Manifold pressure gauge?
 

The aircraft ones are insanely expensive. i want a proper gauge that shows 29.92" at engine off and reads down to at least 5-10 and up to well over 30. Anyone have a source that's cheap?

Why do you want one calibrated in inches of mercury? Do you need that level of precision?

For most of us, a plain vacuum gauge (shows the difference from "atmospheric", not really calibrated though) is cheap and works just fine.

Like these: http://www.amazon.com/b?ie=UTF8&node=15729801

-soD

Uh, 29.92-inches Hg is associated with BAROMETRIC pressure, not MANIFOLD pressure. Typically, MANIFOLD vacuum (normal aspiration)/pressure (turbo or supercharge) are measured in XX-0-14.7 PSI or ±0-30 "Hg...without decimal places.

I meant that with engine off it reads about 30, at sea level and shows absolute pressure just exactly like the one in every airplane with a controllable pitch propeller. I do nit need Kollsman window accuracy.

and when you subtract the difference between barometric and manifold pressure you end up with how much suction your engine is pulling. In a plane it is used for determining a power setting. if you are running the engine at 2300rpm and at 5000 feet and the manifold is showing 23" you are making the same amount of power as if you were at 10,000 feet and showing 23" at the lower altitude you would have less throttle.

For a car it is more difficult to get the relevant data as you are really after vac so you know when you are accelerating efficiently.

If your car has a MAP sensor and is 96 or newer an ultra/scan gauge will give you manifold pressure if you desire it. it will also show psi boost or suction which I found much more useful.

Ebay can be a good source for used gauges and such.

'Econogauges' used to be very common back in the '80s. Wreckers?

I found an absolute pressure gauge in McMaster-Carr for over $400. A little pricey.

Since the OP is close enough to sea level, he might be better off to paint new numbers on a standard vacuum gauge. Add an altimeter and do a mental conversion if ever drive in the mountains.

You can use MPX4250 it reads both vacuum and boost. Combinig that with an ARDuino and some LCD will give you a very acurate pressure gauge that can even calibrate itself to current barometric pressure during a startup.

Data sheet of MPX4250

Prices are below 15$ so it won't kill your budget either :)

One more thing... Combining arduino with any MAP sensor from any turbocharged car should do. You'll just need to calibrate that properly to get accurate readings.