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Re: Official Difference btween the 396 and 402 BB Because you asked.
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Originally Posted by special-K
Why single out the 396 to 402? There were 283s,327s,396s,and 427s. Then they all got bumped up: 283 to 307,327 to 350,396 to 402,and 427 to 454. It was always my understanding this was due to federal emissions regulations kicking in (EPA had just come into existence in '69). They dropped compression ratios and increased displacement. Then there was the smog pump added. The other car companies did the same thing. I figured they jumped the displacement up to compensate choking them out for emissions. It was years later when they started actually building emissions efficiency "into" engines instead of building it "onto" them.
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GM didn't drop compression in 1970. The 375 HP 396 (that was actually a 402) still had 11 : 1 CR that year. Bumping up the cubes happened all the time, but they never put a 350 in car, printed 327 on the SPID, and put 340 fender emblems on it! 
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