I ran a ZDZ-80 with Mejzlik 26x10 prop and J&A pitts muffler. It would tach 6700 rpm. No issues, over 350 flights, no worn out parts inside. I know a flyer who had an on-board tach on a DA-100, it showed the engine unloaded 1000 rpm in the air on a 25x12 Mejzlik 3-blade. I would guess the ZDZ will also unload in the air and it will be in the 7700 rpm range. Another friend had the ZDZ-80 with the K&S tuned muffler (#89) and 12" header. His engine wasn't happy running the 26x10 prop (didn't sound smooth in the air). Switched to the Menz-S 27x10 - 6400 rpm and it sounded better and ran much smoother in the air. Perhaps the header length was wrong for the 26" prop and high rpm.