Farley`s Dodge Diplomat Forum

Forum Index => The Library => Topic started by: Reggie on January 14, 2008, 00:35:20

Title: Engines: Poly 318 "A" Engines vs. LA 318 Engines
Post by: Reggie on January 14, 2008, 00:35:20
Poly 318  "A" Engines vs. LA 318 Engines

Polyspherical (or Poly) "A" engines first appeared in 1956. The term refers to the design of the cylinder heads, all of which had canted valves.  The valves were also arranged so that all of the same type valves were on the same side of the head. Their basic block design was very similar to the early Dodge Hemi and Poly blocks, but no parts interchanged.  Although the Hemi heads were superior breathing heads, they were heavy and more costly to manufacture, so Poly heads were developed across the Chrysler model lines as a cost saving measure.  In the Chrysler and Dodge divisions, the Hemi blocks could be interchanged with Poly blocks and vice-versa. 

The lower priced Plymouth line never received a Hemi engine because it never offered a V8 prior to the 1955 model year. By November 1954 when Plymouth's first V8 was introduced in the 1955 models, Chrysler was planning to phase out the Hemi design. Because Plymouth had no V8 of its own, 1955 model engines were actually Dodge Polys supplied by the Dodge Division. The displacements were 241 ci and 260ci, and they developed 157 HP and 167 HP respectively.

The first Plymouth-designed V8 appeared in 1956 with a 3.74" X 3.12" bore and stroke, displacing 277ci and making 187 HP. It was very similar to the Dodge design, but it had a longer block for better cooling between the water jackets, and larger valves requiring redesigned heads. For 1957, the 277 was bored out to 3.91" to produce 299.6ci (actually called '301' by Plymouth - probably to avoid confusion with the Chrysler "300" series cars). Also in 1957, the Poly 318 was born by changing the stroke of the 301 to 3.31, for a bore and stroke of 3.91 X 3.31, and a displacement of 317.6ci.  1957 Plymouth Fury models got a dual 4BBL 290 HP 318!  Poly 318 engines were phased out of passenger cars after 1966, but were used a few more years in some truck, school bus and industrial applications.

Employing a new lightweight casting technique to save weight, the design of the Poly 318 block was later used for the LA 273 engine which first appeared in compact A-bodies. As a result, the Plymouth 277 "A" engine is the direct ancestor of the entire LA engine family, the Magnum family and even the LA-based Viper V10 engine.
Below are the differences and similarities between the Poly "A" engines (hereafter referred to as "Poly", and the newer LA engines. 

Differences Between Poly "A" engines and LA engines

Similarities Between Poly "A" engines and LA engines

Will Poly Heads Fit an LA Block?

Because of the deep reliefs required in Poly blocks for the straight Poly head pushrod angle, it is not certain whether Poly heads will work on LA blocks.  Poly heads definitely won't work on Magnum blocks, because they are oiled through the deck, and Magnum blocks usually don't have the required oil passages.

Poly Aftermarket Support

There is not as much aftermarket support for Poly engines as with the LA and Magnum series engines, but a few manufacturers are now offering parts for them - notably TTI with their excellent headers. They are not cheap though. The Poly engines will also accept the 4" cranks, which are now the rage with the Poly enthusiasts. Properly set up, they can make some serious power.