The C54 L is more than capable of swinging 21" and even 22" props with authority. Hacker recommends 21" props for the C54 in their specifications. Adequate cooling is a must, I have not experienced heat issues with this setup or previous setups considering I live and fly in SE Texas. When performing maintenance of lubing gearboxes (~75-100 flights) I always take note of how the old grease smells and that it does not appear burnt from excessive heat.