As many had previous stated or eluded, the 455 was the natural progression of Pontiac’s largest full-size engine—which started at 421 and had already moved to 428—to maintain good acceleration as vehicle size and/or weight increased. And as mgarblik had so well explained, everyone else was doing it, too.
Pontiac’s official explanation can be found in the “engine changes” section of the 1970 Pontiac Engineering Manual. It states:
“This year a new 455 cubic inch displacement V-8 replaces the 428 in all applications. Its larger displacement gives it improved performance. The basic block design is the same as the 428, with the bore centers and water jackets remaining essentially the same. The additional displacement comes from a 0.031-inch bore increase and a 0.210-inch stroke increase. The crankshaft is new for the increased stroke while the heads and camshaft are modified from those on the 428.”
Additionally, I once asked Mac McKellar this very question. Why the jump to 455? His answer then was twofold and I’ll summarize.
1. Pontiac had been increasing engine displacement to improve performance ever since its V-8 was introduced in 1955. During the 1960s, its small-car 326 went to 350, its intermediate-car 389 went to 400, and its full-size-car 421 went to 428 and then to 455 (at least developmentally). Its OHC-6 increased from 230 to 250 as well. Mac said the 455’s longer stroke was an effective way at ensuring that plenty of torque was available to satisfy the average customer’s acceleration expectations, particularly at take-off and in low rpm passing conditions.
2. Pontiac’s closest competitors—Chevrolet, Buick, and Oldsmobile—had each stepped up the displacement of their largest engines at that time, and Pontiac—who celebrated most of the 60s as the nation’s third largest automaker—wasn’t prepared to be left behind, especially when GM opened the 455 to the intermediates (A-body). So Pontiac’s Sales department was also pushing Engineering for a 455.
Ironically, the continuous displacement increases at Pontiac (and really with all GM brands) ended around 1970 when emissions and fuel economy became significant issues, and ultimately led to the stark displacement reductions of the later 1970s.
While the explanations above mirror what many had already stated, it gives insight into Pontiac rationale as a company and from an key insider.
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