I am not sure you would see the actual, full vacuum while monitoring the PCM. Doesn't the PCM have a function that tries to scale the MAP signal for altitude. Might be a job for a dedicated vacuum gauge.

 

Just read the MAP at Key-ON and again at WOT. IIRC it is about 3 kpa per inch/hg of vacuum.

 

What I am saying is as you scan when an intake part of too restrictive you see vacuum in upper RPM ranges. You see KPA fall off. Here at sea level it is sometimes as low as 94-95kpa or worse up near shift points. Obviously uncorking that is worth good power.