Chrysler Poly V8 Engines

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Chrysler began production of a less complex, lighter and less expensive polyspheric V8 engine based on the hemispherical Chrysler FirePower engine in 1954. These were used from 1955 through 1958 for installation in the lower-priced Chrysler name-plates in place of the previous flat-head six-cylinder motors.

These new motors used the existing Chrysler hemi blocks and crankshaft parts, but completely different heads, pushrods, intake and exhaust manifolds, pistons, &c. with only a single rocker shaft in each head. They were called polyspheric or “poly” motors (meaning “more than one sphere”), since the combustion chambers were now partially shaped like two separate shallow concave domes: the intake and exhaust valve seats and surrounding reliefs.

In the Chrysler literature, the poly motors were also called single rocker shaft (“SRS”), while the hemi motors were called dual rocker shaft (“DRS”).

Polyspheric variants of the Chrysler 331 and 354 hemi motors were developed, as well as a new 301" motor with an even smaller bore at 3.625" and the same stroke as the others at 3.625". Introduced for 1955 in the low-priced Chrysler Saratoga and Windsor models and continuing through 1958, they were called “Spitfire” motors in the Chrysler literature. The 1958 Saratoga 354 4 bbl. (“58S”) was the most powerful at 310 hp.

All Chrysler Spitfire poly motors were low deck; no poly version of the 1957-58 raised deck 392 hemi motor was produced.