rtprio + su - doesn't work

[Wojciech Puchar]

i run such command

/usr/sbin/rtprio 31 /usr/bin/su centrala -c \
"/usr/local/bin/asterisk -C /centrala/etc/asterisk.conf"

tu run (at startup) asterisk PBX as user centrala with realtime priority.

asterisk is started, but without realtime priority.

how to do this right?

i run asterisk as user (not root), but this server is used to other 
things, so asterisk must have absolute priority over other things.
now i have to do this manually by searching for asterisk's PID and doing

rtprio 31 -PID

thanks