nginx +unicorn+rails+"upstream prematurely closed connection while reading response header from upstream" 502 bad gateway error

Question

I've been scratching my head over this 502 gateway error while converting a video(approx 10mb) using ffmpeg. files of less than 2mb works fine when I try to upload + convert using webrick ,it works fine. I have client_max_body_size 1G set.

for testing purpose I have added these in nginx conf file

proxy_connect_timeout 600s;
    proxy_send_timeout    600s;
    proxy_read_timeout    600s;
    send_timeout          600s;

    proxy_buffer_size           32k;
    proxy_buffers               4 32k;
    proxy_busy_buffers_size     32k;
    proxy_temp_file_write_size  32k;

    fastcgi_buffers 8 16k;
    fastcgi_buffer_size 32k;     

timeout for unicorn is set as 
timeout 300 , in unicorn.rb

Please help.