Dodge Hemispheric vs. Polyspheric Motor Comments

Development and Description
    Less expensive, lighter variants of the Dodge hemi motors were developed, and introduced in 1955 for installation in the mid-priced Dodge cars. These new motors used the existing hemi block, but completely different heads, exhaust manifolds, pushrods, pistons, etc. with only a single rocker shaft in each head; the same intake manifolds were used in many cases (but not the dual 4 bbl.). These motors were called “poly” (for poly-spheric: meaning “more than one sphere”), since the combustion chamber was now formed by two separate concave domed shapes (the valve seats and surrounding reliefs).
    Poly motors are also called single rocker shaft (“SRS”), while hemi motors are dual rocker shaft (“DRS”). Conversion of Dodge Poly Motors to Hemi
    1955-58 Dodge 241, 259, 270, 315 & 325 poly motors can be converted to hemi by substitution of hemi parts using Dodge parts only.
    If the original pistons are used, the motor will run, but have lower compression since the poly piston does not have a compression dome. The crankshaft balance will also be affected, since the hemi pistons are heavier in most cases due to the dome. Heads should be selected from motors of the same or smaller bore size to prevent the chamber from overhanging the bore, but some choices may be available as to port size, valve size and chamber volume. Manifolds must match the deck height (low or raised). Hemi head bolts must be used.

DeSoto Poly Motors
    Although “wide-block”poly motors were used in lower-priced 1955-58 DeSoto models, they are Dodge engines based on the Dodge hemi motor. These and the 1955-56 Plymouths using these motors can be converted by using Dodge hemi parts as described above. There is no poly motor based on the DeSoto hemi series.
Chrysler Poly Motors
    Chrysler 1955-58 Saratoga and Windsor 301, 331 & 354 poly motors are based on the Chrysler hemi motors, which are only distantly related to other brands.
Poly “A” Motors
    There is no hemi motor based on the “A” series poly motor as used in Plymouths 1956-67 and some Dodges 1958-59, and no conversion by direct substitution is possible.
    The “A” poly motor is very similar in design to the earlier hemi-derived poly motors, but parts are not generally interchangable.
Notes
    All 1951-57 Chrysler, DeSoto, Dodge and Plymouth V8 motors, regardless of family, size or type, have the distributor located in the rear of the engine. Only 1958-* “B” and “RB” type motors, both wedge and hemi, have front-mounted distributors.
    There is no commonality of parts or dimensions between the early (1951-58) motors and late (1964-*) hemi motors, which were developed by adapting hemi technology to the “RB” motors.
    The Table below contains “Maximum Compression Distance” data. This is not measured or taken from factory information, it is calculated on the basis of .000” deck height (with the piston's top surface flush with the cylinder bank head gasket surface) with the tallest possible theoretical piston (measured between the piston pin center and the top surface).

Hemi & Poly Motor Data

Motor

Year

Size

Bore
Size

Deck
Height

Rod
Length

Stroke
Length

Maximum
Com. Dist.

Bore
Centers

Plymouth
poly only; 1954-56 Dodge motors used in all applications

1955

241”

3.4375”

9.29”

5.9375”

3.25”

1.727”

4.1875”

259”

3.5625”

1956

270”

3.625”

Dodge, low deck
hemi: 1953-55; poly 1955-56
259” poly only

1953-54

241”

3.4375”

1955

259”

3.5625”

1955-56

270”

3.625”

Dodge, raised deck
hemi: 1956-57; poly 1956-58

1956

315”

3.625”

10.38”

6.618”

3.80”

1.862”

1957-58

325”

3.6875”

Plymouth A
poly only
318” 1958 Dodge also
326” 1959 Dodge only

1956-57

277”

3.75”

9.60”

6.123”

3.125”

1.914”

4.46”

1957

301”

3.91”

1956

303”

3.8125”

3.31”

1.822”

1957-66

318”

3.91”

1959

326”

3.95”

DeSoto, low deck; hemi only; Dodge motors used in 1955 poly applications

1952-54

276”

3.625”

9.54”

6.067”

3.34375”

1.801”

4.3125”

1955

291”

3.72”

DeSoto, raised deck; hemi only
Dodge motors used in 1956-58 poly applications

1956

330”

3.72”

10.37”

6.625”

3.80”

1.845”

1956-57

341”

3.78”

1957

345”

3.80”

Chrysler, low deck
301” poly only; 331” poly 1956
354” poly 1956-58

1955

301”

3.625”

10.32”

6.625”

3.625”

1.882”

4.5625”

1951-55

331”

3.8125”

1956

354”

3.9375”

Chrysler, raised deck; hemi only

1957-58

392”

4.00”

10.87”

6.956”

3.906”

1.961”