Editing Pontiac V8 engine (section)

===421 HO===
First offered as an option in 1963, the 421 HO came in a 4-barrel engine of {{cvt|320|hp|0}} and one Tri-Power HO version with a hotter cam and efficient iron exhaust manifolds and rated at {{cvt|370|hp|0}}. Pontiac offered this to the public as a streetable version of the 421 SD. The engine came with 543797 (4-barrel) and 9770716 heads for the tri-power and special exhaust manifolds and a 7H cam with 292deg. intake duration and later 1964 L with 288deg intake essentially the same as the 068 cam. #9770716 aka "716" heads featured a 170cc intake port volume, and were considered a milder "street" version of the vaunted SD421 Super Duty heads. These same heads were also used on the 1964 GTO 389 tri-power engines. By 1965 and 1966 the same combinations would be rated at {{cvt|338|hp|0}} for the 4bbl and the two Tri-Power versions would be rated at {{cvt|356|hp|0}} and the HO version at {{cvt|376|hp|0}}.