I think a better way is to tap engine intake manifold vacuum. I had a
Precise Flight system on my Piper Archer that provided full instrument
vacuum to 12500 feet at full throttle. I couldn't think of a situation
that
would be an issue with that sort of backup. (If the engine quits while
in
hard IMC, you don't need a vacuum backup - you need a parachute.)

Better still, get electric instruments. Powering gyro instruments with
vacuum should have gone away long ago. Add an avionics bus standby
battery
that will power the panel longer than the gas will last.

Bill Daniels

I am not familiar with the Precise Flight system, but would expect poor
suction based on manifold vacuum at full power.

Venturi vacuum, which is also available from some carburetors, works well
at most power settings including full power; but is probably not adequate
at idle. Therefore, if carburetor venturi vacuum is used, descents and
approaches would require partial power to keep the gyros spun up.

I don't know whether this is a common feature of aircraft carburetors, and
possibly the suction source that Precise Flight uses, as the carburetors
I have seen that provided the second suction source have been automotive.

The Precise Flight (I'm not sure they are still in business) device tapped
the left rear intake riser on the Lyc O-360 with a tube mounted into the
rubber hose . It was manually engaged with a pull knob operated valve that
switched the vacuum source from the vacuum pump to the manifold tap. Of
course, both the vacuum pump and the manifold vacuum sources used the vacuum
regulator to deliver the right vacuum to the instruments.

I was surprised that it provided full instrument vacuum at full throttle -
but it did. Vacuum gyros only need about 3 - 5 inches of water column and
even a clean new engine air filter produced at least this amount of pressure
drop.

It worked extremely well.

Bill Daniels