leaner is to keep the cat converter worker. Some applications have to go to extremes across stoich to keep the converter working right. All this has noting to do with trying to keep the engine happy. An engine running 14.7:1 will be happier then one averaging 14.7:1.

 

If I'm not mistaken you can effectively tune a car to run 14.7:1 (or whatever you prefer) all the time in OL right? I'm having an 02 bung welded in today for some wideband tuning......we'll see how it goes.

 

I'm starting to think about switching to open loop. As doc1of7 said, in closed loop the ecm tries to average 14.7. So when I'm in closed loop, the actual a/f can swing from 18-20.0:1 down to 12:1. BUT in open loop the a/f ratio is steady 14.X, sometimes 15.x(etc) under different loads. It seems that an engine would like the a/f ratio to be steady. "Swinging" a/f ratio couldn't be good for the engine.

IMO,The downside to running open loop is having the a/f ratio change with conditions. In closed loop, the BLM's will vary with ambient temps, humidity, etc. I call the condition "blm drift" . When you are running open loop, there is no BLM. So there will be a/f ratio drift. Now that I have many hours behind a wideband, it seems that a/f ratio drift would be minimal. Really, it should drift a lot less than closed loop's wild a/f ratio dance

yes

 

I'm curious if anyone has collected data to really quantify the AFR swings. I would be *very* surprised if you'd see 20:1 <-> 12:1 swings on a stock motor with a stock tune. With the high sensitivity on the NB O2 sensors, the ECM no more than adds enough fuel to go from 14.9 -> 14.1 before it starts going the other way. My guess given a good state of tune and good sensor would be something well within +/- 0.5 AFR. Maybe even closer.... Can you imagine the surging you'd experience with a 20:1 to 12:1 swing??

Novass, you mention your NB was giving inconsistent results with varying MAP and RPM. You then mention you spent a lot of time to get open loop to 14.7 in all cruise areas. I'm assuming you were tuning VE tables to get things dialed in. Do you think the changes you made would "fix" the NB problems you were having? In other words, was the problem you describe really due to BLM cell changes trying to compensate for the VE tables? Have you tried turning the NB back "on" after these changes to confirm the previous behavior?

 

Yes I turned back on the NB after doing the tuning, actually that what my whole intent to begin with, however it did not help.

So I went back to open loop.

 

My thoughts exactly. Also, just think about a carb! It still can't compensate as well as the computer for certain conditions that can be programmed into the tune even in open loop, such as AE PE etc.

 

What AFR do you think roughly corresponds to 15 MilliVolts?.
It's lean maybe noy 20:1 but at least 18:1. And on the rich side 870mv, would be somewhere around 12s. Lots of variables from car to car so as not to be able to make any true universal statements.

WBs typically have some filtering in them, so you wouldn't see the full range of transiant AFRs.

 

Not universally a true statement. In Holleyworld.com it's somewhat true, but in say weberworld, they do an excellent job of being self compensating. It's the timimg end that gets EFI to outshine Webers. OK, cold start, and a couple of emission driven arenas.

 

You bring up a good point. Maybe we're "fooling" ourselves when we say "Hey, look at the WB output. I'm staying at a 'constant' 14.7. Open loop is the greatest....". Kind of like wideband rose colored glasses....

I'd still think if the engine were *really* seeing swings in AFR at the extremes, we'd experience a lot of surging. Not that there aren't vehicles doing that. It's just that those vehicles aren't tuned well or have bad O2 sensors. I'm sure we could add a heavy software filter to the NB O2 and get a nice consistent reading of 14.7.

Your point about the WB filtering makes me wonder how much "filter" is built into the NB sensors independent of the software filters....? Depending on the location of the sensor relative to the exhaust ports, maybe we're picking up individual "puffs" of exhaust???

Sounds like it might be time for a little experimentation. Now I'm even more excited about getting the WB!

 

Wow! I didn't think the NB would be that far off after the VE tables were set.

So, if you run the NB and WB simultaneously, you see the WB stay resonably close to 14.7 while the NB output changes? And, this change shows up as a "bias" relative to MAP and RPM?

I appreciate your replies. Your experience is making me question things I thought were pretty much set in stone regarding the way the O2 sensor works....

 

Well actually, when in closed loop the NB shows very low blm's saying that it is very rich and tries to compensate. The WB shows a lean condition like 16~17:1 , at the higher rpm levels.

At the lower rpm's (the same MAP level for both low and high rpm) the NB showed about 128 BLM's at higher 2800 ^ it showed way rich. So conversely the WB saw 14.7 at low rpm and way lean at higher rpm's.

 

As far as the NTK sensors go you can see individual "puffs" well above 3,000 RPM, with a scope.

 

My car is far from stock BUT the A/F ratios do swing,in that manner on all cars in CLOSED loop. No surging at all. That's just how closed loop works. Hook up an a/f gauge(like one for the a-pillar) , in closed loop the gauge will swing, open loop the gauge reads steady. The depth of swinging doesn't vary BUT the rate does change with the tune, that would be O2 xcounts. Closed loop does exactly what I said......

FYI, My tune is very good.

 

For years, all the GM service manuals, and magazine articles I can recall were written as if the C/L O2 was just dead nuts accurate, and would keep you at a perfect 14.7 all the time.

Running a WB, you find out real fast that things weren't nearly as perfect as we were led to believe. It's a real eye opener.

 

Many still do, with so many experts out there, you'd think one of them might try to get to the truth. When the mags went to hiring English majors to write tech stuff a dangerous trend began. And then some start OK, but, quickly get impressed with the fact they write for a mag., and then really go downhill.

Not too mention that missing the tune can generate a false indicated 14.7/rich/lean, with many single gas analysers.

 

I'd agree the AFR will "swing" in closed loop. I didn't mean to imply it wouldn't. And, I didn't mean to offend the status of your calibration. I'm just questioning (or rather, thinking out loud in electronic form in as non-offensive fashion as possible) whether the magnitude of these swings are "real" in that they're indicating the AFR that was "in the cylinder". And, that the behavior is exhibited and designed into "production" engines.

I use the "production engines"/"good tune" reference only because I think there's a lot of confusion about what closed loop calibrations are available in addition to how they should be changed for specific engine hardware changes. My ASSumption would be that the production closed loop behavior could be used as a guide. While not necessarily perfect, it should give us a baseline of sorts. I wonder how many people have really paid attention to the "production" system's behavior before making changes. And, while doing that, insured that the system's components were operating correctly (i.e. good sensor). I just wanted to avoid the person, like me, who might never have really looked close at what was going on in the production setting. Then, would bolt on a set of headers with the O2 mounted further downstream of the factory location, start looking *very closely* at what was going on after the installation and say "Well, that must be how closed loop worked before. I know I didn't change anything that would cause it to change......".

I guess I'd put knowledge of these calibrations in a similar light as many people viewed car "computers" in the early 80s. Few took the time to learn how to work with them or what they really did. So, common practice was to blame them for any problems you encountered before ripping them out. Not to say this was never the right thing to do or that the computer never caused a problem. Just that the understanding of *why* was missing. And, potentially, the real cause of the engine problems. In the case of the O2 calibrations, I don't think it's good to blindly leave the "factory" O2 cals in place after changing sensor location, exhaust, induction and/or fuel delivery systems. Then, when the closed loop system acts funny, say "that O2 sensor is garbage". (Then again, it may be garbage.... I think that's the case with NOVASS' sensor!) Just like we wouldn't dream of leaving VE and spark tables completely untouched after most engine modifications, I think there's a need to understand better what effects various engine changes have on the closed loop system and how to compensate in the calibrations, if possible. Ideally, this knowledge would serve us well in the event something like, oh I don't know, a wideband closed loop AFR algorithm were constructed....

Anyway, I've probably hijacked this thead enough on the subject. My intention wasn't to criticize anyone's work, car or experiences. And, my apologies to anyone who really has figured out the O2 calibrations and made the necessary changes. (You are excluded from the "we"s and "you"s I've thrown around above!!) Thanks for the responses! I've definitely learned a lot!

 

No.

https://www.thirdgen.org/techbb2/sho...30#post1749930

There is no way that closed loop is seeing large swings. If this was the case, we'd be able to plot nice sine-wavish curves of the injector pulsewidth increasing and decreasing. but it's just about steady if you log the data...

if theres one thing i believe more than anything else, it's that you can not translate the O2 mV value into ANYTHING related to AFR. You just can't. See the chart in that link. 900mV could be 14:1. or 12:1. or who knows. Log a car going down the highway in closed loop. You'll see O2 mV's into the 800. the car's not in PE, and certainly it hasn't just jumped to 12:1, just looking at constant MAP/RPM/inj pulsewidth tells us that.

 

20:1- 12:1 was just an example. It's not speculation that the o2 mv swings, that is why the ecm counts the condition. Cross counts is when the o2 crosses the 14.7 line. It is not designed to hold 14.7. It's designed to average 14.7 in closed loop.

 

If the o2 didn't swing then o2mv would be steady, it's not.....

 

In the archives at DIY-EFI there's some posts about how catalytic converters work, and how they need varing AFRs to work.

 

starting to wonder if BLM is just for "checking the swing"(would osculation be a better word?)..If so then one could infer that BLMs are a byproduct of catalytic converter function,yes?

 

I agree that there is AFR swing, it's the nature of the system.

My point of contention is that there are not significant AFR changes. I don't believe it's swinging between 13:1 to 15:1 and back or anywhere near that much. On the contrary, I suggest it's swining in a range from maybe 14.2 to 14.9 or so.

If I log data going down the highway at 80mph in sixth on flat land on cruise control, I can see a steady inj PW of (if memory serves) 1.4ms during steady TPS. So, presuming this could be as high as 1.44ms and as low as 1.35ms, that's about a 7% difference. If we assume the highest AFR to be 15.0:1, then 7% lower is 14.02:1. I just don't see the AFR getting out of the 14's (especially on the low end).

The datalog chart I posted in the other thread supports this. there are VERY few data points above 15.0:1, and the number of points below 14.0:1 are pretty well limited to WOT PE/AE.

I think I need to go do another datalog and a chart just like that but ONLY for cruise, steady state conditions, and see what comes of it. Maybe it will prove me wrong

BLM isn't really a byproduct off the cat operation, because the o2 sensor is mounted before the cat and really has no idea what the cat's up to. BLM is simply the correction factor applied to the VE table in order to generate O2 crosscounts (the AFR swing across stoich)

 

So your point of contention is what?, the *exact* AFR changes?.
Does it really matter if it's 15.3 or 20:1?. Or we can get obtuse and worry about if it's really 15.3 or 15.28.

And no, The BLM is to correct the average AFR to 14.7. The code and cal settings set the swings across stoich. There are various min and max O2 settings within the cal data. The cross counts in only in a way, related to what's going on. They're more of a diagnostic tool. It's the reading below the min O2 voltage, and above the max O2 voltage that actually govern things.

Least that's what Doc says.

 

I guess we agree for the most part but it's terminology.

My point about the BLM is that the O2 sensor is in no way responding to/giving data on the cat. you could hack the cat off and not much would change (ignoring backpressure, exhaust temp, clean air reversion back into the pipe...) thus my point being is that it's not a feedback of what the cat is doing, but is certainly involved in keeping the cat alive (by helping to give the afr swing needed by the cat).

If the value of the AFR swing doesn't matter, then i guess the whole thing is moot. i just don't think that the AFR is swinging wildly because at steady TPS/MAP/RPM i have steady inj PW.

don't mean to be obtuse.

 

I never said that. What I said is that BLM/closed loop is designed to average 14.7, for what? To keep the cat happy.All my diagnostic books show a good o2 mv varies from APROX 300-600mv, there is even a chart. That's for a non-modified car, general diag. So in a round-about kind of way the BLMs are just a % number of how well the ecm swings past stoich, to keep the cat happy. It's all related to emissions, closed loop that is.

 

I must add that it's cool that we can disagree and discuss things w/o it getting "messy". This board has changed quite a bit

 

Alright, I have to chime in again.....

I'm not trying to split hairs. Or, obtain any particular geometric orientation. But, I originally asked my questions *because* I thought the implication was that the *actual* AFR was swinging 20:1 - 12:1 in "properly tuned", "normal closed loop", "all sensors working" vehicles as per one of the earlier posts. The quote I originally replied to was :

Not that the general effort to keep things smooth isn't a good goal. Just that things aren't as "not smooth" as what's being implied.

If the real NB swing as we go from 100mv to 800mv is only 14.3:1 to 14.9:1 (or something similarly small, say within 1 AFR of 14.7), it would appear (to me) the drive for people going open loop to "smooth things out" instead of "averaging" really should be well thought out. (Again, I'll separate NOVASS' experiences where the sensor really is doing something strange.) At a minimum, an understanding of the tradeoffs between open and closed loop should be gained. Getting a "smooth" O2 mV reading might not be worth the loss of adapting to operational changes. Especially when the smoothness is only eliminating 0.5 AFR changes.

91L98Z28, thanks for the link to your other post. That graph is phenomenal!!! :hail: The picture is worth 10,000 words as far as I'm concerned. That's exactly the information I was hoping to see.

Again, thanks to all for the good discussion!

 

One major reason people decide to go open loop is that they have modified engines and/or no cat. As many have said, 14.7:1 is primarily an emissions/economy and cat-conv thing. Not every motor will be happy at that ratio.

Regarding "So in a round-about kind of way the BLMs are just a % number of how well the ecm swings past stoich, to keep the cat happy. It's all related to emissions, closed loop that is." ----
I'm not sure I follow, unless you calculate percentage as (BLM/128)/100. BLM is how much 'correction' must be applied to the current VE cell in order to (eventually) obtain an inj PW close enough to stoich that minor variations in PW can cause crosscounts to increase. The BLM is a gross/macro adjustment and in most scans of my cars, stays at one value for a given MAP/RPM/TPS. The INT value might flutter around a bit though.

It would be very interesting to plot the INT against O2 values. If the INT correction factor is fine enough, there should be a correlation (unless there is a third, finer factor I'm unaware of that the code uses to vary the ultimate PW in order to generate crosscounts).

 

The INT and PW should follow the O2 to cause the O2 mv to cross 450. Basically, the O2 reads 750, the pw drops a bit (maybe a really small change, like .05 ms?) to make the O2 go to a value below 450 (say 250?). This is accomplished by changing the INT a small amount. The INT can drive the PW a little bit.

When the average O2 reading is below 450, say 350, the BLM will increase to add a bit more to the PW to try to get back to AVERAGING 450, and the opposite for above 450.

This is the way the STOCK system is SUPPOSED to work.
In the newer, OBD2 cars, they change BLM to LTFT (Long Term Fuel Trim), and the INT to STFT (Short Term Fuel Trim), along to changing the values to 0, then a eprcentage of change from 0, +, or -. It's the same thing, just a different way of seeing things. I'm sure that the code for our older ECM's could be rewritten to reflect such values, if someone REALLY wanted it that way!

HTH
JP

 

One thing that I tried to say there, but didn't is that the changes may just be small enough that we don't see them in the PW displayed in the datastream, they may be as small as .05 ms, and that may be enough for the O2 to swing.

Clear as mud?

JP

 

I don't think you understand, the o2 sensor is deleted with open loop. No o2.


O2 "drives" the integrator, int controls BLMs and blms are just a part of the final PW calculation. Might as well think closed loop is for emissions contol. My new slogan," 128blms= the point when the catalytic converter is happiest."

 

Don't forget about the rate of data capture....or is that what you mean?

 

Yeah, that too, along with the fact that we may not be looking at enough dec places to see the changes.

 

May be saying the same thing here but just a different way, but I thought the INT had a small amount of control to let the O2 swing across 450 mv, then if the average falls off 450, the BLM kicks it to bring the average back? That's the way I was taught by a drivability instructor, but I doubt he's ever look at the first bit of source code ....

 

Not trying to nit pick, just good discussion, but I think the real slogan is "14.7:1 is where the cat converter is happiest".

Why?

Well, lets say you got this stock running vehicle, it's perfect, 128/128 blm/int across the board. so, you've got avg 14.7:1, cool!

now lets say a devious evil "hot rodder" person comes along and wants to tamper with the emissions system, and they install a tamper device that allows them to modify the fuel pressure to an unauthorized high value. Well ok, let's say that the injectors now flow 10% over their rated value because the fuel pressure is higher.

What happens?

Well, at 128/128, suddenly the O2 sensor quits generating crosscounts, because we are now rich. instead of averaging 14.7:1, we're averaging about 13.3:1.

Then, the ECM realizes that the O2 sensor is stuck at a high mV reading, and the BLM's start coming down...

When the BLM hits about 115 (10% less than 128), we start achieving crosscounts again. The effective, tailpipe AFR is now back to avg 14.7:1..the cats are happy again.

BLM is not controlling the final achieved AFR - rather it's the feedback loop adjustment factored into the PW calcs to achieve O2 crosscounts. As long as you are getting rapid crosscounts, the AFR is averaging stoich no matter what the BLM - 110 or 148.

 

Exactly, except that the INT averages a rich mixture, which makes the BLM update to make a *more* permanent change.

The INT is to make the O2 readings swing, and the BLM adjusts to keep the O2 swinging at 450 mv.

Sorry to hijack the post with all this stuff about the way closed loop is supposed to work, but I think it's important to know this before deciding to run open loop because "It's easier". No, I'm not saying that experienced tuner's that run O/L are taking an easy out, just that C/L has it's place, as does O/L, and some vehicles just don't do well with C/L, where other's do well.

 

Closed loop is an easier tune to make. The VE tables can be way far off and the vehicle may still be driven. Disconnect the O2 sensor (forced open loop) and see how well it drives.

As for cross counts and such, the proportional gains are set high on purpose. They are used to destabilize the feedback loop. This is what causes the fueling to switch about the stoich point. The INT and BLM are used to center the fueling about the stoich point. Of course this stoich point is defined by some voltage terms in the calibration, and may or may not truely be 'stoich'.

The GM ECMs (of the C3 and P4 vintage) have an injector PW resolution of 15.26 usec.

RBob.