1965 Offenhauser 273!
Last night I talked to Offenhauser. They told me that the Offenhauser hi rise intake manifold added 18 hp and a dual 4 were 22 hp. Low rise aftermarket intake were 12 hp. They had no documentation left but they worked with camshaft manufacturers like Racer Brown, Edelbrock, Camcraft and Iskenderian.
During the 60ies they worked close with Bob Mosher with Chrysler V8s.
So is there any one who has contact info to him? Unfortunately he closed his shop Mosher Musclecar Restoration 2 years ago.