The power washing may have caused some of the spark plugs to misfire causing fuel to travel into the exhaust system where it ignited, exploded and blew part of the catalytic converter out the tailpipe. And possibly opened up a hole in the exhaust under the car. Further attempts to blow the rest of the cat out may have introduced raw fuel and flame to the underside of the car.

Given the choice of Art's method or power washing, I power wash.

Of note, you wrote, "the cat to which I refer is a catalytic converter". Other than the subject line, the only reference to "cat" was in the video.
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1980 245 Canadian B21A with SU carb, M46 trans, 3:31 dif, in Brampton, Ont.