What is the 426 Hemi and why is it so special?

Question

Mike Sullivan · Accepted Answer

The 426 Hemi is Chrysler's legendary semi-hemispherical V8 engine, produced from 1966-1971 for street use and rated at 425 hp (a conservative figure — actual output was closer to 500 hp). Its hemispherical combustion chamber design — which gives it the name — allowed larger valves and better flow than conventional wedge-head engines. It dominated NASCAR in 1964 and has been the most desirable American performance engine ever produced. The 426 Hemi is the engine that other engines are measured against. Understanding why it's so special requires understanding both its engineering and its history.Why "Hemi"?The name refers to the hemispherical shape of the combustion chamber — a dome shape that, unlike the conventional wedge chamber used by most V8 engines, allows the intake and exhaust valves to be placed on opposite sides of the combustion chamber. This configuration allows dramatically larger valve diameters (2.25-inch intake, 1.94-inch exhaust on the street Hemi) and more direct gas flow paths. The result is better volumetric efficiency — more air and fuel in, more exhaust out — which translates directly to power.NASCAR OriginsThe 426 Hemi debuted at the 1964 Daytona 500, where Hemi-powered Plymouths finished 1-2-3. NASCAR barred the race Hemi for the 1965 season because it was not sold in production cars, which pushed Chrysler to build a street version. The street Hemi arrived in 1966 as Chrysler's response to demands for road-legal access to the race engine — detuned from the racing version but unmistakably derived from it.Street Hemi SpecificationsThe street 426 Hemi (1966-1971) produced 425 hp and 490 lb-ft of torque by Chrysler's conservative factory rating. Independent dyno testing consistently showed 490-500 hp at the flywheel. It ran dual four-barrel carburetors on a cross-ram manifold, required premium fuel, and produced an idle that was unmistakable — lumpy, authoritative, unlike anything else.Production and ValueApproximately 10,000 street Hemis were installed in Dodge and Plymouth vehicles between 1966 and 1971. The Hemi was a $700-$900 option (depending on year and model) — expensive in 1968 dollars. Today, any car equipped with the factory 426 Hemi is worth substantially more than an equivalent non-Hemi car: 3x to 10x the value depending on the specific model. The Hemi 'Cuda, Hemi Charger, Hemi Road Runner, and Hemi GTX are the most valued applications.