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This article is about Hemi based Polyspheric engines. For non-Hemi based Polyspheric engines, see Chrysler A engine.
Manufacturer Chrysler
Also called
  • Poly
  • Poly-head
  • Red Ram
  • Semi-Hemi
  • Spitfire
Production 1955 (1955)-1958 (1958)
Combustion chamber
Configuration 90° V8
  • 241 cu in (3.95 L)
  • 259 cu in (4.24 L)
  • 268 cu in (4.39 L)
  • 299 cu in (4.90 L)
  • 314 cu in (5.15 L)
  • 325 cu in (5.33 L)
  • 331 cu in (5.42 L)
  • 353 cu in (5.78 L)
Cylinder bore
  • 3.4375 in (87.31 mm)
  • 3.5625 in (90.49 mm)
  • 3.625 in (92.1 mm)
  • 3.6875 in (93.66 mm)
Piston stroke
  • 3.250 in (82.6 mm)
  • 3.625 in (92.1 mm)
  • 3.800 in (96.5 mm)
Cylinder block alloy Cast iron
Cylinder head alloy Cast iron
Valvetrain OHV
Fuel system Carburetor
Fuel type Gasoline
Oil system Wet sump
Cooling system Water-cooled
Predecessor Chrysler flathead engine
Successor Chrysler A engine

The Polyspheric or Poly engines were V8 engines produced by Chrysler from 1955 to 1958 as lower-cost alternatives to the Hemi engines. These engines were based on the Hemi engines, using the same blocks and crankshaft parts, but completely different cylinder heads, pushrods, exhaust manifolds and pistons.

They were called Polyspheric or Poly engines, because they featured polyspherical-shaped (meaning “more than one sphere”) combustion chambers. These combustion chambers were formed by the two shallow concave domes where the intake and exhaust valve seats were.

Because these engines needed a less sophisticated rocker setup, with only a single rocker shaft in each head, they were also cheaper and lighter than their Hemi counterparts. In the Chrysler literature, the Poly engines were also called single rocker shaft (SRS), while the Hemi engines were called dual rocker shaft (DRS).

These engines replaced Chrysler's flathead inline six in the division's lower-priced cars, but were slowly replaced by the Chrysler A engine, beginning in mid-1956.

Dodge and Plymouth[edit]

Dodge and Plymouth both offered Poly versions of Dodge's Hemi engine. The Dodge versions were marketed as Red Ram or Super Red Ram (internal code A388).


The 241 (241 cu in (3.95 L)) was Plymouth's Poly version of the Dodge's 241 Hemi for 1955. Bore and stroke were the same at 3.4375 in (87.31 mm) by 3.25 in (83 mm), respectively.


The 260 (also known as 259) (259 cu in (4.24 L)) was introduced in the middle of 1955 by Plymouth. It was a bored-out 241, having a 3.5625 in (90.49 mm) bore and a 3.25 in (83 mm) stroke.


The 270 (268 cu in (4.39 L)) was offered in both Plymouths and Dodges. Like its predecessors, it was a Poly version of Dodge's 270 Hemi. Bore and stoke were the same at 3.625 in (92.1 mm) by 3.25 in (83 mm), respectively.


The 315 (314 cu in (5.15 L)) was a Poly version of Dodge's high-deck 315 Hemi. Bore and stoke were the same at 3.625 in (92.1 mm) by 3.8 in (97 mm), respectively.


The 325 (325 cu in (5.33 L)) was a Poly version of Dodge's largest high-deck 325 Hemi. Bore and stoke were the same at 3.6875 in (93.66 mm) by 3.8 in (97 mm), respectively.


The Spitfire engines were Poly variants of Chrysler's FirePower (Hemi) engine. Chrysler built three Spitfire engines: the 331 Poly, 354 Poly, and the all-new 301 Poly, which did not have a Hemi version. They were introduced for 1955 in the low-priced Chrysler Saratoga and Windsor models and were used through 1958.

All Chrysler Spitfire engines were low deck; no Poly version of the raised deck 392 Hemi engine was produced.


The 301 (299 cu in (4.90 L)) was Chrysler's smallest Poly engine. Having a 3.625 in (92.1 mm) bore and a 3.625 in (92.1 mm) stroke.


The 331 (331 cu in (5.42 L)) was a Poly version of Chrysler's 331 Hemi. Bore and stoke were the same at 3.8125 in (96.84 mm) by 3.625 in (92.1 mm), respectively.


The 354 (353 cu in (5.78 L)) was a Poly version of Chrysler's 354 Hemi. Bore and stoke were the same at 3.9375 in (100.01 mm) by 3.625 in (92.1 mm), respectively. The 1958 Chrysler Saratoga with four-barrel carberutor (58S) was the most powerful at 310 hp (230 kW).