FAR 91.205 Equip Requirements
Wow, this is a tough crowd.
FAR 91.205 prescribes equip required for a given mode of flight, eg VFR day, VFR nite, IFR. Elsewhere in the FARs is found the recurrent inspection and maint requirements for such equip, including nav, static, mode C, xpdr, ELT, etc., regardless of whether the airplane is type certificated or experimental. For IFR, the PIC determines what nav equip is required for a given IFR flight.
Part 91 required backups? Fly hard IFR with only a single EFIS for flight instruments, and no backups, and you may be dead right all the way to the ground, in the event of malfunction.
I would recommend that anyone really needing detailed answers on this subject matter consult the EAA website, elsewhere, when/where it pertains to experimental aircraft.
We all get on this site to exchange info re experimental aviation in a friendly, constructive manner, right?