What should idle vac pressure be in Hg?

 

20-22 hgm i think at idle

 

I used to get 20-22hg, but now I get about 18hg. What could be the cause of this?

 

mine is at 18-20 at idle. I think mine is a little low because of the mile of intercooler piping and intercooler the air has to go through, My compression numbers are good. If anyone has any other ideas it would be appreciated

 

normally a low reading would be due to piston rings not holding compression. Do a compression check to make sure. also your Idle might be off and will give you a inaccurate reading on the boost guage. your idle should be around 700- 800 rpm.