In the early 90's I had a few Fila Brasileiros. At the bite portion of the working weekend the guy being paid to test the dogs asked if anyone wanted to give it a try. I took bites from the Filas. They were somewhat hard mouthed, but it was a frontal bite, and it was a lot of weight (120-160lbs dogs) pulling down. Next was a Malinois about 80lbs. Hard mouthed, lots of tenacity, full mouth bite. The next was a 45lb. pit bulldog of a Snooty/Pool Hall red breeding. The bite pressure of the other dogs felt like nothing in comparison to the pit bulldog. He was sent from about 15-20 yards and he brought a load. The last dog was a 85-90lb American Bulldog. His pressure on the sleeve was more than the others but not like the pit, but he went airborne, hit the sleeve and knocked me on my ass. Quite impressive.
These are just one individual dog from only three or four different breeds. I can't say it crosses all dogs of every breed, but in this case the pit dog had far more bite pressure than the others. I doubt there is anyway to really test bite, like in pounds per square inch, or so many lbs of pressure, as there is no way to tell if the dog is biting all out. I agree with Jack, it is the desire to stay there that makes the difference. That desire, I believe, is exclusive to the APBT. EWO