Understanding the boost vacuum relationship

[tbaumann]

I have a question on wastegate and boosting. I have adjusted the boost up from 14 lbs to 16 lbs by shortening the wastegate actuator rod length and have noticed that the vacuum has dropped from 14/13 down to 12/11 at 55 mph. Is this normal?? Vacuum at idle is 16 to 18. I have an AEM TruBoost electronic booster. It is setup to run the solenoid provided by AEM. The vacuum does not change even if it is “off”
Can someone explain the reason for this ??

 

[Turbotre]

By tightening the actuator rod,the turbo is spooling a little more,even at 55mph.
More pressure equals less vacuum.

 

[tbaumann]

By tightening the actuator rod,the turbo is spooling a little more,even at 55mph.
More pressure equals less vacuum.

To Recap; When the rod is shortened, the boost is increase and it will spool faster (which I noticed)
I am seeing the boost start to trickle on at 70 mph while cruising. I assume this is normal because of the load the car is under at that speed.

 

[scottyb]

That sounds reasonable only in the sense that BEFORE you tightened the rod, the wastegate was so loose that it allowed bypass at all times. If this was the case, then the turbo could be spinning a bit faster now and causing "less vacuum". This could also be the reason it spools faster now, you've completely closed the wastegate until you reach the boost that causes it to push open.