The 3rd generation 6 has a very different head design when compared to the stovebolt. In the 3rd gen the valves sit side by side in the roof of the combustion chamber. Whereas in the stovebolt only the exhaust valve is located in the combustion chamber - a chamber which cannot be fully swept by the piston at TDC. This is why the stock camshaft is a dual pattern grind - with a longer duration on the exhaust lobe. Advertized duration on the post war stock 216 cam is 220/231. The intake lobe duration is purposely shortened to eliminate overlap entirely - which is what would have happened had GM delivered a 231/231 single pattern grind on the same lobe centers.
