"Semi-Hemi" Polyspherical Head V8 Engines:
241, 259, 270, 301, 315, 325, 331, 354
with thanks to Bill Watson
and Hemi Andersen
Chrysler's first V8 engine was the "dual rocker," now known as the original Hemi. Developed from Chrysler's aviation research, the efficient powerhouse had roughly hemispherical heads, opposing valves, and a complex valvetrain that used dual rocker arm shafts for each cylinder.
The Hemi was a premium engine, well engineered but costly, because eight-cylinder engines had mainly been used in premium cars. Most cars had four or six cylinders; spending some extra time and money on building a better V8 made sense. Chrysler had not counted on just how popular the V8 would be, or how much it would drive sales.
It took little time for Chrysler to realize their mistake, and the engineers quickly got to work on dropping costs and increasing output. Their first move was to creating a cheaper and lighter head and valvetrain, for the same engine blocks. Dubbed the "poly" (due to its polyspherical heads) or "semi-hemi," the new design had rounded, circular combustion chambers, like the Hemi; but they only had one rocker arm shaft, instead of the Hemi's two. To make the change, they put the intake valves on the top of the rocker arm and the exhaust valves on the bottom. The results were lower cost, less weight, and higher production; the cost was a slight loss of efficiency.
Poly head design on left; wedge head design on right.
The heads had canted valves in a cross-flow arrangement, gaining some of the advantage of the hemis, but much less expensive to make (this design would make a comeback in the 4.7 liter V8). They also had low friction valve locks to allow for valve rotation, extending their life. Chrysler claimed the rounded combustion chamber prevented carbon deposits.
The 1958 354 engine was available in both Hemi and Poly form; with four barrel carburetors, the Hemi produced 350 horsepower, and the Poly just 310. Still, the Poly was much cheaper and lighter, and the company could make larger engines to replace the lost power.
The bottom of the valve covers were scalloped so the spark plugs were accessible from the top - unlike Ford and Chevrolet V8s, whose owners had to reach under hot exhaust manifolds to get at the spark plugs.
Randy Hicks wrote that the cylinder heads and intake manifolds were interchangeable with any Hemi engines that had the same deck height. However, the new design meant they didn't need spark plug tubes; pistons and rods were less expensive; and the heads were cast rather than being machined. Most used a two-barrel carburetor, because the Hemi was the performance engine.
As Hemi Andersen wrote, the Poly headed engines were an interim solution, not requiring a new block:
The 1956 cars saw a larger-bore version of the 301, displacing 331 cubic inches - but that wasn't the end; it was bored again, to 354 cid, for the 1957 cars, reaching 290 hp and 385 lb-ft in the Windsor (the Saratoga used a four-barrel carburetor to produce 310 hp, 405 lb-ft of torque).
Dodge cars got a A388-coded 270 cubic inch version, with 175 hp and 240 lb-ft of torque.
* Gross horsepower; modern horsepower readings would be roughly 20-40 hp lower. The industry switched in 1971. Only two used four-barrel carburetors: the Chrysler 354 and one version of the 1957 Dodge 325.
The next generation of Chrysler V8 engines were designed to have lower costs and much higher production; they had the same heads, but numerous changes to allow for more automation in the factory. They, too, had polyspherical heads - but while the blocks were an evolution of the Hemi V8 design, they were not the same. These were informally, and later formally, called the "A" engines. With their lower cost and higher production volume, the A engines quickly replaced the older, Hemi-based Poly "semi-Hemis." The 1958 Dodges would be the last cars to carry them; in 1959, Dodge and Plymouth both used the new A-engines. Chrysler and DeSoto had already moved to the new B-engines.
Engineer Pete Hagenbuch wrote:
As Hemi Andersen wrote,
Lanny Knutson added:
Mike Peterson noted,
[A engines • B engines • LA engines]
1958 Red Ram distributor
241, 259, 270, 301, 315, 325, 331, 354
with thanks to Bill Watson
and Hemi Andersen
Chrysler's first V8 engine was the "dual rocker," now known as the original Hemi. Developed from Chrysler's aviation research, the efficient powerhouse had roughly hemispherical heads, opposing valves, and a complex valvetrain that used dual rocker arm shafts for each cylinder.
The Hemi was a premium engine, well engineered but costly, because eight-cylinder engines had mainly been used in premium cars. Most cars had four or six cylinders; spending some extra time and money on building a better V8 made sense. Chrysler had not counted on just how popular the V8 would be, or how much it would drive sales.
It took little time for Chrysler to realize their mistake, and the engineers quickly got to work on dropping costs and increasing output. Their first move was to creating a cheaper and lighter head and valvetrain, for the same engine blocks. Dubbed the "poly" (due to its polyspherical heads) or "semi-hemi," the new design had rounded, circular combustion chambers, like the Hemi; but they only had one rocker arm shaft, instead of the Hemi's two. To make the change, they put the intake valves on the top of the rocker arm and the exhaust valves on the bottom. The results were lower cost, less weight, and higher production; the cost was a slight loss of efficiency.
Poly head design on left; wedge head design on right.
The heads had canted valves in a cross-flow arrangement, gaining some of the advantage of the hemis, but much less expensive to make (this design would make a comeback in the 4.7 liter V8). They also had low friction valve locks to allow for valve rotation, extending their life. Chrysler claimed the rounded combustion chamber prevented carbon deposits.
The 1958 354 engine was available in both Hemi and Poly form; with four barrel carburetors, the Hemi produced 350 horsepower, and the Poly just 310. Still, the Poly was much cheaper and lighter, and the company could make larger engines to replace the lost power.
The bottom of the valve covers were scalloped so the spark plugs were accessible from the top - unlike Ford and Chevrolet V8s, whose owners had to reach under hot exhaust manifolds to get at the spark plugs.
Randy Hicks wrote that the cylinder heads and intake manifolds were interchangeable with any Hemi engines that had the same deck height. However, the new design meant they didn't need spark plug tubes; pistons and rods were less expensive; and the heads were cast rather than being machined. Most used a two-barrel carburetor, because the Hemi was the performance engine.
As Hemi Andersen wrote, the Poly headed engines were an interim solution, not requiring a new block:
The Chrysler version was dubbed "Spitfire," and the first car to get it was the 1955 Windsor - the lowest Chrysler. The 1955 Chrysler poly was a 301 with 188 horsepower and 275 pound-feet of torque. Dodge Truck's version displaced 260 cubic inches, generating 169 hp and 243 lb-ft of torque; it had various measures to increase durability, including added coatings, different metals, and valve rotators.
The 1956 cars saw a larger-bore version of the 301, displacing 331 cubic inches - but that wasn't the end; it was bored again, to 354 cid, for the 1957 cars, reaching 290 hp and 385 lb-ft in the Windsor (the Saratoga used a four-barrel carburetor to produce 310 hp, 405 lb-ft of torque).
Dodge cars got a A388-coded 270 cubic inch version, with 175 hp and 240 lb-ft of torque.
Year | Brand | Size | Horsepower* | Torque |
---|---|---|---|---|
1955 | Plymouth | 241 | 157 | 217 |
1955 | Plymouth | 259 | 167 | 231 |
1955 | Dodge | 270 | 175 | 240 |
1955 | Chrysler | 301 | 188 | 275 |
1956 | Dodge | 315 | 218 | 309 |
1957 | Dodge/DeSoto | 325 | 245 | 320 |
1957 | Dodge (4bbl) | 325 | 265 | 355 |
1956 | Chrysler | 331 | 225 | 310 |
1968 | Chrysler | 354 | 310 | 405 |
* Gross horsepower; modern horsepower readings would be roughly 20-40 hp lower. The industry switched in 1971. Only two used four-barrel carburetors: the Chrysler 354 and one version of the 1957 Dodge 325.
The next generation of Chrysler V8 engines were designed to have lower costs and much higher production; they had the same heads, but numerous changes to allow for more automation in the factory. They, too, had polyspherical heads - but while the blocks were an evolution of the Hemi V8 design, they were not the same. These were informally, and later formally, called the "A" engines. With their lower cost and higher production volume, the A engines quickly replaced the older, Hemi-based Poly "semi-Hemis." The 1958 Dodges would be the last cars to carry them; in 1959, Dodge and Plymouth both used the new A-engines. Chrysler and DeSoto had already moved to the new B-engines.
Engineer Pete Hagenbuch wrote:
As Hemi Andersen wrote,
The early, semi-Hemi Poly has a separate valley cover underneath the intake manifold; A-series engines did not have the separate valley cover, since the intake performed this function. Both series were similar in outward appearance, despite being from rather different engine families.
Lanny Knutson added:
The poly head engines were made under a variety of different names, including Fire Dome (DeSoto), Power Dome (Dodge Truck), and Fire Power (Chrysler). Canadian engines has details on Canadian-built engines.
Mike Peterson noted,
In the long run, the same issues that brought about the Poly also ended their production. As V8 demand kept increasing, the A engines could not keep up with the need for more power; larger displacements were needed. The company designed a series of new large engines ("B engines") that could be made more cheaply and more quickly; even the 392 Hemi was matched by a new 413 cubic inch B engine. Chrysler engineers discovered, while developing this series, that the wedge-head engines were actually more efficient, and much cheaper and faster to build, than the poly design; and the Hemi advantages were overcome with sheer size. Pete Hagenbuch said:
The Poly engines lasted from the 1955 cars to the 1958 cars - a fairly short lifespan, but they were always an interim solution while work proceeded on the A and B engines. DeSoto used the Dodge 325 for just one year (1957); Plymouth used four different displacements of the same engine series from their 1956 to 1958 lines; Dodge and Chrysler each used three displacements, from the 1955 to 1958 cars (though not the same three displacements). It would not take much longer for Chrysler Corporation to stop having different displacements for each brand (and in each successive year), which was a nightmare for dealer parts departments.
[A engines • B engines • LA engines]
1958 Red Ram distributor
Model | IBP-4002 |
Rotation | Clockwise |
Advance Control | Automatic |
Point Gap | .015 to .018" |
Breaker Arm Spring Tension | 17 to 20 |
Contact Dwell (degrees) | 29 to 32 deg |
Automatic Advance curve (Distributor speed) |