The C4B5 is rated lower (2575 RPM / 250 HP) due to the airframe/engine/propeller combo (e.g. Aztec) that it is intended and certified for. Specifically, I believe it was a propeller limitation if I remember correctly. If you look through a parts catalog, you'll find that the crank, cam, rods, and all other moving parts are the exact same part numbers as the D4A5. So, in the experimental world you should have no trouble running a C4B5 up to 2700 RPM and 260 HP on the RV-10, using the appropriate propeller, governor etc.
At least that's my understanding and plan to do the same thing!

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Mike Jimenez & Sarah Hammonds
EAA Chapter 33 Prez & Soc Chair
Marion, IA USA
RV-10 In progress! (N165MJ reserved)
-Emp & Wings complete
-Panel wired up, working on Fuse & Finish
-Blog horribly outdated (sorry)
-Too many distractions, we will finish the plane someday!!!
http://mikeandsarahrv10.blogspot.com/
http://www.eaa33.org/