Front wideband accuracy
 

Just an FYI for you self-tuners out there.

Our initial base maps were done with a wideband in the tailpipe, and generally we see the stock sensor pegged at 11 while our wideband indicates 12-11 or so. This is with expanded scaling using hypothetical numbers. Stock mapping is good only for 12.17 at the richest end.

So lately we took a few cars on the dyno that have less traditional maps. Innovate with small pulley e85, NA but with the 3" MAF, and a few remote folks utilizing the new rear o2 functionality of ProECU. This way we can directly see the AFRs imported into the datalogs. Results?

Well, lets just say this. We won't remote tune your FI FT86 unless it is equipped with a proper wideband or you're willing to eat the motor, or it is 100% stock and similar to the OTS mapping. We're seeing pegged rich stock meters while widebands are showing 13. Last year we might see this a handful of times on local cars, but we'd figure it's post cat or just an unusual one. But now we're having folks put these into the rear o2 port on non-catted setups, and we see the same things. Dyno numbers confirm that we lose power being too lean in those cases.

If you have experience with this please post so we can get a better idea of how off they really are. But we're not going to use the stock wideband for anything except NA setups. We may even do a rental wideband you can clip into the rear harness from now on.

any good and easy wideband setup you suggest for N/A users.

AEM Failsafe if you are boosted is such a no brainer.

for N/A instead?

Best bet is to look for a good brand aftermarket wideband that has an analog 0-5V output. That way you can hook up your regular in car gauge and have an additional output to connect to the rear O2.

http://www.ft86club.com/forums/showthread.php?t=57902

In the next few days I'll log OEM AFR and aftermarket wideband to see how different they are.

Innovate, zeitronix, or aem. Just pick one with a 0-5v aux out. No need for failsafe with NA

This is always a hot button topic. I see widebands, any wideband really, as just another tool. This issue of "accuracy" is even more complicated than most people realize. The reasons are: backpressure issue, engine-out AFR vs exhaust stream AFR, and transient effects.

I've done some informal studies on this issue using lab data. I compared a Bosch LSU 4.9, a Denso Plus 5.1 limiting-current type wideband (similar to what's the front sensor on the FA20), and an emissions bench in a steady state. This particular engine was an n/a, and the data was running a series of steady-state points for a WOT run. Basic result: they were all within about 0.3:1 in most areas, but yes the Denso seemed to deviate a little at the richest AFR's. That's steady-state, lab conditions, enriched operation, no scavenging to speak of.

There are backpressure compensation curves to keep in consideration. AEM now offers a wideband with a backpressure sensor to use, which corrects the value based on the characteristic curve provided by Bosch. That's assuming you don't burn up the backpressure sensor.

http://www.ft86club.com/forums/attac...1&d=1394373505

Now here's where things get nuts: on a boosted engine, especially with a lot of scavenging. The wideband doesn't directly tell you what's going on in the cylinder. You can have a wideband read lean at the tailpipe, in the midpipe or wherever. But when you start looking at the fuel flow meters, the airflow meters, the emissions analyzer CO2%, O2%, and CO%, things get pretty tricky.

When you have overlap (especially with boost), fresh air gets thrown out the exhaust valve and is never trapped in the cylinder. It mixes with the CO and other gases. So literally the AFR changes as the gases are passing through the exhaust pipe. You can only see that with an emissions bench in a lab.

In an actual vehicle it gets tricky because those Denso A/F sensors are actually corrected by the rear o2 sensor located behind the cat. It basically uses the rear o2 sensor and an oxygen storage capacity model of the catalyst to compare what the front o2 sensor is reading and what it should read, based on the response curve of the rear o2.

http://www.ft86club.com/forums/attac...1&d=1394374240

All these complicated effects are why I don't get too caught up in which one is right and which one is "wrong." I do have more faith in the Denso sensors than a lot of people do though.

See attachments with technical specs on Bosch wideband sensors, and an earlier Denso limiting-current type.

How does it apply to ppl that haven't EcuTek and use other software for logging?
I mean... can I just change the OEM Wideband with an aftermarket one that is better and more precise?

@jamesm looked into this when he did full close loop.
It's ridiculous how far off the previous forum golden childs had it so wrong.
A lot of the old dyno plots prior to 6-8 months ago had 13-14 AFR at WOT....

I know I just threw out a long post and people are like "so what, what reading I believe? what should I do?" Let me throw in there that I'm all for having more instrumentation if you're willing to tolerate the science-experiment factor. If you can get a backpressure sensor, a pump-current type (Bosch, NTK) sensor in there, EGT, whatever, go for it. The question is--well now what do I do with this information? How do I interpret it and make decisions?

I have no doubt that one sensor in one location said one thing, another sensor in another location said something else. But what happened inside the cylinder ? We can make some general statements, but without a lab worth of equipment, I say: who knows? And even in a lab, it doesn't always reflect the reality of what's happening in the vehicle on the road.

One of the root causes of all of this hand-wringing are widely accepted internet rules of thumb for tuning. Rules of thumb are there for a reason, and they are still grounded in accumulated experiences of many people. But think about why we're having this debate.

Perfect example: "Don't go leaner than this AFR," subtly implying that you're a bad tuner/you don't know what you're doing/you're going to blow your car up if you are outside that range. That's not a bad kind of rule. We need rules like that, or nobody would accomplish anything doing day-to-day work. The natural result then becomes: "my x sensor at y location says I'm at 11.5, but what if I'm really at 12????? do I have a bad tuner/are people going to accuse me of being an incompetent tuner? Is this engine going to blow up?"

Well... looks like the innovate mtx is going back on.

Is the secondary o2 sensor port an acceptable place for a wideband?

Thanks for the detailed post, not sure how you got all that down so quick! They key here that you hit is "the AFR in the cylinder" is not the AFR in the pipe. The SC makes this even more challenging since we have lots of overlap in spots, and that nice fresh air is being force fed down the exhaust.

Ultimately the dyno is the best for this. You know when you go leaner and lose power you're too lean, and likewise if you go richer, stop dropping, and start misfiring--too rich. I was just amazed to compare the dyno AFR reading, to the onboard AFR, to what we estimate then from the dyno as the 11.5-12.5 best power range. Stock sensor was totally useless as it indicated as high as 14xxx for 12.5, and had little to no variation from there down to rich limit.

But at the same time you do get some correlation with some of the tunes we have done. Just a complete toss up since it is likely dependent on the boost, type of boost, and cam phasing.

20 years ago, Fred from Electromotive told me something I will never forget: "an Air/fuel Sensor ratio doesn't read air/fuel ratio. But rather oxygen presence in the exhaust and that they are all calibrated differently," Simple and obvious but after some thought you begin to realize that focusing on a number or comparing one sensor to an other and calling one "inaccurate" is faulty logic. What you want is repeatability above all else. If you know that the you get most knock resistance with best power X:1 nominal AFR with Y sensor, you shoot for that assuming you don't make any big changes to overlap or boost pressure. That may equate to Z:1 AFR on another sensor but that doesn't matter.

My full time closed loop setup uses a wideband imported into ecutek, not the stock sensor.

I've found that using ecutek's recommended expanded front o2 scaling, the thing isn't just off, it's inconsistent. Sometimes it'll read leaner than a proper wideband in the same car, sometimes richer, and never by a little bit. Suffice to say it's pretty useless for tuning anything. The most common case is the front o2 pegging rich while the wideband is reading something leaner, though it's not always that way.

I've always said if your tuner doesn't require a wideband for FI e-tunes they're doing it wrong. Nice to see someone else agrees..

as @moto-mike is saying, in the end you have to do what's practical. How is the dyno and the knock reading responding to whatever AFR you are reading on your given sensor? The more instrumentation you can compare, the better.

While I'm thinking about it, here is another comment on the cam phasing and what is actually happening in the cylinder on a turbo DI engine:

For those of you who have tuned a boosted EJ and played around with cam timing, more boost, whatever, you expect to see a certain AFR. You have to remember that the port injected fuel gets thrown out the exhaust valve during overlap & open valve injection, but for DI it doesn't! That's raw unburned fuel that can affect readings.

Nobody has come up with a RAM value from the FA20 ECU or a calculated parameter to determine the end of injection timing for the FA20's PFI system. If the end of injection occurs during an open intake valve & overlap, you've got all that unburnt fuel possibly reacting in the exhaust manifold or otherwise skewing the wideband as I said.

This is why if you have an engine dyno you would look at your CO2%, CO, and O2%, and HC ppm. In full stoich part load, CO2% is very very close to wideband AFR, and can be a good indicator of in-cylinder AFR during scavenging too. O2% is related to scavenging pass-through, and can be associated with reactions occuring in the exhaust stream that change the mixture. CO is probably the simplest metric for enrichment. Before wideband o2 sensors existed, all you went by was the CO%. HC ppm can correlate with unburned fuel passing through the exhaust valve during PFI scavenging.

So that's the PFI skewing AFR thing. Meanwhile, the DI start of injection event is timed past the scavenging crank angle window. So that fuel isn't thrown into the exhaust and isn't skewing the reading--this is a major reason why DI turbo engines spool better than port injected engines. They are tuned from the factory for more scavenging because fuel pass-through isn't an issue.

And as a further side note, if the injection system is sophisticated enough there can be multiple injection events for knock relief. The FA20 doesn't seem to do this, but many other DI engines can. Here is among the most sophisticated on the market, the M133 2.0 engine by AMG/Daimler using Piezo A-cone injector:

http://www.ft86club.com/forums/attac...1&d=1394399833

So you've got different things going on actually inside the cylinder depending whether it's PFI, DI, or PFI + DI due to injection and valve timing.

very informative thread thankyou to all

We require a proper wideband for FI tuning as well. I don't trust the stock sensor for anything richer than 12.2. Even then there are inconsistencies.

- Bob

You can calculate this. Take TDC as your starting point. Then add you IPW + latency and you can convert that to crank angle degrees with some math. Generally when factoring in latency (and the distance of the injector nozzle from the valve) I think we're safe in saying that overlap shouldn't play too much a role with PI on the FA20 with reasonable IPWs. With DI though it is a rougher calculation since you can specify start of injection much more precisely with the DI angle, and as a result have a much higher chance of hitting that overlap point. The variable here is that even with overlap, since the piston is moving down you may not necessarily have the fuel go out the exhaust valve. It may move down the combustion chamber instead. You'd need to do some flow analysis to really find this out, but this is exactly the kind of situation we can see on the dyno, and where the AFR in the cylinder vs pipe becomes important.

I don't think its necessary for a supercharged car. You don't have to worry about a boost spike.

Not absolutely necessary, yeah, true. Still good to have as it let's you know everything is working as intended and can let you troubleshoot a problem in the future.

For example, there is no way of telling if you are hitting target boost (only other way is upgraded Map sensor and OBD2 reading).



Whitefrs hates my phone signature

so while using the stock 02. a low 12, should be the best area to keep it?

If you're forced to use the stock a/f sensor, I would think the safe way to do it would be on a dyno, and increase fuel delivery until you start to lose power from too much fuel. Then dial it back a bit.

A decent wideband costs less than an engine, though.

that would jus foul your plugs. you should study tuning a bit more before messing with it. ive edited my cl and ol differently

Running the fuel delivery for maximum power would foul plugs? First I've ever heard of that. Running super rich would foul plugs for sure.

And to further respond to your previous post, in this very thread, posters have mentioned how the stock A/F sensor reads very inconsistently at lower a:f ratios.

sry read your last post wrong, i was on a phone. i was getting my quotes confused because another andrew#### just tagged me in somthing lol but yes that would be better set o a dyno

Correct me if I'm wrong, but I think they're talking about front sensor unreliability for tuning in FI applications, not NA.

Spent a couple of hours wiring the AEM failsafe WBO2 and using our custom map to log the output (within .2 of what the gauge shows).

Note the huge discrepancy and the inconsistency. The AEM unit shows exactly what our dyno wideband was showing (but .4 points richer as it is pre-cat).

That huge spike that looks to be super lean at 15.8 but it is actually just 12.4...

where did u fit the sensor ??? front or middle of the car ??