I have an old 8 hp Briggs with 75 psi. . What is the minimum compression psi. to run?

Well until about the last two or three years, we were always under the impression that you wanted to have around 90psi minimum. Then we were instructed by briggs and stratton that even at 30 psi, there engines would perform. I personally think this is just a theory. They pay engineers too much and they have too much time on their hands.