Andrew Gideon wrote:

Dan Truesdell wrote:

For a 172, it's probably just as good then to get a
manifold-driven backup vacuum. Too bad the electronic AI's are not
certified for IFR (the $1500 ones, not the $6000 ones.)

One of our club airplanes has something like this. But someone told me that
it works least well under full throttle...which means down low executing a
missed approach.

Was I told correctly? How serious is "least well"?

That's exactly right. (From memory) the STC recommends you maintain at least 3.5
inches of vacuum to the gyros. That means the intake manifold pressure has to be
at least 3.5 inches less than the ambient pressure.

At sea level on a standard day, you will have to adjust your power to have a
manifold pressure no higher than (29.92 - 3.5) = 26.42 in Hg.

If you have a fixed prop and no manifold pressure gauge, you have to figure out
what that means for a throttle setting. There will be a calibration placard
giving maximum RPM for different pressure altitudes. At high pressure-altitudes
(low ambient pressure), you will need to reduce power correspondingly to
maintain the required vacuum to keep the gyros spinning.

Specifically, during a missed approach, you'll have to choose between keeping
the gyros spinning and having full power available.

My limited experience is that 3.5 inches is not enough for good reliable
instrument indications. That may be a function of which models of instruments
you have installed and how new the bearings are.

Of course, as I pointed out elsewhere in this thread, you don't get to practice
managing the vacuum under realistic conditions. Pulling out the knob on the
panel doesn't activate the system if you have a working vacuum pump. There is a
shuttle valve that selects the vacuum source that provides the most suck
(manifold or pump) when the panel control is activated.

Said shuttle valve is also a point of failure. You can check it on the ground
with the engine idling. At idle, the manifold will pull more vacuum than the
pump. I can count on the fingers of *no* hands the number of pilots I have ever
seen perform this check routinely.

At risk of beating a dead horse, get the dual-rotor pump from
http://www.aeroadvantage.com instead.