is it normal to have my vacuum gauge thats mounted in dash to go down when i step on the gas then shoot back up after a second? it idles around 15, then whne i step on the gas it goes down to like around 5 or so and then jumps back to 25 or so! is this normal?

 

it should be the opposite, seems like the gauge if hooked up wrong... it should be at 0 at idle and increase from there

 

Wrong the gauged is fine. How the could you tune idle mixture if the gauge always read zero? Why would they sell vacuum canisters for cars that don't make enough for power brakes? How in gods name would the pcv system work? Or vacuum advance?

The gauge dropping when you step on it is right too. My T/A idles at 15 in of vacuum too. Your fine. Your reading vacuum because the cylinders are creating low pressure by sucking and the trottle blades are closed. Open the throttle and now the air is moving through the manifold much faster.

 

Damon is 110% right! I've always tuned my cars using a vacuum gauge to achieve maximum inches of vacuum at idle. One of the gauges mounted on the hood of my '71 is vacuum, and I watch it often while driving. If I'm careful to try and keep the vacuum at it's highest peaks during takeoff and cruising, I can actually see my mileage go up.
Most modern mileage readouts on the dash of today's computerized cars are based on vacuum readings. As you accelerate the vacuum goes down, as does the mileage.

 

25 pounds of vacuum must be nice. I can't get over 15 at idle with mine thanks to the cam.

Your gauge is working fine just like Val (1971bb427) and Damon (77Nomad) said.

 

My stock cammed 403 Olds only pulls 15. Hmm then again the brakes do suck lol.

 

I know i am wrong, and understand y... so was i thinking of boost then? or am I completely misinformed on something?

 

Well it depends. Turbo cars can and usually do run at idle with vac. Then when you stomp on it the turbo spools up and now your into boost. vacuum is negative and boost is positive. A roots blower should be into boost all the time. Centrifugal blowers I'm not sure about but I would think its more like a turbo than a roots.