I searched and found only few info on it. With a stock engine, my vacuum number was in the -20mmHg during the idle. But now with a built engine, when the engine is cold, it's in the low -17~ mid -18s. Once the engine warms up, the reading is in the mid -18s to low -19s.

Is it normal to have vacuum readings change by the temperature or do I have a vacuum leak? Also, seems as though -18 is about the average vacuum readings for the built engines, why is that?

 

I don't know this answer, but I would think it was because of the lower compression on the engine. My stock block held vacuum at 21-23 and fluctuated midly in colder weather, but never under 21.

No with that said, I think I remember someone telling me that 15-18 mmHG was normal with lower compression.

 

hmm my car only gets about 10 hg at idle. This reading comes off my hks evc VI

 

Mine is @ -18.1 at idle, and @ -17.4 under load near redline using UTEC data logs. I haven't data logged my engine while it was cold, so I'm not sure what the vacuum reading would be cold. My compression is @ 145 +/- across all cylinders.

 

I think its normal....at warm up I'm at the same as chris...but once its warm it goes to 18-20. 8.5:1 compression here.

 

That sounds about right.

 

How could your vaccuum be at -17.4 under load? Under load or WOT it should be at 0 or + if you are FI...

 

If I don't hammer the gas, I can stay in vacuum all the time.

 

Sorry, should have been more clear that I'm NA.

 

Its normal. The vac flucuates because as the temp of the engine rises, the rings seal better.

 

I have both Zeitronix and the HKS EVC VI. When my Zeitronix reads -18~-20s, HKS EVC VI reads about -8~-10s. I think the HKS EVC VI boost sensor isn't as sensitive or something..

 

The HKS is reading its vacuum in PSI, not in inches of Mercury. the conversion is roughly 1psi equals 2 inHg. so backsically, 9 psi of vacuum equals 18 inHg.

 

Yeah the UTEC is weird about this, if you look at the dashboard it reads -9 psi at idle? Its basically just saying 18 inHg of vacuum.