what does a manifold pressure gauge tell you. when is it needed. please help me on this if you can. input does help

 

For one I would say that with that gauge it is a good way to avoid having the eng. bust at the seams when the boost gets to much, depending on mod's your limit will vary .

 

The boost gauge u have is messuring your manifold pressure. They are just two different names for the same guage.(manifold press) (boost) So your all ready covered.

 

Manifold pressure, also known as "boost", is an indication of how well your turbo is working.
Higher boost allows more fuel to be injected, allows that fuel to burn more thoroughly, and helps keep egt's down.
Each turbo is designed to be efficient within certain parameters. Generally a stock turbo is efficient to about 35psi.
Knowing what your boost normally is, helps you find and fix some problems, such as loose or blown intercooler boots.
Too much boost (45psi+) on a stock motor will eventually blow the headgasket, thats the reason some guys use studs and o-rings.
I hope this helps, and go ahead and ask anything else you can think of.