I had an interesting discussion with other pilots about converting my
Piper Cherokee 6 to all electric. This would basically require
replacing the AI and DG with electric AI and elec. HSI. So, then,
there would be no use for vacuum pump. But, one pilot said that the
certified elec. AI only allows for installation of the unit, and that
the FAA would *NOT* approve full removal of the vacuum instrument if
it was delivered and certified with the aircraft. Any experiences on
this?

My goal would be to put an elec. AI and HSI in the panel, dump the two
vacuum guages and also dump the vacuum pump (some weight savings!).
Has anyone done this successfully?

Aircraft manufacturers are doing it, but only with some redundancy in the
electrical system.

Call your FSDO and ask them if they would approve it on a 337 as a matter of
equivalent level of safety or better.

Make sure that after the modification you can fly for min 30 minutes on the
battery using essential equipment. The new electric AI and HSI will be part
of that essential equipment. You'll need to find out from your battery
manufacturer what the Amp-Hour rating is at 30 minute and 60 minute
discharge rates. Add up all your essential loads for night IFR and see if
it fits. Consider putting a larger capacity battery and in general sprucing
up your electrical system cables, terminals, etc where anything is wearing
out.

Take a look at what Diamond Aircraft does with their DA-40. It's got a
emergency battery pack full of a bunch of alkaline C cells hidden under the
panel. If you lose your alternator, you get power to everything from the
main battery. If the main battery dies, you flip the emergency battery
switch and the attitude indicator and some panel lights are run off this
emergency battery pack for another hour or more. Relatively inexpensive
compared to the cost of a new electric AI and HSI and would help you get it
approved. Batteries in the pack are automatically replaced at annual.