Ok, I am trying to make my own digital boost gauge, and I need to clear something up in my head. vacuum. now it makes sense that vacuum is actually negative pressure in the intake manifold from the engine sucking in air when turbo is not boosting. but I have heard elsewhere that intake manifold pressure is always positive, and vacuum is simply considered anything under 1 psi or so. anybody care to elaborate? maybe am i confusing absolute pressure and atmospheric pressure. like its always positive absolute pressure, but negative atmospheric pressure?

also, I can handle reading positive pressure, and while i know I CAN read neg pressure, i'm not sure how to go about it yet. how important is showing vacuum in a boost gauge. To me it seems pretty useless information as long as you know you're not boosting. any comments or corrections? should i take the extra time to get it to show it, or ignore it? thanks.

 

Hi,

Absolute pressure is gauge pressure plus atmospheric pressure. Gauge pressure is the pressure measured by a gauge less atmospheric pressure. Physics is cool.

I believe you will want to monitor vacuum as your car spends a lot of time NOT under boost. Changes in vaccuum at idle could indicate small boost leaks or other problems. Normal readings are 18 to 20"hg of vacuum at idle with stock cams on my Talon.

 

I think you will have a hard time showing vacuum on a digital guage due to the whole issue of atmo presure vs. absolute pressure and getting the necessary equipment to calibrate to atmo. I am assuming this is an elecrical gauge. Pressure should not be a problem.

 

what about when reading kPa? Im in negetives all the time when not under acceleration...?

-Shahul

 

my gauge is a digital that reads in dkPA (i hate using deca-kilo prefix). The vacuum at idle is usually around -0.40- (-0.50)

When letting off throttle at higher engine speeds I've seen it peak at -0.7x before. Seems completely normal.

 

even if you are showing negative pressure there is still positive manifold pressure........for example if your guage is showing 10psi of vacuum then you still have 14.7-10= 4.7psi of positive manifold pressure. if boost guages were calibrated for absolute pressure the guages would read from 0 to 40 psi or thereabouts. in that case under boost our cars would register a peak boost of almost 34psi. an engine is basically a pump. so when you are accelerating and suddenly close the throttle(close throttle body) the engine will still be trying to pump air through it. the obstruction of the closed throttle body and lack of boost from the turbo will cause the pressure to dip below atmospheric.