"Cub Driver" wrote...

"Actually, a little bigger. There are two outflow valves that work in
tandem. On the 747 they're located on the aft belly, and each is a
touch smaller in area than one aircraft window -- an oval about 4in by
12in. There are also two relief valves on the left side of the
airplane, and they are about 8" in diameter."

I don't know about the 747 Classic, but the -400 has 2 rectangular outflow
valves, each about 1 x 3 feet. The -400 also also has 3 A/C packs vs the 2 in
the Classic for pressurization.