Plymouth Polyspherical Head Explained

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The 1956-1967 Plymouth Polyspherical Head Explained

The Plymouth Polyspherical or “A” engine, was a unique design, unlike the Dodge and Chrsler versions which were “Poly” heads on the Hemi block(s).

277 Through 318 A Series Polyspherical Head V8s

As is well known, the first OHV V8 engine produced by Chrysler Corporation was the original “Hemi”, actually called the “double rocker” V8. While this engine had great power potential and volumetric efficiency, the hemispherical head and complex valve operation added cost, size, and weight, and even lengthened the engine build time. Plymouth was the lower cost “value brand” so there was no way their management could justify a Hemi V8, but they recognized that a V8 was going to be necessary to compete with Ford and Chevrolet.

So not willing to add a Hemi to the Plymouth line, and to address the cost, complexity and weight issues, Chrysler adapted the Hemi engine design by substituting a polyspherical shaped head, with valves activated by a single rocker shaft. The shaft was centered between the valves and actuated intakes and exhausts on either side. What it wasn’t was a wedge chamber with inline valves.

It was not quite the full hemisphere chamber, of the Hemi but it still had a cast, but rounded, circular combustion area that could be used with that single rocker arm. To take advantage of this head design, Chrysler engineers put the valves in a diagonal arrangement, with intake valves on the top side of the rocker arm and the exhaust valves on the bottom side. (See diagram at right).

As such, the bottom of the new engine’s valve covers were uniquely scalloped around the exhaust valve spring. The head design allowed that the spark plugs were accessible from the top – unlike Ford and Chevrolet wedge chambered V8s. The “Poly” engine had hydraulic lifters and came with a two-barrel carburetor, as standard equipment.


The first engine released to Plymouth in 1955 was the Dodge block of 241 inches, which was designed to use both Hemi and Poly heads, with Plymouth getting the latter design. In its original form, this “Poly” developed 157 HP, a good solid number. An optional 260 cu. in 167 HP version was also offered – both with a 2 barrel carb. Shortly after, a 4-barrel version was delivered that produced 177 HP.


For 1956, a completely new, designed from the ground up, Polyspherical head engine was introduced, called the “Hy-Fire” V8. It had it’s own head and block, unique from the Dodge designs. Plymouth engineers made the new block slightly longer, with more “meat” between the cylinder bores for future bore size increases. They also designed a larger crankshaft with larger bearing surfaces, and added enlarged the valves for better breathing.

The result was the all-new Plymouth 277 cubic inch V8. This 187hp V8 was exclusive to the Belvedere and Sport Suburbans and had a bore and stroke of 3.75″ x3 .13″. For those wanting more power, the 277 was available in a “Power Pack” version that included a four barrel carburetor and dual exhausts. It developed an even 200 hp.

Later in the year, the engine was expanded to 303cubic inches and would make its appearance in the Fury model. The 303 was actually a Canadian-sourced engine, with a bore and stroke expanded to 3.8125″ x 3.3125″, and a compression ratio of 9.25:1 using the 277’s heads. With its single Carter WCFB 2442 S four-barrel carburetor, it pumped out an honest 240 HP at 4800 rpm. It featured reinforced dome pistons, a high performance camshaft, high load valve springs, balanced connecting rods, and high-speed distributor.

In the newly developing horsepower race, Plymouth released a dealer-installed “High Performance Package” in the spring of 1956. This kit included dual four-barrels, special air cleaners, an aluminum intake manifold, and high performance camshaft. The kit was available for both the 277 and 303 V-8s. The kit raised the 277’s HP to 230, while upping the 303’s power to 270.

The old Dodge Poly, now expanded to 270 cubic inches, was carried over and used in the lower priced Savoy and Plaza models. Both engines shared a 8:1 compression ratio and developed�its 180 or 187�HP at 4400 rpm.


In 1957a new series of Polys was introduced. The Canadian 303 was replaced with a 301 cubic inch version which was available in all car lines except the Fury. The performance line received a 318 cubic inch version. The 301 was a one-year offering and was based on the 318 block, but used the the same 3.125″ stroke as the old 277, but had a 3.91″ bore vs the 3.625″ bore of the former.

Plymouth also released the “V-800” option for the Fury only. This equipped the 318 V8 with a 2-four barrel intake and a high performance camshaft.

1958 and Beyond

Starting in 1958, the only Poly offered was the 318. This would now be rated at 225 HP in 2 barrel form, and 250 in 4 barrel and 290 in 2 4-barrel format. But from now on the new “B” series engine would be the performance option. From 1959 thru its phase out in 1967, the Poly would be the yoeman V8 for Plymouth.

Eric White Digital Library

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