wrote:
Anyone have any experience with or comments about the manifold pressure
based standby vacuum system vs a separate electrical driven vacuum
pump? Other than the price difference. I want to install a backup and
am interested in getting some info.

I have the precise flight vacuum backup which relies upon the difference
in manifold pressure Vs. static pressure. It was on the AC when I bought
it but since there is a wet vacuum pump on the AC (highly reliable) I'm
curious as to why it was installed. There are several ADs out for the
precise flight alternate vacuum system which makes it more of a PITA
than anything else but it does work and it is fairly fool proof. The
thing you have to keep in mind is that you have to have sufficient
difference MP VS static pressure so if you are up at altituded running
wide open and your pump fails you will end up having to decend to a
point where you can obtain this difference to run the gyros.

If it were me I'd put the extra coin into having an electrical driven
backup pump system.... especially if I did not have a wet pump.