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Author Topic: Engines: Poly 318 "A" Engines vs. LA 318 Engines  (Read 56278 times)

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Engines: Poly 318 "A" Engines vs. LA 318 Engines
« 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

  • Heads are very different:
    • Intake and exhaust arrangement for the Polys is I-E-I-E-I-E-I-E, where the LA/Magnum arrangement is E-I-I-E-E-I-I-E.
    • Poly valve arrangement is canted with intake and exhaust valves angled opposite each other, while LA valve arrangement is wedge style with all valves aligned in a row at the same angle.
    • Poly heads have the intake valves on one side of the rocker shaft and the exhaust valves on the other side, while LA engines have all of the valves on the same side of the rocker shaft.
    • Poly and LA Rocker arms are different.
    • Poly heads are much heavier than LA heads.
  • Valve covers are different:
    • Poly valve covers have two bolts in the middle of the cover. LA covers have five smaller bolts around the outside edge of the cover.
    • Poly valve covers are also scalloped at the lower edge, while the lower edge of LA valve covers is straight.
  • Pushrod angle is different. Poly 318 blocks had the original 59 degree lifter angle to work with the Poly heads, so the pushrods are much closer to being in line with the lifters than in LA and Magnum engines. Because Chrysler carried over the basic Poly block design with different head designs for the LA and Magnum families, the pushrod angle is much worse.
  • Poly blocks are different (although they will accept LA heads):
    • Poly blocks have deeper reliefs on the intake side of the deck to clear the straighter pushrod angle required by the Poly heads.
    • Poly blocks are a bit heavier due to their non thinwall castings.
    • Poly blocks can sometimes take up to a .130 overbore - much more than LA blocks.
    • There is a possibility that one bolt hole in the bellhousing area may be a little different on earlier versions.
  • Poly and LA camshafts are different due to the different valve arrangement
  • Poly lifters were solid.  Except for early LA 273 engines, all LA lifters were hydraulic.
  • Poly "A" Pistons are different due to the different cam timing, so the valve reliefs are in the wrong places for LA engines.
  • Intake manifolds are different
  • Exhaust manifolds are different

Similarities Between Poly "A" engines and LA engines

  • Poly "A" blocks can be used with newer LA and Magnum heads.
  • Transmission bellhousing bolt pattern is the same.
  • Crankshafts interchange with 273/318 and internally balanced 340 LA engines.
  • Oil pump and distributor drive shafts are the same.
  • Oil pans from 273/318/340 LA engines will fit
  • Dampers from 273/318 and internally balanced 340 LA engines will fit.
  • Distributors from all LA engines will fit.
  • Poly timing chain and sprockets are the same as LA engines.
  • Poly block deck height, bore spacing and 59 degree lifter angle are the same as LA blocks.

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.



  • Poly vs. LA valve arrangement (Poly illustration courtesy of Small Block "A" Engines, LA head photo and comparison ©  2009 Reginald A. Royster, Sr.)
  • Hemi vs. Poly vs. LA Head Design Comparison (Contributor unknown)
  • Poly 318 Valve Design (Courtesy of Chrysler Corporation
  • Poly 318 in a 1964 Dodge (Contributor unknown)
« Last Edit: February 18, 2011, 23:41:39 by Reggie »
"There is only one success - to be able to spend your life in your own way." - Christopher Morley 1890-1957, American author and editor

Library format and content © 2007- 2012 Reginald A. Royster, Sr., unless otherwise authored or noted.

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