My engine analyser (EI CGR-30P) comes on with the Master Switch. It does have RPM, Manifold Pressure and Tacho.
It is also connected to the fuel flow system and records fuel used at Start Up before the Avionics Master is on. It also communicates (RS232) with the IFD550 to give fuel usage and range info on the map display and Datablock.
My CGR-30P also comes on with the master. It is needed at start-up as it is the only engine instrument, including tacho.
Mine was installed in the Netherlands, but I seem to recall this is how it is designed to be installed according to the manual supplied, so I would expect anyone to do it like this.
This wiring instruction appears in a number of installation manuals.
For example, the Sandel SG102 AHRS gyro can be wired direct, per the IM, so that it is halfway to having stabilised by the time you get around to switching on the avionics and sorting them out. Mine wasn’t done that way because it does expose it to the starter motor spikes, and I judged that to be not worth the risk.
What I was wondering is whether there is a regulation requiring this type of wiring.
Peter wrote:
What I was wondering is whether there is a regulation requiring this type of wiring.
Maybe, I have no idea. I’m just wondering what kind of reason should warrant such a requirement?
Peter wrote:
Who installed it?
Mine was fitted by Mike Walker (retired BA Avionics Engineer), at the same time as other new avionics were installed. He did a good job. Everything worked first time after a fairly complex installation.
I’m just wondering what kind of reason should warrant such a requirement?
With a tachometer it is obvious – you want it to work when the engine starts.
Admittedly such an instrument will still stop working if you turn off the electrics master during flight, and that is reportedly a popular tactic by renters to reduce their billing 
But it is less obvious for an engine monitor which does only CHT and EGT.
In the meantime no avionics installer has made any input here and my enquiries elsewhere have not produced a response, so maybe the requirement is found in the STC IM only i.e. the STC won’t be approved otherwise.
The G500 STC for the DR400 has an option to have the system operated by a switch so you don’t have it connected to the electrical system for engine start.
However the standby attitude indicator is connected directly to the main aircraft DC bus bar.
I just got this from someone who found it on a US site:
