Flight testing for engine change
 

In a few months I should be ready to test my RV-6A with a rebuilt O360. Previously I had a Subaru flown for 170 hours. Do I only need to re-enter phase 1 for 5 hours by notifying the local FSDO?

Thanks

You need to sufficiently flight test the new engine.
The minimum flight time is 5 hrs.

Flight test new engine
 

Thanks Mel, so I notify the local FSDO by letter that I am re entering phase 1 for 5 hours and include the flight area for testing?

Check your operating limitations.

When I changed out my O-290D2 for the O-360 I had to sign it back into to Phase I for five hours. However, I just read some new Operating Limitations and they specifically stated that the FSDO must be notified and the test area to be used. I don't recall if they wanted other details included in that letter.

Good luck with the new engine!

Ops Limits
 

The operating limitations specific to your aircraft trump any opinions. Read them and do as they say. There have been many changes over the years. But just about all of them have required working with the FSDO for a major change, which of course you have done with the engine swapout.
Besides throughly testing the engine, you really need to flight test the aircraft, gathering the new data such as Vx, Vy, Vso. Most likely this engine will have an impact upon flight characteristics and W&B that were different than the first. As Mel stated, 5 should be the minimum, but I suspect it could, and should, be longer.

Be safe.

Vic