Gauges Help: What does this mean...

[GGKauten]

"Boost pressure 30 Hg vacuum/15psi"

I know this is probably a total noob question, but I know the typical boost gauge measures the pressure using the psi output on the display of the gauge.

However I do not understand these gauges where the 0 is in the middle and on the left you have 0-30 Hg, and then on the right side of the 0 you have the 0-15 psi.

Whats exactly does the Hg mean in terms of how the gauge functions and is this the most beneficial path?

The guages I was looking at can be found here...

[The basic PSI gauge...0-60 which is excessive cause I wont really go over 15 ever]
http://www.nopionline.com/nopistore/...ost%20Pressure

[The PSI/Hg one...still really confused...]
http://www.nopionline.com/nopistore/...ost%20Pressure

Any possible insight into this matter would be greatly appreciated!

[paultakeda]

A boost gauge is there to measure pressure. Pressure is measured in a variety of ways, and it's simply a matter of history that the boost gauges we buy measure pressure in PSI (pounds per square inch) when it is "positive" and in inHg (inches of mercury) or just Hg for short when it is "negative".

When a gauge is at zero, which is what is read when the engine is off, you effectively have the same pressure in the system as that of the atmosphere. At sea level, this is 14.7 PSI. When you turn on the engine, the needle drops to a "negative" pressure. At idle this reading is usually 20 inHg or if converted to PSI, 9.82 PSI. This means that an idle engine at sea level has a pressure 9.82 PSI less than atmospheric. The reason is you have the throttle closed, so the engine pistons are sucking at what little air is supplied so long as you don't open up the throttle plate, and therefore causing a bit of vacuum (hence that odd combination of connecting a boost gauge to a vacuum line).

If the car is stock, a max pressure of 15 PSI over atmospheric is all right. But if you intend to modify the fuel map program by "chipping" it, then you should get one that reads up to 25 PSI. And you should, of course, get a gauge that measures below 0 (PSI or inHg, doesn't matter so long as it is on the gauge), simply because when you are driving normally your car will never exceed 0 PSI -- the only time it does is when you accelerate or are driving at fairly high RPM.