The image below is straight out of the POH issued in 1944 for the P-51D.
60 inches is two atmospheres… quite a lot to ask for even a brand new Merlin, let alone one that’s pushing eighty year old.
Note too that the published operating limit was predicated on the availability of 115/145 (we used to call it “Grape Juice”) which I haven’t seen in the wild in nearly fifty years.
