My first 911 was a non-runner 74. The PO sold it after a big bang stopped him cold on the freeway. Turns out the rocker shaft on cyl 1 (exh) walked out and while mix was able to enter chamber it couldn't get out unti intake valve opened again. Despite the loud backfire the engine suffered no damage and had 170 psi on all six.

Has to be some difference given the "air pump" would not be breathing correctly.

Mind you, just thinking out loud, not from a wealth of experience.