Tuning larger LT-5 cams

[XfireZ51]

Ron,

Running O/L forces the fueling to be based strictly on the VE tables. O2 sensors feedback to the ECM for pulsewidth mod for the Integrator and BLM calcs. There are some threads on ThirdGen.org that discuss Closed Loop tuning for larger cams. The greater overlap does cause fresh air to be detected in the exhaust by the O2. Consequently the ECM interprets that as a lean condition and adds fuel. That's why you see the BLMs drop to minimum 115 that is in the calibration. In fact, the INT goes lower than that. What most people don't realize is that a 128 BLM DOES NOT necessarily correspond to 14.7:1 AFR. When running C/L, you'll see the AFR toggling rich/lean in order to not oversaturate the cats. Probably a reason why yours may very well be shot.
The magic 128 BLM for C/L indicates where the O2 swing points are. The voltages can be manipulated based on airflow. It means you can modify the calibration to tell the ECM that what appears to be lean actually is not. And it can be done
in such a way to affect principally only the idle. Being able to use C/L with the larger cams also allows use of additional parameters such as proportional gains which make a difference in managing surging for example.
At Bowling Green, Graham said something very interesting, if I recall correctly, in that the LT-5 wants to stall. Increasing the overlap exacerbates the oscillation bringing the LT-5 into areas where it will stall. One reason why people raise the idle when going to larger cams. So being able to control idle more tightly becomes even more important when using larger cams. C/L presents finer control of the motor which is why I believe Graham suggested C/L for your emissions calibration.