1956 354cc Chrysler Hemi
The 354, released in 1956, had a bore of 3.9375 in and stroke of 3.625 in. It was used in the 1956 New Yorker, Imperial Custom and Crown, and Chrysler 300B. Dodge used a modified version of this engine in the 1957 D-501. The 300B engine was rated at 340 bhp, while the New Yorker and Imperial 354 engine configuration produced 280 bhp. For the 300B an optional 355 hp version was available, making it the first American V-8 to be rated at one horsepower per cubic inch.
A hemispherical combustion chamber allows the valves of a two valve-per-cylinder engine to face each other across the chamber, rather than opening side-by-side. This creates more space in the combustion chamber roof for the use of larger valves and also straightens the airflow passages through the cylinder head. This creates what is known as a cross-flow head, where the intake charge flows directly across the chamber to the exhaust valve located directly opposite it. These features significantly improve the engine’s airflow capacity, which can result in relatively high power output from a given piston displacement. But the design can also significantly increase the flow of incompletely combusted air-fuel mixture straight out of the exhaust valve. With a hemi combustion chamber, there is minimal quench and swirl to burn the fuel-air mix thoroughly and quickly; the spark plug is frequently located at or near the centroid of the chamber to facilitate complete combustion. Hemispherical combustion chambers, because of their lack of quench, are more sensitive to fuel octane rating; a given compression ratio will require a higher octane rating to avoid ping in a hemi engine than in a wedge engine. Engines with hemispherical combustion chambers often use dome-topped pistons to attain the desired compression ratio.
The hemi head usually has intake and exhaust valve stems that point in different directions, requiring a large, wide cylinder head and complex rocker arm geometry in both cam-in-block and overhead cam engines.
Chrysler developed their first experimental hemi engine for the Republic P-47 Thunderbolt fighter aircraft. The XIV-2220 engine was an inverted V16 rated at 2,500 hp. The P-47 was already in production with a Pratt & Whitney radial engine when the XIV-2220 flew successfully in trials in 1945 as a possible upgrade, but the war was winding down and it did not go into production. However, the exercise gave Chrysler engineers valuable research and development experience with two-valve hemi combustion chamber dynamics and parameters.
In addition to the aircraft engine, Chrysler and Continental worked together to develop the air-cooled AV-1790-5B V12 Hemi engine used in the M-47 Patton tank.
Chrysler applied their military experience with the hemispherical combustion chamber to their first overhead-valve V8 engine, released under the name FirePower, not “Hemi”, in 1950 for the 1951 model year. The first version of the FirePower engine had a displacement of 331 cu in (5.4 L) and produced 180 bhp. Eventually, each Chrysler divisions had its own versions of the FirePower engine, with different displacements and designations, and having almost no parts in common. Chrysler and Imperial called their versions the FirePower. DeSoto called theirs the FireDome. Dodge had a smaller version, known as the Red Ram. Only Plymouth didn’t have a version, instead retaining the poly-head engines: there was no Plymouth hemi engine until the 1964 426.
Collectively, the 1951-’58 Hemi engines are now commonly referred to as first-generation Hemi engines,and the group can be identified by the rear-mounted distributor and the spark plugs in a row down the center of wide valve covers.