Change contents BEFORE writing to DB

Hi all!
I want to process the post content before it is written to the database upon saving a post. This means I cannot use filters because they only work on rendering the data (when retrieved from DB)… I must thus use an action hook. add_action(‘save_post’, ‘myFunction’); only parses the post_id and not the content. does anyone have any pointers how I should take care of this?