For Plymouth in 1955 the biggest news of the year was, of course, the “new”
V-8 engine. The engine was not really new and was supplied to Plymouth by Dodge
division until a new engine foundry could be completed to supply Plymouth with its
own engines. Plymouth’s first true V-8 would not appear until 1956, and then only
slightly changed from the Dodge version.
 At first the Plymouth V-8 was offered in two displacements-a 241ci rated at
157hp with a bore and stroke of 3.44x3.25 in. and a 167hp, 260ci engine with a
larger bore of 3.563 in. Both engines featured a compression ratio of 7.6:1. Shortly
after introduction, a four barrel 177hp version of the 260 hit the streets. Maximim
torque was achieved at 2400 rpm on all engines, the 241 pumping out 217-ft and the
260 claiming 231lb-ft whether 2 or 4 Bbl equipped. Called the Hy-Fire V-8, it was
Plymouth’s first version of a Poly engine.
 The Hy-Fire engine achieved some of a Hemi’s free-breathing characteristics
by placing intake and exhaust valves opposite each other in the combustion
chamber. This provided better breathing while not restricting valve size as was the
case in some other small block engine designs. The design not allowed adequate
“breathing” on the intake cycle but allowed the exhaust valve to open further into
the combustion chamber to exhaust spent gasses. Thus creating one of the famous
features of a Poly head.
 In 1956 the big news was that the new Hy-Fire V-8 was designed and built by
Plymouth. Plymouth engineers made the block slightly longer, with more “meat”
between the cylinder bores, installed a larger crankshaft with larger bearing, and
enlarged the valves for better breathing.
 The new 277ci. 187hp was exclusive to Belvedere and Sport Suburbans and
had a bore and stroke of 3.75x3.13 in. This was the first of the 2nd generations
Poly’s. The old Dodge 270ci was carried over for use in the Savoy and Plaza models.
Both engines shared a 8:1 compression ratio and developed their max Hp at 4400
rpm. For those wanting more power, the 277 was available with a power pack that
included a four barrel carburetor and dual exhausts and developed an even 200 hp.
Later in the year, a potent 303ci. engine would make its appearance in the Fury. The
303 was a Canadian-sourced engine pumping out an honest 240hp at 480 rpm. A
bore and stroke of 3-13/16x3-5in. produced a compression ratio of 9.25:1, and air
fuel were mixed by a single Carter WCFB 2442 S four-barrel carburetor.
Reinforced dome pistons, a high performance camshaft, high load valve springs,
balanced connecting rods, and high-speed distributor rounded out the engine
package.
 For high performance enthusiasts, Plymouth released a dealer-installed High
Performance Package in the spring of 1956. Retailing for $746.90, the kit included
dual four-barrels, special air cleaners, an aluminum intake manifold, and high
performance camshaft. This kit was available for both the 277ci. Belvedere V-8 and
the Fury 303ci. V-8. The kit  raised the 277’s hp to 230 while upping the Fury’s 303
to 270hp.
 In 1957 the 303 was gone and in its place was the 301ci. which came in all car
lines except the Fury which offered the 318 as its smallest V-8. The 301 was only
available in 1957 and was based on the 318 block, but had the same 3-1/8in. stroke
as the 277.( the 301’s bore was bigger though) What Plymouth called the V-800
option was the dual quad, high performance camshaft 318 and it was available only
in the Fury. With the advent of these engines in 1957 the 277,301,318 plus the
Canadian 303 was the beginning of the 2nd generation Poly’s.

 Taken from Plymouth 1946-1959 by Jim Benjaminson
 

The 270-cubic inch Hy-Fire engine is used on Detroit-built Plaza and Savoy models
unless a power package is specified by the customer. Basically the same engine as
used last year, the bore of this Hy-Fire engine has been enlarged to increase its
displacement by tem cubic inches. It can be identified by its spider-like intake
manifold, stamped steel tappet chamber cover, and an oil filler neck protruding
centrally from the tappet chamber. Its automatic choke is the same carburetor-
mounted integral unit used in 1955. Hydraulic tappets are standard.

The 277-cubic inch Hy-Fire engine is used on all Belvedere and Suburban models,
and also on Plazas and Savoys built outside of Detroit. Every model with the power
package also has the larger engine. The 277-cubic inch engine can be identified by
its combination intake manifold and tappet cover, its oil filler neck located on the
left-bank rocker arm cover, and its new Mixturemizer automatic choke, built into a
well in the intake manifold. (This was the first of the 2nd generation Poly engines.)
 

Design Features of the 1956 V-8 Engines
New on both Plymouth engines this year are several improvements designed to give
them more power, more economy, longer life and greater smoothness. Recalibrated
carburetors help give substantial improvements in fuel economy. Aiding them are
higher compression ratios, new design spark plugs and redesigned Polysphere
chambers. Longer life is promoted by low-friction valve locks on intake and exhaust
valves, chromium-alloy cylinder heads, and integral valve seats.

The New 277-Cubic inch Engine
Completely new for 1956, the 277-cubic inch Hy-Fire engine is the latest addition to
the Plymouth engine line. It is produced in a brand-new “Qualimatic” engine plant
and incorporates a number of outstanding new features. A longer cylinder block
gives more space for cooling water to circulate between cylinder bores for better
cooling. Bigger valves give greater volumetric efficiency by offering less restriction
to the flow of incoming mixture and outgoing exhaust gases. This improves
“breathing” at higher engine speeds for more power. Mechanical tappets are new on
the 277-cubic inch Hy-Fire engine. This simplified tappet design is made possible by
the light weight of the valve train, the geometry of the linkage and the superior
quieting contour of the cam. Self-locking adjusting screws on the rocker arms
maintain correct tappet clearance.

A power package is available at extra cost on the 277-cubic inch Hy-Fire V-8. It
consists of a 4-barrel carburetor, a different distributor and a dual exhaust system,
and raises horsepower to 200, torque to 272.

In either the 270 or the 277-cubic inch form, the new Hy-Fire V-8 is a power plant
matched to the demands of today’s driving. Its greater displacement gives higher
torque and more power. The built-in quality of the Hy-Fire V-8 again brings to the
low-price field the same fine engineering found in the most luxurious of the Chrysler
Corporation line of automobiles.
Courtesy of Chrysler Historical Society