When introduced in 1951, the Chrysler Hemispherical combustion chamber V-8 was a radical piece.
In continuous production from 1951 through 1958, it went through constant changes that
increased horsepower from 180 to at least 400 in the fuel injected version of 1958.

Chrysler’s 1951-58 Hemi – An Astounding Engine

Chrysler applied their WWII experience with the hemispherical combustion chamber and used it to develop their first overhead-valve V8 engine, released in 1950 for the 1951 model year under the name “FirePower”, not “Hemi”, (the spelling is correct with the “P” capitalized. The early Hemi was never called “Hemi” by Chrysler. It was referred to as the “Double Rocker” V-8, to distinguish it from the “Polyspherical” head single rocker shaft V-8 introduced a year or two later. The first version of the FirePower engine had a displacement of 331.1 cu in and produced 180 HP.

All 1951-1958 Chrysler FirePower Hemi engines have a bore center distance of 4.5625″, larger than any other Chrysler engine except the later B “wedge head” engine introduced in 1958, and RB (raised block B) a year later. All Chrysler FirePower engines are “over square”; meaning that the bore is larger than the stroke. This version of the Hemi was released in three displacements for the period 1951-1958: 331, 354, and 392 cu. in.

For a chart of the specifications of the different configurations of the 1951-1958, click HERE.

331 Hemi (1951-1955)

This print advertisement introduced the 1951 Chrysler 331 CID Hemi V-8

The 331 CID V-8 was first FirePower engine and was used from 1951 to 1955. It had a deck height of 10.38″, a bore of 3.8125″ and a stroke of 3.625″ for 331 CID. For the time, the valve sizes were very large at 1.8125″ for the intake, and 1.50″ for the exhaust.

The first versions used a two-barrel carburetor and produced 180 HP in the years 1951-53. The camshaft timing would be 252° intake and 244° exhaust, with 30° overlap and used hydraulic valve lifters. This camshaft would be used in the non-300 letter series cars through 1956.

For 1954, the engine was offered in 2 and 4-barrel versions making 195 and 235 HP respectively, this HP increase due to an increase in Intake and Exhaust valve sizes to 1.94″ and 1.75″ respectively. Dual exhausts were available. The deck height was reduced slightly to 10.32″ which did result in a small increase in compression. Except for these changes, all other specifications remained the same.

In 1955, the standard engine’s horsepower increased to 250 HP with a slight compression boost to 8.0:1. A “Poly” version of the block was released at 301 CID and produced 188 HP.

Chrysler 300
In 1955, the Chrysler 300 was released. This engine was “hot rodded” with the following changes:

• • Compression ratio increased to 8.5:1
  • Mechanical valve lifters.
  • Camshaft Duration: Intake 280°, Exhaust 270°, Overlap 60°, Lift (Intake) 0.444″ (Exhaust) 0.435″.
  • Carburetors: Two Carter WCFB 2118S (front) 2317S (rear).
  • Horsepower: 300 @ 5200 RPM.
  • Torque: 345 ft. lbs. @ 3200 RPM.

354 Hemi (1956)

The 1956 Chrysler 300B was equipped with the 354 CID Hemi V-8 with optional 355 HP beating Corvette to the 1 HP per CID by one year.

The 354 CID version was released in 1956 in response to Cadillac, Packard, and Lincoln V-8 increases in engine displacement. It used the same deck height as the 331 but had a bore increase to 3.9375″. It used the same stroke of 3.625″. Using the same heads, the compression ratio increased to 9.0:1. It was used in the 1956 New Yorker, Imperial Custom and Crown, and Chrysler 300B. No 2-barrel version was offered from this point forward, with “economy versions of the block running “Polyspherical” heads instead of the more complex and expensive Hemi heads.

The Chrysler 300B engine was rated at 340 HP, while the New Yorker and Imperial 354 engine configuration produced 280 HP. Dual 4-barrel carburetors and a more aggressive mechanical camshaft resulted in the increased power.

Chrysler 300B
As in 1955, the 300 B was modified with the following specifications:

• • Compression ratio increased to 9.0:1 (10.0:1 in the 355 HP version)
  • Mechanical valve lifters.
  • Camshaft Duration: Intake 280°, Exhaust 270°, Overlap 60°, Lift (Intake) 0.444″ (Exhaust) 0.435″.
  • Carburetors: Two Carter WCFB (front) 2444S, (rear) 2445S.
  • Horsepower: 340 @ 5200 RPM (355 @ 5200 RPM)
  • Torque: 345 ft. lbs. @ 3200 RPM.

The 300B could be had with an optional 355 HP high performance version, which had its compression ratio increased to 10.0:1, making it and the DeSoto Adventurer the first American V-8s to be rated at one horsepower per cubic inch.

The 354 was a short-term Hemi, being in place for only this one year. The 354 block would be used through 1958 using the Polyspherical heads – called the Chrysler “Spitfire V8”. This was used in the Chrysler Windsor and Saratoga models from 1955-1958.

392 Hemi (1957-1958)

The 1957-58 392 CID Hemi was the last of the original design. In 300C and 300D configuration it made 375-400 HP.

In 1957 the engine was bored and stroked, The stroke increased to 3.906″ and the bore was increased to 4.00″. The result was 392 CID. The deck height was raised to 10.87″, 0.5″ taller than that of the previous blocks. Because its deck was taller, Chrysler cast the 392 heads wider in the intake side so that earlier manifolds could be used with this tall deck block. Camshaft timing was increased to 252° intake, 252° exhaust, 30° overlap.

The 392 was used in the 1957-1958 New Yorker, 1957 Chrysler 300C, 1958 Chrysler 300D, and 1957-1958 Imperial and the 1959 Imperial Crown. In 1957, the standard 392 made 325 HP with 9.25:1 compression for the New Yorker and Imperial. In 1958 it had 10:1 compression and made 345 HP using a single four-barrel carburetor.

Once again, the 300C and 300D Series cars received a dual four-barrel version of the 392. The standard 300 series engines were rated at 375 HP in ’57 and 380 HP in ’58. In 1957, a special high-performance Hemi was offered for the 300C and this engine was rated at 390 HP. It required a 3-speed manual transmission. It used higher compression and a very radical camshaft. Though rated at 390 horsepower, it is suggested that it made well over 400.

Chrysler 300C and 300D
The changes to the standard Hemi for the 300 Series in 1957 and 1958 were:

• • Compression ratio 9.25:1 in 1957, 10.0:1 in 1958
  • Mechanical valve lifters.
  • 1957 Camshaft Duration: Intake 280°, Exhaust 270°, Overlap 60°, Lift (Intake) 0.444″ (Exhaust) 0.435″.
  • 1958 Camshaft Duration: Intake 276°, Exhaust 276°, Overlap 55°, Lift (Intake) 0.444″ (Exhaust) 0.435″.
  • 1957 Optional Engine Camshaft Duration: Intake 300°, Exhaust 295°, Overlap 95°, Lift (Intake) 0.455″ (Exhaust) 0.455″.
  • Carburetors: Two Carter WCFB: (1957) Front 2534, Rear 2535 (1958) Front 2741S, Rear 2742S.
  • 1957 Horsepower: 375 @ 5200 RPM (optional 1957) 390 @ 5200 RPM)
  • 1958 Horsepower 380 @ 5200 RPM (optional) 390 @ 5200 RPM
  • 1957 Torque:  420 ft. lbs. @ 4000 RPM
  • 1958 Torque: 435 ft. lbs. @ 3500

1958 Fuel Injection Engine

Fuel injected 300Ds received these unique medallions on their rear flanks in place of the standard emblems.

A special high-performance engine was again offered in 1958. It was an extremely rare piece and only delivered for 16 300Ds. This option used Bendix “Electrojector” electronic fuel injection. When so equipped, the high performance 392 was rated at 390 HP. Other than the fuel injection setup, all other engines specifications were the same. Bendix reported that the system, with no other changes to the engine, would result in a 10-15% increase in power. Chrysler’s rating of 390 for this engine was likely conservative – educated guesses hover around 435-450 HP.

Due to reliability problems, at least 12 of the 16 300D cars with this option were recalled and retrofitted with the dual 4-barrel carburetor setup. One of the remaining cars that did not have the system removed has been located and was restored.

From 1959 forward, the Chrysler line only was fitted with either the corporate “B” or “RB” (raised deck B) wedge V-8, thus ending the early “Hemi” era for Chrysler cars.