boost guage questions

[Brett Dikeman]

At 6:24 PM -0400 4/28/03, luke wrote:

>i just installed a boost gauge in my 90tq (kh, mac-07a) and had some
>intresting results... the guage reads -20psi (vacuum) at idle. when
>i rev it up a bit it goes around -10 then after 3000 rpm when i
>juice it the guage reads about +5 psi (boost). i didnt think it was
>supposed to read like that... whats it supposed to look like?

That's correct, but the gauge is mislabelled if it shows the same
units and scale for both vacuum and boost, and goes down to "30",
since as Huw noted, -14psi is a perfect vacuum.

Most boost/vacuum gauges are labeled in inHg(inches mercury) on the
vacuum side if they go "down" to about 30; if they go "down" to more
like 70, that's cmHg(centimeters Mercury).

Here's a decent picture to illustrate this, of an autometer vacuum/boost gauge:

http://jeepsunlimited.com/zj/kennebell/boost.jpg

(ahh, the wonders of Google's image search :-)

Note the:
-different units at the bottom
-difference in scale on the vacuum side due to the different units
-30inHG is almost 180 degrees from 15psi boost on the dial face.

Brett

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