I stumbled back across the Thirdgen Prom Tuning Guide Book - what a resource! I need to not forget about it again.

And what do you know, in chapter 5, under the 7730 ECM, there's a topic titled "P730: BPW and Target AFR, how it is used and calculated at WOT in PE Mode". Perfect!

Curiously, the author states that target AFR is only a factor of stoich and the two PE modifiers (temp and RPM). Either I'm missing something big, or this statement isn't quite true, as the target AFR is clearly less than stoich on a cold start when I'm not in PE. I'll skip over this inconsistency for now. Later, the author describes how injector BPW is calculated - it's (grams of air per cylinder)/((target AFR)*(injector flowrate)). Target AFR and injector flowrate are hardcoded into the tune, but grams of air per cylinder is a function of MAP, EGR partial pressure, cylinder volume, VE, and MAT. Cylinder volume is fixed, VE is fixed, and MAP is essentially constant at WOT.

However, I can't believe I didn't think of MAT before! My first two runs, which had the same MAT, still don't perfectly agree (avg norm BPW of 8.88 and 9.12), but the big outlier is the third run with an avg norm BPW of 8.39*. There's still something here I don't understand, but it makes sense that MAT is a big factor. Apparently there's a lookup table that goes into figuring out the MAT's contribution to cylinder air mass. "MAT Compensation Counts vs. MAT" (0x7FD / L87FD for my bin). My XDF gives no description, so now to dig a little deeper to figure out what this does and how to calibrate it.


*I realized I wasn't accounting for the slightly-different BLMs in the three runs (133, 128, 129). I think multiplying by an additional factor of 128/BLM truly normalizes BPW for the purpose of this comparison.