Poly 318 History and Debunking Myths

Poly A-block Engine Assembly Plant

The 1950s came with the most confusing engine decade for the Mopar divisions but saw technological advancements that would become mainstays of the muscle-car era onward. There is a lot of confusion about the history and terminology surrounding the A-block that I try to clear up here based off factory brochures, manuals, talking with dedicated enthusiasts, and my own experience with different years/models.

Combustion Chamber Design

Before leaping into the A-block changes throughout the years, a lot of people might ask how a “poly” engine differs from a “hemi” and a “wedge.” It’s all in the combustion chamber and for Mopar developed in the 1950s. For 1951, Mopar introduced a V8 with hemispherical combustion chambers (Figure 1) whose intake and exhaust valves were opposite each other. They designed another head for 1955 that bolted to the same hemi-block engines (Figure 2) whose intake and exhaust valves were diagonal each other down in a well. This design became known as polyspherical design. For 1956, Plymouth designed a new engine also with polyspherical combustion chambers that they named the A-block (Figure 3). For 1958, Mopar aligned itself more with GM and Ford designs by introducing the wedge-shaped combustion chamber on B-block engines (Figure 4) whose intake and exhaust valves are inline in a wedge-shaped chamber like the vast majority of popular Mopar, GM, and Ford engines. Now that the terminology is set, onto focusing on the A-block.

Historical Timeline with Changes


The Plymouth division introduced the new A-block 277 for 1956 USA models, overboring it to a 303 for export models. It was not available in any other Mopar division. The design borrowed the idea of polyspherical combustion chambers of Chrysler and Dodge hemi-block poly engines (Desoto never offered a poly hemi-block engine), but the A-block was a completely new and advanced design with multiple features surviving in Mopar engines into the 21st century.

1957 – 1958

1957 brought a lot of changes, so try and keep up: Plymouth was still the only division offering the A-block but revised the block into a 318 that came standard in USA Fury models with a 313 in exports (an under-bored 318). They also used up the remaining 303s in exports and discontinued the 277. Unique to 1957, Plymouth used up the leftover 277 crankshafts in the new 318 blocks to make an oddball 301. Now we need to address the confusion around the “V800” A-block. Many people online claim the V800 is the dual-quad carburetor version of the 318, but this isn’t accurate. In fact, all 1957 – 1958 318s were called “Fury V-800.” The 1957 – 1958 Fury line offered three different versions of the A-block: “Dual Fury V-800,” “Fury V-800 Super-Pak,” “Fury V-800” (Figures 5 and 6). Let’s break these down:

  • Dual Fury V-800 318: The only factory “high-performance” A-block from 1956 – 1966 and optional in 1957 – 1958. It boasted a cast-iron dual-quad intake manifold with Carter carburetors, a hotter cam, higher compression of 9.25:1, dual exhaust, dual-point distributor, and resistor spark plugs. It put out an advertised 290 HP at 5,200 rpm and 330 ft./lbs. torque at 3,600, which enthusiasts have confirmed on dynos to be rather accurate.
  • Fury V-800 Super-Pak 318: An optional engine with cast-iron single Carter four-barrel intake manifold, compression of 9:1, dual exhaust, and performance-curve distributor. It put out an advertised 250 HP at 4,400 rpm and 340 ft./lbs. torque at 2,800.
  • Fury V-800 318: The standard engine for 1957 – 1958 (except Fury models) that would go on to be the standard V8 for most Mopars through 1966. Single cast-iron two-barrel induction with compression of 9:1. It put out an advertised 225 HP at 4,400 rpm and 330 ft./lbs. torque at 2,800.


In 1959, Plymouth continued with the A318 (A313 export), Chrysler began offering the A318, and Dodge overbored the A318 to 325 (advertised as a 326) and added a hydraulic flat-tappet camshaft, lifters, and nonadjustable rocker arms. This was the first time A-blocks were offered with a hydraulic cam and lifters, and many industrial, heavy truck, and marine A318/A313 engines would utilize similar parts through end of production.


By the 1960 model year, Plymouth, Chrysler, and Dodge offered only the A-block 318 in the domestic and A313 in export models as their small-block option. 

The 1962 Redesign

In 1962, the A318 saw major changes to the block and components that would last through the end of production in 1966 and in many ways continue into the LA-block and then the Magnum. Of particular importance for those interested in running an A-block, 1957 – 1961 A-blocks use the same bellhousing bolt pattern and alignment position as many of the 1950’s hemis and hemi-block polys. The crankshaft flange is 1/2″ longer on these earlier A-blocks to mate to cast-iron PowerFlite and TorqueFlite transmissions. Therefore, the 1956 – 1961 A-blocks will not accept a 1962 onward aluminum TorqueFlite transmission without an adapter (see the crankshaft and transmissions page for more details and photos on transmission options and the year differences). The redesigned 1962 A-block bellhousing bolt pattern and alignment position would go on to become the LA and Magnum patterns. Mopar engineers would use many of the internal and external parts of the 1962 redesigned A-block in the LA 273 released in 1964 followed by the 318, 340, and 360, which I detail in the parts interchange section.

Industrial and Marine Versions

Starting in around 1959, Mopar began producing both industrial and marine versions of the A-block that I detail on the engine specifications page.

Debunking “Poly,” “Junk,” “Wide Block,” and “Boat Anchor” Myths and Confusion

Now that I have hopefully cleared up some of the general history, I want to address other general misconceptions about the A-block that have survived through the years regardless of the facts.

What, There Are Two Poly Engines?

Many people throw around the term “poly” as if there was only one type of 1950’s Mopar engine family with polyspherical heads. What we call the “poly 318” is quite simply an A-block family that includes the 277, 301, 303, 313, 318, and 326. All factory A-blocks have polyspherical heads. The confusion stems from Mopar having multiple 1950’s V8s based off the hemi block that have polyspherical heads including Dodge/Plymouth 241, 259, 270, 315, 325 and Chrysler 301, 331, 354. These engines are typically referred to as “polys” or “semi-Hemis” but are extremely different from the A-blocks with almost no parts interchangeable. I find that using the “A-block” and “hemi-block” distinctions clears up the “poly” confusion.

Throw Away that Junk Engine!

As early as 1956, hot rodders began removing their A-blocks to replace with the larger Hemis, B-blocks, and eventually RB-blocks. In 1968 with the introduction of the LA340, people had even greater options to replace their A318s with a small block of larger displacement. Performance LA parts that do not fit A-blocks were more readily available as soon the LA was born in 1964, and the A318 largely fell by the wayside. Due to all the engine swapping and streets lined with A-blocks awaiting the junkman, myths started and have perpetuated that the A-block is a terrible engine design and cannot produce power. Going up against a 450 or 600 HP A-block 390 or 402 stroker turning 7,000 rpm at the track might change one’s perception or at least give them a run for their money down at the traps. A-blocks in fact have strong castings with high-nickel iron, factory forged crankshaft, full-floating pistons, and an advantageous combustion chamber design.

The “Wide-block” Poly 318 Is Wider than an LA and RB!

I regularly come across forum posts of someone enthusiastically interested in pulling their LA or B/RB and replacing it with an A-block just to get responses, “Good luck fitting it. Those wide-block 318s are wider than a 440!” No they aren’t, and they in fact are not wider than an LA318. The A318 and LA318 blocks are nearly identical in dimensions to where most people would not be able to tell them apart side by side. I have measured an A318 and LA318 with factory valve covers side by side, and the A318 valve cover protrudes 1-7/8″ horizontally more than the LA valve cover on each side, but there is more to explain about this measurement (Figures 7 and 8). While the valve cover width differs, both the A318 and LA318 are the exact same width of 24-1/2″ from outside of factory exhaust manifold to outside exhaust manifold. The protruding A318 valve covers extend out about flush with the exhaust manifolds, whereas the LA318 (and B/RB) exhaust manifolds stand proud of the valve covers. This visual difference gives the A318 an optical illusion of being wider than an LA and with enough crossing of the eyes a B/RB. I have also measured and detailed in Figure 9 a B/RB to further prove that and A-block isn’t wider than an LA and certainly not a B/RB.

Those A-blocks Are Heavier than a 440!

Mopar’s marketing team did their job very well advertising the LA273 as the lightest Mopar V8 ever made. What they neglected to publicize was what some Mopar sleuths, myself included, have confirmed by actually weighing parts. All in all, the A318 is between 29 and 60 lbs. heavier than the LA318 depending on intake, water pump, and accessories used. The heaviest A-block is the A318 with a “Dual Fury V800” 2×4 intake, and that engine loaded from oil pan to carburetors, pulleys to rear crank flange (without alternator, power steering, or air conditioning) weighs about 561 lbs., which is about the same as an LA360 and about 100 lbs. lighter than an RB. Here’s the weight breakdown of parts that are unique to the A and LA:

  • A318 bare block: 192 lbs.
  • LA318 bare block: 169 lbs.
    • Difference: 23 lbs.
  • A318 heads (w/ valves installed, no rockers): 110 lbs.
  • LA318 heads (w/ valves installed, no rockers): 102 lbs.
    • Difference: 8 lbs.
  • A318 pistons: 10.55 lbs.
  • LA318 pistons: 10.42 lbs.
    • Difference: 0.13 lbs.
  • Total difference: for these parts, the A318 is about 31 lbs. heavier than the LA318.


By working through the history and some of the misconceptions about the A-block, I hope to have educated those interested and set some of the record straight. There are of course logical, reasonable arguments for running one engine over another engine, and the A-block will not fit everyone’s purpose, but the myths above should have no place in making that decision.


Mopar hemi head combustion chamber
Figure 1: 1950’s Hemi Combustion Chamber (photo of 392 Hemi)
Mopar Poly Hemi Head
Figure 2: 1950’s Hemi-block Poly Combustion Chamber (photo of Chrysler 331)
Mopar Poly Combustion Chamber A Block Poly 318
Figure 3: A-block Poly Combustion Chamber (photo 1964 318)
Mopar Wedge Head Combustion Chamber
Figure 4: B, RB, LA Wedge Combustion Chamber (photo of LA273)
Poly A-block Fury V800 SuperPak Engine Options
Figure 5: 1957 – 1958 Plymouth Fury Engine Options (from 1958 brochure)
Poly A-block Fury V800 SuperPak Engine Options
Figure 6: 1957 – 1958 Plymouth Fury Engine Options (from 1958 brochure)
Plymouth Dodge Chrysler Poly Polyspherical Engine Dimensions
Figure 7: A-block Poly Dimensions
Plymouth Dodge Chrysler LA 273 318 340 360 Engine Dimensions
Figure 8: LA 318 Dimensions
Plymouth Dodge Chrysler B RB 350 361 383 400 413 426 440 Engine Dimensions
Figure 9: B and RB Engine Dimensions
Vintage Hot Rod A-blocks

Please send in your vintage A-block photos!

Vintage poly 318 race car
Ian Bell & Alf Mullins Australian Rail (circa 1962)
Vintage poly 318 race car
Ian Bell & Alf Mullins Blown Australian Rail (circa 1964)
Vintage poly 318 race car
Unidentified Rail with Pre-1962 A-block & Crank-driven Blower
Vintage poly 318 race car
Larry Ferguson’s Rail with Pre-1962 A-block & Mechanical Fuel Injection
Vintage poly 318 race car
Larry Ferguson’s Rail (body off) with Pre-1962 A-block
Vintage poly 318 race car
Roadster Hot Rod (photo courtesy owner/driver Tony Cooke)
Vintage poly 318 race car
Export Poly A-block Stock Car (photo courtesy owner/driver Steven Wickham)
Vintage poly 318 race car
Export Poly A-block Stock Car Wrecking (photo courtesy owner/driver Steven Wickham)
Vintage poly 318 race car
Poly 318 with Tunnel Ram (car/driver unknown)