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The factory stuff is only making changes to an overall multiplier in the short and long term trims. It's only "trimming" the fuel where it sees you drive it all the time in closed loop all the time - and changing the overall multiplier. At WOT it's going off the established fuel table - while using that same multiplier, but it can't change the table itself based on O2 sensor info. If it trimmed fuel at idle and cruise - it's going to pull fuel away at WOT too.

The modern standalone stuff looks at what your target AFRs are throughout the rpm range and load settings, and re-writes the tune to get it closer while it "learns". It learns at WOT as well as idle and cruise. Depending upon how much authority you let it have - it can change the fuel table slowly or quickly to get it pretty darn close. Of course, you can also pull up the logs and manually change the fuel tables to speed up the process.



It still has seperate fuel tables for idle and cruise and WOT, while the O2's are narrow band(unless factory turbo car) it still tries for a predetermend fuel trim from fuel maps in the software. Now it can alter the fuel maps over time from learning how you drive, but OE's are based on fuel milage and emmisions production and don't set for best performance.



My Dakota would slow down by 2 full tenths if I let the factory PCM "learn" while idling up the return road. It trimmed the multiplier - that also affected WOT operation. So every time I was in the staging lanes - I'd reset the PCM and let it start fresh. I finally just "forced" it to stay in open loop by unplugging the factory O2 sensors when I was at the track. It was very hard to go rounds with it slowing down every pass!