Thanks for the detailed post, not sure how you got all that down so quick! They key here that you hit is "the AFR in the cylinder" is not the AFR in the pipe. The SC makes this even more challenging since we have lots of overlap in spots, and that nice fresh air is being force fed down the exhaust.
Ultimately the dyno is the best for this. You know when you go leaner and lose power you're too lean, and likewise if you go richer, stop dropping, and start misfiring--too rich. I was just amazed to compare the dyno AFR reading, to the onboard AFR, to what we estimate then from the dyno as the 11.5-12.5 best power range. Stock sensor was totally useless as it indicated as high as 14xxx for 12.5, and had little to no variation from there down to rich limit.
But at the same time you do get some correlation with some of the tunes we have done. Just a complete toss up since it is likely dependent on the boost, type of boost, and cam phasing.
Quote:
Originally Posted by arghx7
I know I just threw out a long post and people are like "so what, what reading I believe? what should I do?" Let me throw in there that I'm all for having more instrumentation if you're willing to tolerate the science-experiment factor. If you can get a backpressure sensor, a pump-current type (Bosch, NTK) sensor in there, EGT, whatever, go for it. The question is--well now what do I do with this information? How do I interpret it and make decisions?
I have no doubt that one sensor in one location said one thing, another sensor in another location said something else. But what happened inside the cylinder ? We can make some general statements, but without a lab worth of equipment, I say: who knows? And even in a lab, it doesn't always reflect the reality of what's happening in the vehicle on the road.
One of the root causes of all of this hand-wringing are widely accepted internet rules of thumb for tuning. Rules of thumb are there for a reason, and they are still grounded in accumulated experiences of many people. But think about why we're having this debate.
Perfect example: "Don't go leaner than this AFR," subtly implying that you're a bad tuner/you don't know what you're doing/you're going to blow your car up if you are outside that range. That's not a bad kind of rule. We need rules like that, or nobody would accomplish anything doing day-to-day work. The natural result then becomes: "my x sensor at y location says I'm at 11.5, but what if I'm really at 12????? do I have a bad tuner/are people going to accuse me of being an incompetent tuner? Is this engine going to blow up?"
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