How do you verify AFR?
 

I'm starting to run into some issues with AFR inconsistency across dynos. A car I tuned a few weeks ago went to another shop today and they show 14:1 AFR accross the board, the LC-1 shows 12.7-13.1 on this stock 1.6 car. How do you know which one to believe?

I believe the car, not the dyno. Typically the dyno operators shock me by either admitting their gauge is wrong and they don't care, or they don't use a clamp in the tail-pipe for a good ground, and therefore have miserable readings.

Thoughts?

dyno operators HATE to admit the dyno is off.

"this is a $3,000 controller, we calibrate it weekly..."

:jerkit:

lol, good thing he didn't buy a $200 LC-1 that's accurate. Still, this stuff bothers me. I'd expect MER to run a tight ship with an accurate WBo2. Now I have to do more work on this tune so I can sleep at night.

A tail pipe probe will read a bit leaner than a sensor mounted further up in the exhaust at idle and light load. Generally the readings should start to line up as the engine starts to work.

If your wideband tells you 14:1 and you don't believe you're really that lean, easy test is to add fuel. If you make more power, you were too lean. Generally speaking, the 'correct' a/f ratio is the one that makes the most torque. That's the whole point of tuning on a dyno.

I had a dyno operator use my in-car sensor. He said the tail pipe reading can be up to a point leaner than the upstream sensor.

He couldn't do that today because it was a HP check. I put the car on the dyno a few weeks ago and everything looked good. The new "perfected" VE table numbers didn't change enough to move a full-point leaner. Thanks for the confirmation. Would you expect a stock 1.6 with 28* peak spark to make more than 100lb/ft at 14:1? I don't.

Does the car have a cat?

no

Sensor01 is a narrowband sensor. The generic sensor input levels are set so that the horizontal "middle" line for both AFR and Sensor01 is 14.7 AFR. You can see that when Sensor01 switches rapidly, my Wideband is hovering around 14.9.

The sensors are a couple inches from each other. Notice how much faster the NBO2 reacts. It's a solid 200-300ms more immediate.

https://www.miataturbo.net/attachmen...ine=1315841566

I guess my calibration is close but not perfect. Not that I can calibrate the PLX wideband controller.

Y8s, can you repost that and make the min/max values of AFR and Sensor01 match?

what's the point? narrow band doesn't tell you anything in the min and max values. min and max useable are basically what I set them to.

A narrowband is a nernst cell. The wideband is a nernst cell plus oxygen pump and control circuitry, so it's going to be not as quick to "respond" by nature. Interesting plot, but I can't say it's surprising.

And it's ok if it's slower too because the MS has an O2 sensor delay setting for tuning.

The log post was mostly to satisfy hustler's question of how to prove it.

how do I get my AFRs so nice and smooth? Mine oscillate all over the place :(

In you datalog as a whole, or just in certain points like idle? First I would make sure that the scale you have isn't making the oscillation look worse that it is. If it's oscillating a lot by .1 afr, then to me that isn't a big deal. The other option is to adjust the o2 sensor sampling rate.

idle:
https://www.miataturbo.net/attachmen...ine=1315858124

wot in 4th:
https://www.miataturbo.net/attachmen...ine=1315858124

scale is same as y8s. I'm running 550cc injectors on otherwise stock motor, so I think those fluctuations are perfectly acceptable.

actually yeah I guess he just has it zoomed in all the way, making the changes appear "smother"

his point was to show NB vs WB