Mopars 1968 '383 Engines'

YY1 said

IIRC, de-rating of A body 383 was due to more restrictive exhaust necessary for clearance.

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I heard that as well. The horsepower rating standards of the day did not require any accessories to be installed. If we were talking about using the SAE standards that came into use in 72, there would be no question about the downrating.