guys,

i was wondering... what are your reading of vacuum on idle ?

mine seems to be a little bit high side comparatively...

i've seen my fren ones running at -0.39cmhg should be BAR i guess..
some of my fren also running at -0.43 bar...
my country has it most of them is using BAR... or CM/hg.. not sure it's 43 mmhg or not..

but my reading shows 0.29cm/hg or my boost meter writes -0.4 bar.

does the vacuum level indicate any health of the engine ?

 

http://www.onlineconversion.com/pressure.htm

-0.4 bar = -11.8 inches mercury, which is pretty bad for a stock port or mild port motor.

What rpm are you reading this value at?

 

Trying to use an intake vacuum reading to gauge the health of your engine is basically a crapshoot (unless it's really low and you suspect a fragged engine). There are too many variables that can affect vacuum including intake leaks, idle speed, throttle plate position, etc.

 

this is something im curious about as well. ive heard one should see 16 psi of vacuum at idle. is that correct?

 

You mean 16 inches of mercury, of course. That's about what mine reads at 750 rpm.

 

^ maybe i do. from what you said, i dont think i know what im talking about.lol. I was under the impression a boost gauge should read a set # of vacuum at idle? i may be very off. please correct me.

 

Yes, there will be numbers in vacuum. However, the vacuum reading is in a different measurement which is usually inHg, not PSI.

 

mine reading at 900 rpm...
EBC reads -29cmhg

 

just wondering how does it related to the health of the engine....

 

^ i would assume it proves how healthy the compression is. back to my stupid question. so what should i see on a BOOST gauge at vacuum? anything?

 

It doesn't really, unless you have a baseline of 'your' engine when you know its in good health.

 

Im curious aswell, my FD has good compression but when idle at around 750ish it will sit at 14inHG and occasionally drop to 12inHG and back again..

 

Unless you know what inHg intake vacuum your engine pulled when new and fully broken in, and at exactly what idle rpm, and you're absolutely positive you haven't made any changes to the throttle plate position or air bleed screw position, and you're absolutely positive you don't have any intake leaks, it's not going to prove anything. Try dropping the idle rpm just 200 rpm and see what it does to the intake vacuum reading...you've gone from "my engine's compression is healthy" to "OMG, my engine's popped".

On a standard non-metric boost gauge, with no intake leaks or excessive air bleed screw settings, and a smooth idle at 800 rpm on a stock engine, you should see between 15-19 inHg.

 

mine then is around 12 to 13 inHg then...
just a curiosity.. i dun have any problem with my engine

maybe it's my air bleed valve opened too big :P or maybe something else :P

sorry for digging some of your curiosity up :P

but it's a good discussion

 

The only sure-fire way to gauge engine health is with a compression test. Low vacuum readings could indicate a vacuum leak just as much as poor compression. If anything, changes in vacuum readings only indicate that something is wrong. Also, electrical load seems to have an effect on my vacuum readings, for example, when I turn on the A/C. I usually pull somewhere around 15-16inHg, and when the A/C is on, it drops to 13-14inHg.

At one time, my vacuum readings were 18inHg. I had some boost issues, and a bad IAT sensor. After removing the UIM and swapping out some hoses, my vacuum had dropped to 16-17inHg. I messed with it some more, and eventually it dropped to where it is now. I believe now that the source of my problem is in the UIM gaskets, as they did get a little bent up when they were out of the car. But when I first noticed the drop in vacuum, I was almost sure that I needed a rebuild!!



*edit*
I see now that I have provided information that has already been posted....

I hate it when I do that.

 

19inHg in my stock engine for 7 yrs for me.