I'm ripping a DVD, and I can get mencoder to use 1 or 2 cores, but it won't take advantage of all 4!

If I don't give it a threads= it rips at about 45 fps. If I give it threads=4, or threads=8, or threads=auto it rips at around 80 fps (Should be doing about .9 * 45*4) top also shows me only getting to 200% CPU usage, not 400%. And system monitor never shows all 4 cores anywhere near 100%.

Please help! Have an Intel Quad Core Q600
mencoder -v\
        -alang en\
        -vf crop=720:352:0:66\
        -ovc x264 -x264encopts threads=auto:subq=4:bframes=4:b_pyramid:weight_b:pass=1:psnr:bitrate=1400:turbo=1\
        -oac copy\
        -ofps 24000/1001\
        -vobsubout subtitles -vobsuboutindex 0 -slang en\
        -o /dev/null
was the command I used to rip (after dumping the vob to hard drive)