I know taking compression test would tell the health of an engine...but if compression test is not available could one tell by vacumm reading?

 

Yes, it's useful as a general indicator.

Just remember engine porting affects idle vacuum, as does the actual idle speed.

Dave

 

Also the position of the air bleed screw underneath the throttle body. The larger the opening, the more air that gets past the throttle body plates, and the less vacuum generated.

 

is there general #'s that u can follow? stock/ported?

 

also depends on if you run a stock air box

street port with K&Ns: 12-14 in Hg
race port: 10 in Hg
bone stock is more like 18 in Hg

 

about 15 and over is ok stock, not to many people are rocking 18 vac stock. Some are doing more but 15 to 16 is good.

 

im pullin 17 with a nice street port.... pretty funny

 

i run 12-14 on a huge street port

 

it's pretty useless...just do a compression check and know for sure

 

+1...as shown by the wild variance of vacuum numbers people are already posting...

Also, the vacuum reading will change with idle rpm as well. One person's "17 in Hg" will be another's "14in Hg...is my motor blown?"

 

Mine has been at about 13 inches at 750 rpm (stock build as far as I know) for the past three years. Every time I read threads like this, I used to think my engine only had days of life left.

I took my vert into the dealer to get some work done. He told me my engine had bad compression (and bad vacuum readings) and that I should be ready to replace it soon. That was ~7 years and 60K miles ago.

 

yup comp and vac readings are bullshit!

on my street ports with all new engine parts and full blue printed assembly (much higher tollerance than from factory) you will get 10 to 11" @ 1000rpm idle and 89psi comp after run in of 2000km.

Porting has a massive effect and as such no generalizations can be made except those by your engine builder who has experience of your particualr specification engine set up.

 

Now I think everyone will agree that you should know where your idle vacuum is, so that if one day it's not running right you can check to see if the vacuum has changed.

Dave

 

how do you check each face for compression? i only have a regular compression gauge...

 

Agreed, the only useful information is to see what the vacuum reads for your car, and then watch it closely for changes over time.

Just like the crank RPM has a huge factor in a compression test, so does the rpm/intake/port setup with the reading.

-R4tw

 

Exactly. My motor would usually pull 16". After a hard run, the motor was only pulling 12-13" and bouncing slightly. I knew it was blown before i even got out of the car even though i was hoping it was just the vac line from the MAP. Turns out it was a chip on one of the seals in the rear rotor.

 

i wish i bothered to look at the vacuum my motor pulled just after it blew.
i dont even remember for sure what it pulled when it was "healthy", erm i guess best way to put that is not blown.

had 130k on a original bone stock s4 t2. i think i remember about 14" which seemed really good for the miles and low compression s4 turbo motor. not positive though

 

so i guess i just hold the release button on the gauge and try to look at the reading really quick for the 3 faces...?

 

faq should hold what your looking for.
or just click here http://www.fd3s.net/compression_test.html

 

Dunno if it's been mentioned already but your vacuum will also change +/- to a large degree depending on what your idle A/F is. E.g. richer side of stoic will give you more vacuum, leaner - less, generally.

Comp readings bullshit! How so???

Even if they were (given compression tester result can vary widely), at the very least comrpession test is extremely important to compare compression on the front / rear rotors for example.