Within the airflow, turbo section of the Cadillac ATS-V is the wastegate open boost table. This table looks like below. This may be the table Cadillac is thinking of when they specified that the ATS-V (under the right conditions) can hit 18 psi of boost.
The left axis is barometric pressure. At sea level that is 14.7 Psi which is 29.9 inHg. If you check the local weather often it includes barometric pressure. However, we are interested in non-altitude compensated barometric pressure. The LF4 has a couple of PIDS that relate barometric pressure; there is Barometric (SAE) and Barometric Pressure. Sometimes they are the same, sometimes they vary. My impression is they have different refresh rates
The columns are engine RPM.
So if you were at sea level, the table appears to limit boost to 15 psi at 1000 rpm, rising to 18.1 psi at 6750 rpm. So far so good.
What if you are not at sea level? Our local barometric pressure here in Texas at 700 ft altitude is 14.4-14.55 psi instead.
Here is the table with a new row added for an interpolated value at 14.4 psi.
The maximum boost available goes down with the delta between 12.3 and 14.7 (2.4) by 2.4 psi, so basically every 0.1 psi drop in baro gives us a 0.1 drop in maximum boost attainable.
Note that one can change the units for the rows to inHg or PSI in Hptuners, whatever is easiest for your analysis.
Now, actual boost at various RPM depends on how the engine is brought to that RPM. Turbos like spin time — a long pull on a dyno in 6th gear gives plenty of spin up time for the turbos. With only a WOT burst on a highway, boost will rapidly build but will not match the maximum values in the table until the turbo has time to spin up and pressurize the manifold.
Barometric pressure SAE during a WOT run for my ATS-V falls to 13.5 psi, while the barometric pressure PID stays around 14.5 psi. This may reflect vacuum at the intake of 1 psi? Regardless, if the engine uses the baro SAE it may further limit boot by a whole point, so a max of 14.7-13.5 = 2.2 and 18.2 – 2.2 is 16 psi max. That look like what people often see on the boost gauge.
For tuning, some examples I have seen simply max this table out. I prefer not to do that, as I would like controlled boost. If we are re-tuning for adding 3 psi of boost, one could simply add 3 psi to the 14.7 row. So if the ATS-V had pulled 14-16 psi in your conditions, it would now pull 17-19 psi in those same conditions. However, now the 2.4 PSI delta from 14.7 to 12.3 covers a delta max psi of 5.4, so more would be pulled at local conditions. We would need to change both the 12.3 row and the 14.7 row to add 3 psi to
Now that gives us 2.4 psi of boost delta over 2.4 psi of baro delta again, so that each 1 psi drop in baro gives a 1 psi drop in boost. If my engine is using a baro of 13.5 psi at WOT it will be 2.2 psi under 21.2 and max at 19 psi of boost.