Ed Pawlowski wrote in :

There were some
bad years though when the first pollution stuff was hung on engines.
IIRC it was about mid 70's.

Two things happened there in the early 70's. One was the
emissions controls (and the US industry has no-one to blame
but themselves for that, hoping they could band-aid their
existing designs instead of investing the money to design
a proper fix). The other was the SAE deciding horsepower
should be specified using the "net" method instead of the
"gross" method.

The net horsepower rating eliminated a lot of the tricks
the manufacturers used to cheat on horsepower ratings.
So you'd see a "400hp" engine suddenly drop to maybe
220hp net, and then to maybe 165hp after the emissions
controls were added.

John