2011 LT, no super charger. Torque has a "boost" display, so I set it to look see. at idle it is showing -9.2 psi, when I rev it a bit it goes to -11.0 psi.

What do you suppose it is really measuring? Doesn't seem to be manifold pressure that is different numbers.

 

Vacuum somewhere ?

 

Vacuum...under a load that number will go to zero, atmospheric pressure...under lite/no load the vacumm created exceeds the A/P and will show larger neg numbers..good tuning tool once you understand how it works...the results of the pistons pulling in air on the down stroke...reading taken from MAP sensor...

 

OK, I drove it and you are right , it seeks to zero. So that would actually be manifold vacuum, versus the other manifold pressure PID?

 

Same reading from the MAP sensor I would think. Choosing the turbo/vacuum option just creates a display with positive values for when its in boost.

 

right..same as a vacuum gage..only it also goes positive press

 

I'll bet that 's why some SSs need the electrical vacuum pump for early use of brakes then.

 

That would be correct...cold start up has the turbo applying low pressure (about -10 on boost gage).. so vacuum is limited for build up until the idle settles down, then is shows around -20,..for some odd reason ,the brake booster loses its charge aftera few hours...