I run ZDZ's - - and using the new ignition tach setup -I get direct rpm readings -on the ground or saved from flying -as max rpm -which the tach will read out on landing .
because I don't have other engines to directly compare -I stick with just reading what the tach says.
The 23x8 that reads 6300 may be a very good number -for the conditions reported
How can you know?
My ZDZ50 NG is piped with a AMT pipe and the rpm numbers I have are accurate and repeatable -but obviously not under same conditions as the report above.
My 23x8 is a JAS -a wood copy of Mejzlic - which runs 7200 on the ground.
A Bambula 22x8 - 6700 and 7600 ground and level flying numbers
A Mejzlic 22x8 7790 and 8340 ground and level in the air. In 90 degree not muggy days -typical at our field in summer mornings at 4350 ft elevation.
The model is an EF YAK so ---- not a racer type
I like the 23x8 prop best tho the Bambula is very quiet and only suffers from less inital acceleration --