The HO engine of 71-2 is just a 455 with th 4 bolt main as the 70 version was, but the 70 modle didn't have the round port heads. The Sd was a new type of engine as they were also working on the twin turbo 301 while Mac McCeller was still alive but the amount of power that the twin 301 put out to much HP & was going to be a threat to the Corvette so Chevy had it killed but the SD was already built by then & the money was spent so it became the big Gun. As we all know it has provisions for dry sump oiling & other features that would have made it even stronger then it is. But the emission std's jumped in & there were some changes as the RAIII cam was the 1st cam to be used in the SD but it got pulled because it made to much HP & was hard on the emission side to. But it got through with the other cam & a delay switch on the dist so it would pass. The HO engine was a low comp. RAIV engine so it could keep up with the HP that chevy was putting out to stay as the power house for Pontiac , without hurting chevy in the way.
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