Put my ultragage in, and it shows 6.5 at idle, and 9.25 at 1500 under load. It was showing 15 out the back door at the track.

It claims to be reading/displaying as PSI. But this doesn't sound right at all, regardless of units. MAP should go down as the throttle body opens.

The only thing I can think of is my plenum gasket is gone and the oil or crankcase pressure is pressurizing the intake manifold?

Edit: I'm reading up, and apparently a MAP works inverted of a vacuum gauge. Not easy to wrap my brain around.

~ AP is 14.7 psi.
The present vacuum on the sensor is read as an absolute pressure (suddenly this makes sense!) relative to AP.

Thus, high vacuum = low pressure relative to AP and low vacuum = high pressure relative to AP.


I feel stupid now.

Now to convert PSI to InHG

PSI.......inHG
18..........36
15..........30
12..........24
9............18
6............12
3............6
1............2

Notice a pattern?

So 5 PSI with butterflies closed and 15 with them open = 10inHG and 30 inHG respectively.... equals 20 inches of vacuum.

So whatever the little display thingiemajig says, double it and subtract from 30 for manifold vacuum.