I just googled 'pit bulls bite force' and a wealth of info popped up.. including psi pressure and comparisions with other breeds bite force.. Hope that helps.. have some fun searching with your keywords..
I would do a search on the internet. I can't remember any resources off the top of my head, but there are several books about it and websites. If you could find the books, definitely buy them, as they're always better sources of information.
You could also go to your local animal control and get statistics from your local area. Just remember those statistics aren't going to be 100% accurate bc of obvious reasons.