Another good use of a vacuum gauge would be to determine of your air filter, mass air body, throttle body and plenum inlet are flowing enough to supply the engine. If you are at full throttle, max rpm and do not read zero vacuum then you have a restriction supplying air to the engine.
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88 347, Twisted wedge heads,Comp Cams 224/230 cam, 1/4=10.718 @ 123.02 mph dyno=
http://www.fbody.com/members/LarryS/mustdyno.jpg