Pontiac Super Duty 421 vs real fine 409. No comparison, why the difference? How did Butler get 80 hp with stock parts?

The factory rated HP was highly underrated. Butler could squeeze out 20-30 more by smoothing intake runners, bigger carb jets and an over bored block.

“ Pontiac officially listed its Super Duty 421 V-8 as an expensive, limited option on the Pontiac Catalina and Grand Prix in 1961. This particular motor was factory rated for 405 hp @ 5600 rpm, but actual output was rumored to be closer to 460 hp.”


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