You mention the 1963 Pontiac Tempest 326 engine actually measured 336 ci and that the reason was the rumored 330 GM limit for mid size cars. I was a new Pontiac District Service Rep at the 1964 new model convention and at the service meeting the Pontiac Chief Engineer, Pete Estes, took questions at the end. Someone in the back asked "How many cubic inches are in the 326 engine?" I my thought was OMG, where did we get this moron but Estes immediate answer was 336ci. The reason was that the new Pontiac 326 engine line was not ready but it could be built on the GMC engine line as a 336 till Pontiac could build it for the following years. No one would complain that they got extra ci. The rumored 330ci GM limit may be the reason for the 326 design but was not the reason Pontiac downgraded the 1963 Tempest designation.
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