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Mopar A Engines

The A engine family was a modern, efficient, and durable design produced from 1956 through 1966; the LA series of V8 engines was based on a lightweight casting of the A engines, appearing in 1964. The A and LA blocks are similarly sized and hard to tell apart from the outside, but the A's polyspherical heads were considerably larger than the LA's heads.

Willem Weertman, the legendary engine designer, started his Chrysler engines career working on the A-type V8s at the then-state-of-the-art Mound Road plant.

The A engines shared little with the Hemi-based Poly V8s; since Plymouth’s sales were far larger than Dodge or Chrysler, and pricing was lower, the A-type V8 engines needed to be made far more quickly at the new Mound Road engine plant. That was a strong influence on its design.

valvesThe “single shaft” (Hemi-based) Poly can be identified by its separate valley cover underneath the intake manifold; Randy Hicks wrote that the cylinder heads and intake manifolds were interchangeable with any Hemi engines that had the same deck height. A-series Poly engines did not have the separate valley cover underneath the intake manifold, since the intake itself performed this function. Both series of Poly engines — A-based and Hemi-based — were similar in appearance, with scalloped valve covers and a rear-mounted distributor, despite being from completely different engine families.

The A engines went under trade names including Red Ram. It was an overhead valve design with dome-shaped heads. Dodge wrote, “So durable are they that the standard 25,000­ mile laboratory tests for wear had to be changed to 50,000-mile and 100,000-mile checks-because no discernible wear showed up until long past the 25,000-mile figure.”

EngineYears (USA)*BoreStroke
303 1956-573.813.31
326 1959 (Dodge)3.953.31

The A-engines slowly replaced the polyspherical-head (“single-rocker”) V8 engines: 277, 301, 331, and 354. A key difference between the A engines and the older poly engines — essentially, Hemi engines with modified combustion chambers and single rocker arms — was in the ease of manufacturing.

While Dodge, DeSoto, and Chrysler had relatively small sales numbers and could live with the low pace of building the poly V8s, Plymouth’s high production required something cheaper and faster to build. The A engines fulfilled that function, with little, if any, performance penalty. Any loss in performance could be made up in cubic inches, and the A engines ran larger than most of the polys.


1957 Plymouth Figures 277 301 318 red ram engine
Gross Horsepower 197@4,400 215@4,400 n/l
Torque (lb-ft) 270@2,400 285@2,800 n/l
Compression 125-165 psi
Bore x Stroke 3.75 x 3.125 3 29/32 x 3.125 3 29/32 x 3 5/16
Compression 8:1 8.5:1 9.25:1
Firing Order 1-8-4-3-6-5-7-2
Bill Watson noted that the LA-based 273 used the poly (A) 318 crankshaft, bearings, bearing caps, vibration damper, and conecting rods. The 273 and 318 both had a 3.31" stroke. Pistons were different, though, as the bores were different while camshafts could not be shared due to the different valve arrangements. The sharing of parts was one of the things Chrysler bragged about when the 273 was introduced, though little on the 273 was shared with the earlier 277/301 A-engines.

Red Ram engine

1962 318 figures 318 truck 318 2-barrel 318 4-barrel
Gross Horsepower 200, 202 230@4,400 260@4,400
Torque (lb-ft) 286/288@2,400    
Compression   9.0 9.0

Canadian notes (Bill Watson)

318 cubic inch A-engineThe 303 was introduced in Canada before the U.S. Plymouth Fury came on the market. It was used in the Canadian-built 1956 Dodge Custom Royal (2-bbl) and 1956 Chrysler Windsor (4-bbl). In 1957 is was used as the standard V8 in the 118" wheelbase Plymouth and Dodge models.

The 313 V8 appeared in 1957 in the Canadian Custom Royal. For 1958, the 313 became the standard V8 for the 118" wheelbase Plymouth and Dodge models. It was available in Plymouth and Dodge models through 1964, and was replaced by the 318 for 1965. The poly 318 was last used in Canadian production on the 1967 models.

The Canadian 1958 Dodge Custom Royal used the poly version of the 354, shared with the Canadian-built DeSoto Firedome and Chrysler Windsor. Chrysler of Canada did not start using the B block V8 until 1959.

The A engines: 277, 301, 303, 318, 326

(Horsepower figures are gross until 1971.)

Bill Watson noted:

The poly head first appeared in 1956, with Plymouth using a 241 cid version and Dodge using a larger-bored version with 270 cid. In mid-year Plymouth increased the bore of the 241 to 3.56" for 260 cid. Plymouth used the 270 with poly heads in 1956, but replaced it with a new 277 engine, the first A block. It shared little with the Dodge V8 and came only as a poly. The early hemi heads will not fit on the A block.

So, the 303-cid V8 was used, based on the 277 V8 block, with the 3.75" bore replaced by a 3.81" bore. The adoption of the 277 size over the 270 reduced costs - they could share crankshafts and piston rods.

Chrysler also offered a poly head engine in 1955 on the Windsor, using a small bore version of the 331 (3.63" vs 3.81") for 300 cid. The Windsor never came with a hemi engine, using poly heads through 1958.

The A engine was built through 1967, actually, with 1966 being the last year in the U.S. and 1967 in Canada. The US-built 1967 LA 318 has engine number prefix C318 while the Canadian 1967 poly A 318 has engine prefix CC318. (The "C" stands for the 1967 model year while the second "C" for the poly stands for Canada)

1960 318
vs GM and Ford
Ford 292 Dodge Red Ram 318 Chevrolet 283
185 hp 230 hp 170 hp
292 lb-ft 340 lb-ft 275 lb-ft

Joshua Skinner wrote:

In 1957, the 277 grew to 301 ci with a 3.91" bore and the same 3.12" stroke. Also available beginning in 1957 was 3.91" bore and 3.31" stroke engine netting 318ci. The hottest version of the early A engine was the 1957 Fury 318 with dual quads producing 290 horsepower, the same rating as a 340 6-pack. The lesser versions were rated at 230 hp (318 2v) and 260 hp (318 4v, made from 1958 to 1962). The 1959 Dodge "Red Ram" engine was part of the A family (325 or 326 cid).

hyfire v8

Bill Watson noted:

Chrysler produced three different hemis during the 1950s, with different blocks for Dodge, DeSoto, and Chrysler. DeSoto never built a poly engine, although they did use the A-block poly in the export DeSoto Diplomat, the Dodge-based poly in the 1957 DeSoto Firesweep and Chrysler's 354 poly in the Canadian-built 1958 DeSoto Firedome.

1960 Dodge Figures 277 301 318
Gross Horsepower 197@4,400 215@4,400 230
Torque (lb-ft) 270@2,400 285@2,800 340
Compression 125-165 psi
Bore x Stroke 3.75 x 3.125 3 29/32 x 3.125 3 29/32 x 3 5/16
Compression 8:1 8.5:1 9.25:1
Firing Order 1-8-4-3-6-5-7-2

(Drew Beck reminded us of the 303 V8 used only in the 1956 Plymouth Fury, 240 hp with a four barrel carb; and the Dodge-only 1959 326 V8.)

In 1960, the 318 cubic inch Red Ram V8 produced 230 gross hp at 4,440 rpm, and 340 lb-ft of torque at 2,400 rpm, fed by a two-barrel carb and using a 9:1 compression ratio.

valvesNicholas Challacombe wrote:

I have a 1962 Bristol 407 with an A type engine of 313cu inches or 5130 cc. It has a four barrel Carter carb 3131s and the engine bore is stated in the work shop manual as 3.875/3.877 inch. These engines came from Canada. Apparently the 318 is a 313 bored out by 30 thou.

All Bristol 407s have the 313 as did all 408s except for the last 18 which had 318. All the Bristol 313s had a performance pack and produced 250 hp.

(The photo at the start of this section is the 313, rated at 220 horsepower (gross) in Australian Royals.)

The A-engines were supplanted by the closely related, but lighter, LA V8 engines. Bore center sizes and bore lengths on the 318 A-series and LA-series engines were identical, but the A-engine was 55 pounds heavier.

The most clearly visible differences is in the heads: the LA engines used wedge heads, while the A-engines used much wider, heavier polyspherical heads. Willem Weertman said,

engine adI was in the production plant when we were first building the A engine series and then I had been transferred to engineering when we were building the LA engine series. I was in charge of design for the LA engine conversion.

The biggest difference between the LA and A engines is really the valve arrangement. We went from a skew valve type of arrangement on the A-engine which had the exhaust valve parallel to the bore, and the intake valve tipped toward the intake manifold giving what has been described as a poly-spherical chamber. That was the A engine.

When it came to the LA engine we made all the valves tipped to the intake manifold and inline as viewed from the front of the engine giving it a wedge shaped combustion chamber. The reason we went to such a change, which triggered totally new cylinder heads and manifolds for the engine was that the engine was designed to go into the Valiant car. The Valiant car was originally not designed to take a V-8 engine. So we were really limited in every which way about getting the engine in place and the older A engine was simply far too wide at the cylinder heads in order to go into the car. So we put the wedge head engine, cylinder heads on top of the A engine and that was what we needed to do in order to get that engine into the Valiant.

A-engine cross-section

engine brochureIn the process we also wanted to take a lot of weight out because the Valiant, I guess the Dart was the companion car of Dodge, wanted to have engines much lighter than what a conventional A engine would be. So we took as much as we could out of the cylinder heads and the intake manifold and the cylinder block which is of course the largest and heaviest piece of an engine. That triggered a new casting process for the cylinder block that allowed us to make all the walls thinner and we took a lot of the weight out of the block.

As for the performance drop in going to a wedge head, Pete Hagenbuch noted that there wasn't one:

The performance improved by getting rid of the silly polysphere. A wedge chamber does have some advantages. One of the advantages is that you can build in a lot of what we call squish, where the chamber is just part of the cylinder head surface and the piston have a flat area that matches up with it, because squish is why you can run 12 to 1 on a wedge head because without squish you would have to run 9 to 1. It gets the charge moving and mixed, you know moving through the chamber at high velocity which means the flame travel is fast and there isn't anything left to burn by the time it gets to top dead center where you expect the detonation. Anything that reduces detonation also helps reduce pre-ignition which is catastrophic.

Detonation can lead to pre-ignition because of the rise in temperature. If you are in more or less constant detonation then sooner or later you going to hole a piston, its going to become pre-ignition which means it starts burning the minute the intake valve opens which means that the full force of the combustion is working against the piston going up and things get really hot in there, and aluminum melts at around 1200°.

Also see

Distributor for 1958 Red Ram V8 (318)
Model IBP-4002
Rotation Clockwise
Advance Control Automatic
Point Gap .015 to .018"
Breaker Arm Spring Tension 17 to 20
Contact Dwell (degrees) 29 to 32 deg
Automatic Advance curve (Distributor speed)
290 to 410 0 Deg
410 0 to 2 Deg
650 4 to 6 Deg
1650 8 to 10 Deg
Vacuum Advance Curve: Manifold vacuum in inches of mercury
6.1 to 7.3 0 Deg
10 4.6 to 6.6 Deg
14 10 to 12 deg
Condenser Capacity (Microfarads) .25 to .285
Timing Mark location (marks 2 deg apart) Fan Drive Pulley
Engine Idle Speed (RPM) 475 to 500
Firing Order 1-8-4-3-6-5-7-2

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