MP4 encoding problem

Question

Hello all,

I'm encoding a wmv file to MP4 (h264) but the bitrate of the file is shotting to much. if i'm giving 256k it goes around 1025k or even 411k. How can we solve this problem using ffmpeg?

Here is the command line i'm using

ffmpeg -i "18959.wmv" -vcodec libx264 -b 256kb -bt 100k -s 640x480 - r 25 -aspect 4:3 -acodec libfaac -vpre ".\libx264-ipod640.ffpreset" -profile aac_low -ac 2 -ar 44100 -ab 32k -y "test.mp4"

I have checked with vpre no effect, event -bt don't effect any quality

Thanks.