The Winrar v3.6 readme has the following section in it:
Multithreaded version of RAR compression algorithm improves
the compression speed on computers with several CPU,
dual core CPU and processors with hyperthreading technology.
Multithreading is enabled by default, but you can disable it
in "General" part of "Settings" dialog.
In the command line mode you can control multithreading with
Haven't tried it myself though ....