Chrysler’s Polyspherical Head V-8s were odd but great performers. They filled the gap between the Hemi head and the later wedge V-8s.

Understanding the Poly Head V-8

When the Chrysler Corporation entered the Overhead Valve V-8 era, starting in 1951, all their research pointed toward the hemispherical combustion chamber as the best design for power and torque. The result, of course, was the release of the famous 180 HP Chrysler Hemi. HP grew from 180 to 400 in the years up to 1958, with displacement only rising by 60 cubic inches!

As this was the corporate approach, both Dodge and DeSoto followed with their own Hemi designs (they were each unique with few parts interchangeable).  But as the engines came into being there was an understanding that the valve train design and the head size and weight added to production costs.  This came into focus when it came time for Plymouth to produce its own V-8.  Since Plymouth was the “economy” line, these costs needed to be mitigated as much as possible.

Understand that the engineers did not want to completely abandon the advantages of the Hemi combustion chamber but wanted to reduce parts and weight. What they created was a hybrid – going from the complex “double rocker” shaft of a Hemi to a single rocker shaft of a wedge head. After much experimentation and evaluation, the “Polyspherical” head (“Poly”) was designated for Plymouth.

What’s Different

The Poly head is truly unique in that the intake side of the combustion chamber is hemispherical, and the exhaust side is a wedge.  The engineers chose this approach because they felt that cylinder filling before combustion was more significant than exhaust gas exit after firing.  We could get deeply into this approach, but it’s not appropriate for this level of the story, so let’s just assume that they were on to something – after all millions of of these “A” Series V-8s were produced between 1956 and 1966.

Using this design, only a single rocker shaft was used, but not all the rocker arms faced in the same direction.  The intake rocker faced the carburetor, and the exhaust faced the outside, just like a typical wedge head engine.  The reason for this approach was that the intake valve had to be tilted toward the carb to allow that side of the head to be hemispherical. This required that the pushrod had to move across the head to the exhaust side to activate the rocker. Surprisingly, this angle of attack was similar to what happens in the double rocker shaft Hemi’s intake. (More about this later). Using this odd intake rocker allowed the head to use only one shaft, reducing size and weight – and complexity. The engine would be designated the “A479” V-8.

The Poly head (left) next to the Hemi head as shown in the Dodge block.

History of the Poly – It’s Not That Simple – But It Is

While this is the story of the Plymouth “A” engine, the real truth is that the Poly design was actually developed as a lower cost cylinder head for both  Chrysler and Dodge Hemi head engines. These Poly engines were developed to fill a gap in these two brands where the old L-Head six cylinder had resided for Chrysler, and for a non-performance V-8 could be used for the entry level cars at Dodge. Rather than build an entirely new engine for both brands, the Poly head was designed to be bolted on the two unique Hemi blocks with only pushrod changes!

Chrysler Spitfire
The Chrysler Windsor received this engine, designated “Spitfire” beginning in 1955 and used it through 1958. The Spitfire utilized a 301 CID block in 1955 (3.625” bore),  a 331 CID (3.8125” bore) in 1956, and a 354 CID block (3.94” bore) in 1957-58. These blocks were exactly the same as the Hemi head blocks with only a different piston. It was that the heads would be different.

In another cost-savings trick, the Poly heads were designed to accept the Hemi intake manifold. It did allow hot rodders to bolt the dual 4-barrel manifold from the 300 Series Hemi onto the Spitfire!

Dodge Red Ram
In the same fashion, Dodge designed a poly head for its Hemi block coded A388.  It was attached to 1955 Dodge 270 CID V-8s. All that utility that was designed into the Chrysler Spitfire was designed into the Red Ram, including using the same pushrod angle for the intake valve and the adaptation of the Hemi intake manifold. The Dodge Poly would be the base V-8 engine in the line from 1955 through 1958 and grow from 270 CID to 315 and then 325 CID. Dodge would use the Plymouth “A” Series Poly engine in 1959 with a slight overbore to reach 326 CID in 1959.

Starting in 1955 and designating the Poly engine as “Red Ram” added confusion to what engine was under a Dodge hood, because the original Hemi had been given this name.  Hemi engines starting in 1955 would be called “Super Red Ram”.  It is said that many buyers were disappointed to see a Poly head engine under their hood in those years due to this naming scheme.

Plymouth’s First Poly
We talked about the Plymouth-designed “A” engine initially, but the first issuance of a Poly in Plymouths happened with the introduction of the V-8 for the line in 1955. It was the Dodge Poly V-8, de-bored to both 241 CID (initial release) and 259 CID (late release) that year. Plymouth’s own “A” series would debut in 1956.

The Plymouth Poly Growth
At its introduction in 1956, the A engine was 276 CID (3.75” x 3.125”) . It grew to 303 CID (3.8125” x 3.3125”)  for the Fury that year, using a Canadian supplied block. The base Poly grew to 301 (3.9125” x 3.125”) for regular Plymouth models in 1957 and to 318 CID (3.9125” x 3.3125”) for the Dual 4-barrel Fury in 1957-58.  Thereafter, it would stay at 318 CID and would be the workhorse V-8 for Plymouth through 1966. Dodge used this engine in 1959 with a slight overbore to reach 326 CID and used it as their base V-8 since their old Hemi/Poly block had been retired.

NOTE
The architecture of the old A Series block was so good that when the “LA” lightweight block was released in 1967, much of the parts and pieces from the old block were retained – and that engine spawned the famous 340 and 360 V-8s.

The Last Oddball

While not a Poly engine, few know that Chrysler offered a 383 CID V-8 in 1959-60 that was not the famous 383. As before, Chrysler wanted their entry-level Windsor line to use the 383 Series B wedge that had been assigned to DeSoto, Dodge and Plymouth as their top dog engine.  However, production was not up to speed to meet this requirement.  As a result, the 413 CID RB Series engine was de-bored from 4.18 inches to 4.03 inches, resulting in that same CID as the real 383. It was a two-year only V-8, replaced by the true B Series 383 when production came up to speed.

Please see each tab section for a breakdown of the available variants of the engine over time.

1955-1958 Chrysler Poly Head V-8s

1955-1959 Dodge Poly Head V-8s

1955-1960 Plymouth Poly Head V-8s

NOTE: After 1960 the Plymouth “A” Series Poly Head was Shared as the Base V-8 with Plymouth and Dodge
See the Fact Sheets Below for Specific info