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Closed Loop - Isn't it just chasing the AFR?

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Say your AFR is WAY off @ WOT. Your AEM Infinity ECU is in Open Closed Loop and chasing & correcting the AFR. Right? I mean it's not predictive, instead it's reactive so didn't you just cruise through cells with wrong values and ECU was behind chasing & correcting errors from the past?

How does the ECU go about actually correcting AFR in the cell you're in or about to hit?

Are these ECUs so fast that they catch and correct AFR and modulate injector during the combustion stroke?

It can correct pretty quickly, particularly at high RPM. Since the VE values are interpolated between cells, as you move to a cell that is too lean, then AFR starts to go lean, and the correction starts to go rich. As you reach the center of the cell, your correction will be almost all the amount of extra richness required. So yes, while it's reacitive, it's a computer so it's reaction times are much faster (and more accurate as long as the injector characterization is correct). Best bet it to tune the VE so the closed loop doesn't have to work so hard.

it's not during the combustion stroke, as there is a transport delay between when the combustion occurs and the exhaust gasses (containing the residual oxygen to be measured by the O2 sensor) reach the sensor and are scanned by the ECU.

First let's just clear up that in open loop the ECU will not chase any error - It just blindly supplies the amount of fuel it's programmed to deliver, regardless of the measured AFR. Closed loop is when the ECU will make corrections. As you've correctly guessed, this is a reactive process, not predictive, so there needs to be an error before it can be corrected, and there is some level of delay in the ECU applying trims. Basically the closed loop fuel control is not a bandaid for properly calibrating the VE table. The closer the base fuelling is, the less work the closed loop control needs to do and the more accurate the fuelling will be overall. Closed loop rally is there to pick up the pieces if small corrections are needed due to atmospheric conditions etc.

It's the same process the 'self mapping' ECUs use - they correct the values as they react to the irregularities, further refining them over time.

Ooops, Yes yes I meant to say closed loop that is doing the correcting. Thank you all for the detailed response. It's amazing what these ECUs can do and how quickly however once all fuel trims are dialed in the responsiveness of the vehicle is fantastic vs being sloppy.

On my 13PSI SC S2000, I just finished buttoning up yet another tune using the Greddy eManage Ultimate piggyback. I leveraged OEM ECU fuel trims for partial throttle tuning and got LTFTs to +/-2% and STFTs to be +/-3%.

Then I polished off the Open Loop operation with the help of my WB... so I'm @ .82 LAMBDA at WOT.