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Im about to go 550s on jdm Atisto. Considering 550s are 25% more than 440s., Im a bit confused as I expect O2 sensor to do some trimming. I do have an safcbut would prefer to go one step at a time. Any guidance would be most appreciated
 

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You absolutely must have some kind of piggyback like an SAFC if you want that to work well at all. What you are suggesting is not 'one step at a time' but doing one third of the needed work for your goal. You will need the 550's, an SAFC or similar piggyback, and a wideband O2 sensor to get it anywhere near properly tuned. Also, if you have not installed an upgraded fuel pump, that is highly recommended as well.

Closed loop fuel trims will only adjust for ~15% of the difference and then only in lower throttle areas of the map, O2 sensor feedback is not applied to in-boost/high throttle mapping areas because the targeted air/fuel ratio cannot be accurately tracked by the old narrowband O2 sensor.
 
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