Chris - The first flight on my airplane was last May and I have just over 100 hours on it.... RV8, Showplanes fastback conversion, XP400, and Whirlwind 74HRT prop.
After some initial troubles that turned out to be the result of an assembly error (cam to crank timing was a tooth off) the engine/prop have performed well though I've always felt there's a bit more vibration than expected. It's been dynamically balanced at 2300 RPM and while fairly smooth there, it's a bit "buzzy" above or below that RPM. Not bad but noticeable.
Regarding the prop, within 5 hours of the first flight, I noticed a "scar" on one of the blades near the spinner prop cutout. As you know, the spinner/prop assembly is fabricated by WW as a pair so I immediately called and discussed the situation and forwarded photos and some accurate measurements of the scar (approximately 2 inches long and not deep enough to cut into the carbon fiber).
WW said this was a non-issue from a structural perspective so with their concurrence I continued to fly. They said to send it back over the winter and they would "0-time" the prop and refinish the effected blade. Due to the XP-400 buy-back, I pulled the prop and engine this weekend and will ship it back soon. I spoke with WW about the slight vibration issue and they indicated they would take a close look at the prop for issues that may cause this (though it may or may not be prop related).
VERY happy with the customer service/support from WW! Also, WW recommends the HRT over the 74RV for engines over 200 hp.
Lastly, regarding the XP-400, they're all subject to the buy-back program and Superior wants them all out of service. I'm working with Barrett on an IO-390 replacement. It will be interesting to see how the 390/74HRT combination compares to the 400/74HRT.
Good luck with the rest of the build!
