I logged STFTs today and it goes to .82 and stays there, which, again if you multiply that out it is 11.5 AFR but this is a from a narrowband. My wideband was reading the same as it normally does anywhere from .86-.9 (which is super lean and I don't think the pistons would survive being that lean...). I'm welding my bung on the new midpipe tomorrow night and will put the new sensor in and let you all know what the results are.
If it helps, ECT were in the high 170s, and IATs were under 100 degrees (closer to 80 degrees). Total Timing is always around 14-15 degrees ...it was a cool morning today.
Thanks for all the input.
STFT is the short term fuel trims. It going to .82 means that short term the long term trim is lean, and the car is richening up. As Wicked said, .75 is the rich limit on the car for STFT, so it can not enrich (add fuel) past that. Your lean vs. rich understanding of AFR is also backwards, AFR larger than 14.7 (standard stoich) or in your case 14.03, is considered LEAN afr. AFR less than that, is rich. 11.5 for instance is pig rich, much richer than needed. BUT STFT is not a lambda value so you cant compare it directly, it is a trim, that the ECU uses to add or subtract fuel, STFT lower than 1 means that the ECU is adding fuel to hit the fuel target, STFT larger than 1 means that the ECU is pulling fuel to keep the target.
What this means, and what I should have noticed before but was skimming, is that your either running rich, OR your sensor is reading rich. You said your lambda was .86 to .89, that is rich AFR. Rich is better than lean to some degree, but can still cause cylinder washing, etc. but wont melt a piston the same way.
TLDR; your STFT supports that fuel is being added to hit your target lambda. Do you have the ability to log anything named Lambda or Lambese from the SCT tuner? IDK if its on cars that are factory narrowband, but it would be nice to see the targets set in your tune if you can see them, as well as what the car is seeing and doing on the narrowband sensor.
Another thing to check, you could try and change the narrowbands, and see if your STFT goes closer to 1, if they are throwing off the reading, and causing it to richen up, that could explain some of what you see. Do you recall which STFT you looked at (there should be bank 1 and bank 2, one of which is driver side and one passenger side). Are both the same?